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Pulse radiolysis of aqueous solutions of carboxy, carbamido and pyridyl derivatives of pyridine
Znt. J. Radiat. Phys. Chem. 1971,
Vol. 3, pp. 259-272. Pergamon Press. Printed in Great Britain
PULSE RADIOLYSIS OF AQUEOUS SOLUTIONS OF CARBOXY, CARBAMIDO AND PYRIDYL DERIVATIVES OF PYRIDINE M. SIMIC * and M. EBERT Paterson Laboratories, Christie Hospital and Holt Radium Institute, Manchester M20 9BX, England (Received 21 December 1970)
Abstract-Rate constants of the reactions of the hydrated electrons with carboxy, carbamido and pyridyl derivatives of pyridine were found to be diffusion controlled (1-2 x lV” dm8 mol-l). Some rate constants of the reactions of OH radicals and H atoms were measured also from the formation kinetics of the resulting transient absorption spectra. The k(OH+S) values were found to be dependent on the state of protonation of the solute. The transient absorption spectra resulting from the reactions of e.,,-, OH radicals and in some instances H atoms with these. compounds were recorded and compared to reactions of more complex derivatives [e.g. (nicotinamide adenine dinucleotide)+]. It was concluded that e,,-, OH radicals and H atoms add to the pyridine ring, the state of protonation of the adducts being dependent on pH of the solutions.
INTRODUCTION ONE ELECTRON reaction with nicotinamide adenine dinucleotide (NAD+) and N-methylnicotinamide(l) positive ions has been studied by pulse radiolysis in aqueous
solutions and a correlation of the observed transient absorption spectra to the possible free radicals(2) proposed.
(0 esq- (or*co3-)
+
Q-
_ QCONH?
h
B
In this work the free radicals obtained through the reactions of the primary radicals produced on radiolysis of water, the OH radicals, and in particular hydrated electrons and H atoms with the simpler basic units of NAD+ such as nicotinic acid and nicotinamide have been studied over a wider pH range to elucidate the role of these units in the redox steps of the respiratory chain. In addition, the reactions with the other corresponding isomers of the pyridine derivatives substituted in the 2 and 4 position
6,
6
0
t3
f2
have been investigated in an attempt to discriminate between the electrophilic properties of these isomers and spectral properties of the resulting isomeric free radicals. * Laboratory of Radiation Biology, University of Texas, Austin, Texas 78712, U.S.A. 259
M. SIMIC and M. EBERT
260
Beside the general interest in the reactions of hydrated electrons with the carboxy and amino derivatives of pyridine, we were also interested in other pyridine derivatives which form complexes with various metal ions. The reactions of the primary radicals of water with these complexes are greatly dependent on the reactivities of the ligands and the properties of the ligand free radicals. Some of the dipyridyl derivatives were investigated from this particular point of view. EXPERIMENTAL
All chemicals used were of first grade supplied by Koch-Light. Water was Dissolved air was removed by bubbling argon redistilled from alkaline permanganate. containing < 2 x low6 of 0,. N,O was of medical grade containing negligible amounts of 0,. The pulse radiolysis equipment used has been described by Keenec3). Electron pulses of 0.1-l ps duration were used and the solutions were irradiated in cells with either 2.5 or 5 cm optical path length. Two Bausch 8z Lomb monochromators were coupled to eliminate the scattered light. A band width of 5 nm was used unless otherwise stated. At low wavelengths and in the regions where the parent compound absorbed strongly a pulsed 250 W xenon-lamp was used as the analysing light. The absorbed dose per pulse was monitored with a secondary emission chamber calibrated with solutions of KCNS, using E[(CNS)~-] = 7600 dm3 mol cm -I (760 m2/mol) at 500 nmt4) and G(OH) = 2.8. RESULTS
The optical absorption spectra of the transient species were produced radiolysis by the reaction of the solutes with the primary species of water:
on pulse
HZ0 ------yuh- OH (2.8), H (0.6), eaqm (2.8), H,, H,O,. The G-values, i.e. the number of species produced per 100 eV absorbed, used in calculations, are indicated in the brackets. When argon-bubbled solutions are pulsed the overall contribution of the eaq , OH and H reactions are recorded. The OH contribution was recorded in the presence of N,O. Under these conditions the eaq- were converted almost quantitatively into OH radicals :
(2)
t-w
e aq-
-----•,
OH+OH-+N,.
The H atom contribution is in general relatively small (< 12 per cent) and could be either ignored or corrected. In most of the systems studied the spectra of the transients resulting from the reactions of the eaq- were resolved by the difference method [total absorption minus the (OH + H) contribution]. In some of the systems the OH radicals were removed with excess l-methyl 2-propanol (~-BuOH)‘~). Both the difference and the t-BuOH method yielded the same spectra for the transients produced in the reactions of eaq -. The H atom reactions were studied in acid solutions at pH = 1 adjusted with perchloric acid in the presence of excess t-BuOH (usually 1 moldm-3). Under these conditions, provided the solute concentration does not exceed 1O-2 moldm-3, all the e,,- are converted into H atoms and the OH radicals removed by ~-BuOH(~J.
Pulse radiolysis of aqueous solutions of derivatives of pyridine
261
In the spectral regions where the extinction coefficient of the parent compound was comparable (> 5 per cent) to that of the resulting transient a correction was applied for the increased transmission of light due to the destruction of the parent compound using G(radica1) = G(solute) = 6.2. The rate constants for the reaction of the hydrated electrons were obtained by following the rates of disappearance of the hydrated electron absorption at 650 nm in solutions containing 1O-5 to lOa moldm-3 of the particular solute. Neither the eaq- nor the OH reactions with the investigated solutes resulted in an absorption big enough to interfere with these measurements. The pH of the solutions was adjusted with NaOH and was either self-buffering or buffered with borax. The results of these measurements are presented in Table 1. TABLE 1. RATE CONSTANTS OF THE REACTIONS OF THE HYDRATED ELECTRON WITH SOME PYRIDINE DERIVATIVES AND THE pK VALUES OPTHEPARENT COhiPOUNDSINAQUEOIJSSOLUTIONS
PK, S
_.__~_~
J&q-- + S) (dma mol - s-3
PH
~~~ _ _
Pyridine
7
Picolinic acid Nicotinic acid Isonicotinic acid Nicotinamide Nicotinuric acid Isonicotinamide 2,2’-Dipyridyl 4,4’-Dipyridyl Dipyridylamine
1.0 x 3.1 x 1.1 x 1.0 x 2.4 x 2.4 x 2.1 x 3.2 x 2.5 x 3.3 x 1.4 x
9.1 10.5 10.5 7.5 9.2 9.0 9.2 9.3 9.1
Pyridine ring
100 10s 1010 l@O lOlo lV” 1010 1O’O 10’0 1O’O l@O
5.23 5.32 4.81 4.86 3.40 3.61 444 4.82 -
Substituent 1.01 2.07 l-84 3=17 -
The rate constants for the reactions of the H atoms and OH radicals with some of these compounds were obtained from the formation kinetics of the resulting transients at the maxima of absorption (Table 2). CONSTANTS OF THE REACTIONS OF OH RADICALS AND H ATOMS WITHSOMEPYRIDINEDERIVATWES
TABLE 2. RATE
S
____ Pyridine Nicotinic acid Nicotinamide Nicotinuric acid 2,2’-Dipyridyl 4,4’-Dipyridyl
PH
k(OH + s) (dma md - s-l)
7.0 2.0 9.1
1.8 x 10s 2x10’ 2.3 x log 2.6~108 1.5x1og 1~1x10~ 6.2 x lo* 5.3 x 10s
;:; 7.5 9.3 9.3
PH
k(H+S) (dma mol s-3
1
1.7 x 108 5x108 1.5 x 10s 2x108
-
1 1 1
The transient spectra produced from the reaction of eaq- and OH radicals with 2-, 3- and 4-carboxyl pyridine are presented in Figs. 1, 2 and 3. At about pH 7 these
M. SIMIC and M. EBERT
262
0 200
0 300
Xnm
400
500
FIG. 1. Pulse radiolysis of 2.5 x 1O-4 mol dm3 picolinic acid (Zcarboxy pyridine) in aqueous solutions, pH 7, 1.3 krad/pulse. Transient spectra resulting from the reactions of e,,- (0) and OH (0) (in the presence of N,O, normalized to E scale).
4000
.03
5 7 z E "E -0 u
g .02 2000
.Ol
0 200
300
Xnrn
400
500
0
FIG. 2. Pulse radiolysis of 2.5 x 10d4 mol dm3 nicotinic acid (3-carboxy pyridine) in aqueous solutions, pH 7, 1.3 krad-pulse. Transient spectra resulting from the reactions of e,,- (0) and OH (0) (in the presence of N20, normalized to 8 scale).
Pulse radiolysis
of aqueous solutions
of derivatives
of pyridine
263
compounds exist in the C,NH,COO- form. In the case of nicotinic acid, the biologically relevant isomer (3-carboxy pyridine), the absorption spectra were investigated at different pH’s (Fig. 4). An increase of pH as high as 12.7 did not result in an observable change of the spectra of the transients formed in a reaction with the e W-. On the other hand a decrease of the pH with a consequent conversion of the e,,- to H atoms produc+d a drastic change. The H atoms react quite efficiently with nicotinic acid in the CBNH,COOH form (the pK, values for the carboxy group and the nitrogen in the ring are 2.07 and 4.81 for nicotinic acid, Table 1). In addition to
-a
0.0
0
d
-4
I
200
300
400
500
-c
Xnm FIG. 3. Pulse radiolysis of isonicotinic acid (4-carboxy pyridine) in aqueous solutions, pH 65,1.3 krad/pulse. Transient spectra resulting from the reactions of e*q- (a) and OH (0) (in the presence of N,O, normalized to E scale).
these relatively short-lived transients a strongly absorbing long-lived species (t> 10 ms) or product resulted from the interaction of the e,,- and H atom with nicotinic acid from pH 1 to 12.2 with a X,,, = 340 nm. These species were observed in the presence and absence of excess t-BuOH in argon-purged solutions. But they were absent in N,O-saturated solutions where the reactions of OH radicals were present only. In 1 mol dm-3 CH,OH+ 1O-s mol dm-3 nicotinic acid saturated with N,O at pH = 7 only l H,OH radicals are produced. The methanol radicals do not react with the nicotinic acid to yield transients equivalent to those obtained from the H atom or eaq - reactions. They react with each other in a second-order process. Some amides of carboxy pyridine derivatives were investigated: nicotinamide (Fig. 5), isonicotinamide (Fig. 6) and nicotinuric acid (Fig. 7) which is a peptide (nicotinyl-glycine) but falls into the same class as nicotinamide when the e,,- reaction is considered. These compounds were investigated only in neutral solutions due to their instability at low and high pH’s.
M. SIMI(: and M. EBERT
264
400
300
Xnm
PIG. 4. Pulse radiolysis of 1 mol dm-3 r-BuOH+2 acid in aqueous solutions, 5 krad/puIse. Absorption and the resulting products (read after 1 ms) of eaqinitial; A, product; and H adduct at pH 1; 0,
x 10e3 mol dm-a nicotinic spectra of initial transients adduct at pH 5-12-7; Lz, initial; 0, product.
Xnm
FIG. 5, Pulse radiolysis of S x 10m4 mol drne3 nicotinamide in aqueous solutions, pH 7,1.3 kradlpulse. Transient spectra resulting from the reactions of cap- (e) and OH (0) (in the presence of N,O, normalized to the F scale).
Pulse radiolysis of aqueous solutions of derivatives
of pyridine
265
loo0
iOO0
Anm
FIG. 6. Pulse radiolysis of 5 x lo-‘mol dm-s isonicotinamide in aqueous solutions, pH 7.5, l-3 krad/pulse. Transient spectra resulting from the reactions of e*,- (0) and OH (0) (in the presence of N,O normalized to the E scale).
FIG. 7. Pulse radiolysis of 5 x IO4 mol dm-8 nicotinuric acid (nicotinyl glycine) in aqueous solutions, pH 8.5, 1.3 krad/pulse. Transient spectra resulting from the reactions of e,,- (0) and OH (0) (in the presence of NeO, normalized to
266
M. SIMICand M. EBERT
FIG. 8. Pulse radiolysis of IO-3 mol dm-3 dipyridyls in aqueous solutions in the presence of 2.5 x 1O-2 mol dmA3 N,O, 0.5 krad/pulse. Transient spectra resulting from the reactions of OH radicals with 2,2’-dipyridyl (0) at pH 9.3 and 4,4’-dipyridyl (0) at pH 7.
FIG. 9. Pulse radiolysis of 2.5 x 10e3 mol dm-” 2,2’-dipyridyl t- 1 mol dme3 t-BuOH in aqueous solutions, 0.55 krad/pulse. Transient spectra resulting from the reactions of e,,- at pH 7 (O), pH 12.7 (o) and H atoms at pH 1 (0).
Pulse radiolysis
of aqueous solutions of derivatives
of pyridine
267
D
6
0
300
400
500
600
700
xnm FIG. 10. Pulse radiolysis of 2.5 x lo-* mol dm-s 4,4’-dipyridyl+ 1 mol dm-s GBuOH in aqueous solutions, 055 krad/pulse. Transient spectra resulting from the reactions of e,,- at pH 3-7 ( x ), pH 7 (O), pH 12.7 (o) and H atoms at PH l(O).
The reactions of the primary species (H, OH and eaq-) were investigated only with the symmetrical 2,2’ and 4,4’ dipyridyls. The spectra of the OH adducts of these compounds in the presence of N,O are shown in Fig. 8. The transients resulting from the reactions of the eaq- and H atoms were studied over a wide pH range in the presence of 1 mol dm-3 t-BuOH and are given in Figs. 9 and 10. The spectral change for the electron adduct to the 4,4’-dipyridyl as a function of pH was investigated (Fig. 10) and pK, = 10.5 found. DISCUSSION
We shall discuss separately the results of the carboxy and carbamido derivatives of dipyridyls which are important biological molecules and the dipyridyls which are of interest in the chemistry of metal complexes. The pyridyl group, -C5NH4, will be designated as Py. Carboxy and carbamido pyridines
The state of protonation of the nitrogen in the pyridine ring as well as the state of protonation of the substituents have to be considered in the study of reactions with H atoms, OH radicals and hydrated electrons. In near-neutral solutions the carboxy derivatives are present in the PyCOO- form while at pH 1 in the H+PyCOOH form. In the intermediate pH’s they exist as the zwitter ions H+PyCOO-. The reaction of the OH radicals with the pyridine derivatives is expected to lead to addition@) to the ring as in the case of other aromatic systems@). In near-neutral solution, therefore, *Py(OH)COOOH + PyCOO- B 19
M. SIP.& and M. EBERT
268
Besides this obvious reaction it was argued (s) that the OH radicals in aqueous solutions of pyridine could add to the nitrogen in the ring giving C,H,N-OH radicals which absorb at lower wavelengths than the adducts to the ring. This reaction may also take place with the carboxy derivatives. The spectra show beside the easily recorded maximum in the 320-340 nm region an additional band at much lower wavelengths (A<260 nm). The evidence for the addition of OH radicals to the nitrogen in the ring is still lacking. The other absorption maxima could be due to the formation of isomeric ring adducts. The distribution of the isomers probably depends on the state of protonation of the parent molecules. The amides of the 3- and 4-carboxy derivatives gave very similar spectra when attacked by OH radicals in near-neutral solutions. In the case of isonicotinamide (Fig. 5), for instance, the 340 nm peak was more pronounced than the equivalent one for isonicotinic acid (Fig. 3) indicating a possible enhancement of one of the isomers due to the effect of the substituent. The transient spectrum resulting from the attack on the OH radicals of nicotinuric acid seemed at first somewhat puzzling. Kinetic data suggest that OH radicals also attack the side-chain : /.Py(OH)CONHCH,COO-
@a) OH + PyCONHCH,COO-
-PyCONHCHCOO-.
(3b) The overall rate for the OH reaction
is 1.1 x log dm3 mol-l
s-l while the rate with the
side-chain is about 4 x lo* dm3 mol-l s-l. The RCONHCHCOOradical has two absorption bands with A,,, at x 260 and z 340 nm, and E z 1000 m2/mol and 300 m2/mol, respectively(lO). Superposition of the spectra of an OH adduct which should absorb about 320 nm as in the case of nicotinamide and of the sidechain (peptide) radical could lead to the overall observed spectrum. The reaction rates of the OH radical with the deprotonated pyridine ring of various pyridine derivatives reported here is of the order of 10s dm3 mol-l s-l compared to k = 1.8 x log dm3 mol-l s-r for deprotonated pyridine@). On the other hand, the rates of reaction of OH with the protonated ring is at least an order of magnitude less for nicotinic acid (pH 3.15) and about two orders of magnitude less for pyridine (pH 2). The higher OH rate for the protonated carboxy derivative could be due to the activation of the ring through the effect of the carboxy group, i.e. the change of electron densities at various positions in the ring. Other carboxy derivatives of pyridine are expected to behave very similarly. The reactions of esq- with the carboxy derivatives give pyridine ring adducts which are quickly protonated in solutions at pH 4-12.7. (4)
e,,-+
PyCOO-
11f
____f
PyHCOO-.
The reaction of eaq- is diffusion controlled (1 - 2 x 1O1Odm3 mol-’ s-l) and is about twenty times faster than for pyridine. This considerable increase in rate is probably due to the perturbation of the r system of the ring by COO- and CONH,. This is analogous to the electron reactivity of the benzene ring in benzene and its carboxy derivatives (Z 10’ and 3 x 10” dm3 mol-r s-r, respectively)“). The similarities are extended further to the spectra of these compounds. The electron adducts of the
Pulse radiolysis of aqueous solutions of derivatives of pyridine
269
carboxy derivatives of both benzene(ll* 12)and pyridine have two bands, the stronger at around 300 nm and the weaker at around 400 nm. While the electron adduct of benzoic acid shows spectral transition in highly alkaline solutions (pK, = 12) attributed to the appearance of the non-protonated electron adduct’12), the electron adduct of nicotinic acid does not show any marked difference up to pH 12.7. This is probably due to a higher charge concentration in the electron adduct to pyridine around nitrogen in the ring, while the charge in the benzene ring is more evenly distributed, resulting in lower pK, value for the benzene derivatives. Radicals similar to those described here have been prepared previously’2) by reduction of the corresponding pyridinium iodide with Zn or Mg and were found to be relatively stable in non-aqueous solutions. The spectra of these radicals with maxima at 300 and 400 nm strongly suggest that the reaction of eaq- with the carboxy derivatives of pyridine leads to similar transients, e.g.
o- l
(5) eaq- + I
’
N’
coo-
\
H+
coo-
1 /
Q
The interaction of the unpaired electron with the carboxy group in the 2-, 3- or 4position could lead to different optical transitions. The spectra of the electron adducts of these three derivatives are characteristic, i.e. the separation of the two bands and the ratio of extinction coefficient (E) of the bands being different. It is interesting to note that the spectra of the corresponding amides (Figs. 5 and 6) are almost identical except for a somewhat higher E value. The disappearance of the electron adducts through a second-order reaction is followed by the formation of a long-lived species with h,,, = 340 nm and another band at lower wavelengths which was not investigated. A similar product was observed in the steady-state and pulse radiolysis of N-methylnicotinamide and NAD+(l). It has been suggested that the product is a dimer of a radical formed through one electron reduction of these solutes(13). The original notion that the product might be NADH which was based on purely spectral considerations had to be discarded since the radiation product lacked enzymic activity(14). We may conclude now that in a one-electron process nicotinic acid, nicotinamide, N-methylnicotinamide and NAD+ behave almost identically. Furthermore the other two isomers, the 2- and the 4-carboxy derivatives, are in that respect very much the same. If this is so, what is the role of adenine in NADf and why are the 2- and 4-derivatives not equally efficient? Obviously the charge, the steric hindrance and the electric potentials play an important role. The reaction rates of H atoms with the pyridine derivatives are of the order of 10s dm3 mol-l s-l and are considerably lower than those of electrons. They are very similar to the reaction rates of H atoms with the heterocyclic systems containing two N atoms in the ring, e.g. the pyrimidine derivatives’15). The transient spectrum resulting from the reactions of H atoms with nicotinic acid is very different from the spectrum of the isomeric protonated electron adduct. The H atoms are likely to be adding to the ring since abstraction reactions have in general low rates which, in an aromatic system, could be even further reduced. Assuming spectroscopic features of the H atom adducts to be similar to those of the electron adducts, two absorption
270
M.
SIMIC
and
M.
EBERT
bands could be expected for these transients. The apparent presence of three bands at 410, 340 and < 290 nm perhaps indicates more than one transient formed. It is possible that the H atoms are less selective in the site of attack than the conjoint adding of an electron and proton. Here again the long-lived species (or product) which is formed from the reaction of the transients absorbs at 340 nm, although the absorption is only half of that in neutral or alkaline solutions. A higher state of protonation of this long-lived species at pH 1 could lead to a lower extinction coefficient. On the other hand, only one of the isomers might give this particular product while the products from the other isomers lack an absorption in this region. Dipyridyls The reactions of the OH radicals with the dipyridyls are expected to proceed at different rates depending on the state of protonation of the two rings. For the fully deprotonated dipyridyls the rates are close to those for deprotonated pyridine and its derivatives when the two-ring structure is taken into consideration. Here again the OH radicals are expected to add to the ring. While the 4,4’-dimer has only one major absorption band in the 280-700 nm region with &,,, = 365 nm, the 2,2’-dimer has two bands with X,,, = 365 and 305 nm. It is probable that the OH radicals add at two different positions in the latter case. The extinction coefficients of the dipyridyl OH adducts are considerably higher than for the compounds with a single pyridine ring. The reactions of the electrons with the dipyridyls are also diffusion controlled of the order of lOlo dm3 mol-l ssi. The spectrum of the electron adduct to the 2,2’dimer is pH independent between 4 and 12.7, with h,,, = 365 nm and another weak band at 460 nm. The spectrum of the electron adduct to the 4,4’-derivatives is pH independent only between 3.7 and 7 with h,_, = 385 and 570 nm (Fig. 10). The spectrum of the electron adduct to the 4,4’ dipyridyl is very similar to the spectrum of methylviologen cation radicalt16)
which was found to have two major bands at 400 and 600 nm and Ed,,,,= lo4 dm3 mol-l cm-l (103 m2/mol) in acetonitrile solutions. We would like to suggest therefore that the reaction of hydrated electrons with dipyridyls leads to transients with the same structure, e.g.
The small shift of x 20 nm in X,,, probably arises from the difference in solvents and slightly different structures of the radicals. The disappearance of the methylviologen cation radical in the presence of watero6) is in keeping with the transient nature of the dipyridyl electron adduct in aqueous solutions. The products of the decay reactions are unknown for both radicals.
Pulse radiolysis of aqueous solutions of derivatives
of pyridine
271
The electron adduct to 4,4’-dipyridyl has at pH 12.7 a noticeably different spectrum with both bands shifted towards lower wavelengths (pK, = 10.5) (Fig. 10). It is our belief that this spectrum at pH 12.7 belongs to the unprotonated electron adduct of the 4,4’-dipyridyl. The absence of the unprotonated electron adduct to 2,2’-dipyridyl at the same pH could be explained on the basis of a higher resonance stability of the para-para form. In these, as in some other dimeric systems, such as diphenyl where a highly resonating structure of the electron adduct is present, the molar extinction coefficients are very high ( > 104 dm3 mol-1 cm-l (= 10s m2/mol), and the pK, values for the protonation of the negative electron adduct are considerably lowered. It is interesting to note that the reaction of electrons with the CO(P~Z)~+ complex did not lead to the formation of the electron adduct to the ligand in spite of relatively very high electron rates for Py2, but rather to reduction of ColI1 and an intermediate assigned to a low-spin complex with one unpaired electronu7). It is of interest to find out which substituents of Py, could retain the electron in the ligand as in the case of the [Co(NH,),-benzoato] complex where the NO, group was found to be in that respect quite efficientus). The H atom adducts to the dipyridyls which are formed at pH 1 have lower extinction coefficients than the electron adducts, although much higher than, for instance, the H atom adduct of nicotinic acid. This behaviour of H atom adducts follows a general trend observed for many other systems(lv).
REFERENCES 1. E. J. LAND and A. J. SWALLOW,Biochem. biophys. Acta 1968,162,327.
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.
E. M. KOSOWERand E. J. POZIOMEK,J. Am. them. Sot. 1964,86,5515. J. P. KEENE, J. sci. Instrum. 1964, 41, 493. J. H. BAXENDALE,P. L. T. BEVANand D. A. STOIC, Trans. Faraday Sot. 1968, 64, 2389. M. &MI&, P. NETA and E. HAYON, J. phys. Chem. 1969, 73, 3794. M. SIMIC and M. Z. HOFFMAN. J. Am. them. Sot. 1970.92.6096. M. ANBAR and P. NETA, Znt. i appl. Radiat. Isotopes, i96$ 16, 227. B. CER~EK and M. EBERT, Trans:karaday Sot. 1967, 63, 1687. L. M. DORFMAN.R. E. BWLER and I. A. TAUB. J. them. Phvs. 1962.36. 3051. M. SIMIC, P. NE;A and E. HAYON, J. Am. them: Sot. 1970,92,4763: ’ D. F. SANGSTER,J. phys. Chem. 1966,70, 1712. M. Z. HOFFMANand M. SIMIC, Fourth International Congress of Radiation Research, Evian, 1970, Abstract No. 383. G. STEIN and A. J. SWALLOW,J. them. Sot. 1958, 306. A. J. SWALLOW.Biochem. J. 1955. 61. 197. G. SCHOLESand M. SIMIC, Biochik. biophys. Acta 1968, 166, 255. E. M. KOSOWERand J. L. COTTER,J. Am. them. Sot. 1964,86, 5524. J. H. BAXENDALE,Ric. Sci. 1970, 68, 43. M. Z. HOFFMANand M. SIMIC,J. Am. them. Sot. 1970,92,5533. M. SIMIC and M. Z. HOFFMAN,to be published.
Resume---On montre que les vitesses des reactions des electrons hydrates avec les derives carboxyliques, carbamidiques et pyridyliques de la pyridine sont limit& par la diffusion (k(e,,-+S)a l-2 x 1Orodm* mol-l s-l). On a aussi mesure quelques constantes de vitesse des radicaux OH et des atomes H a partir de la cinetique de croissance des spectres d’absorption transitoires correspondants; les valeurs de k(OH + S) dependent du degre de protonation du solute. On enregistre les spectres d’absorption transitoires resultant des reactions de e,,-, des radicaux OH et, dam quelques cas, des atomes H avec ces composts, et on les compare avec les reactions des derives plus complexes [par exemple (nicotinamide dinuclCotide adtnine)+]. On conclut que e,-, OH et atomes H s’ajoutent sur l’anneau pyridinique, le degrt de la protonation des radicaux form& dtpendant du pH des solutions.
212
M.
SIMIC:and
M.
EBERT
Pe3lOMe -
CKOpOCTHbIe KOHCTaHTbI peaKL@iti NApaTH3HpOBaHHbIX 3JIeKTpOHOB C Kap6OKCK; mipqqun-nepnaaraMu nkipnAnrisa 06HapyxRHbI KaK YITpaBAReMbI AlW&$y3liek aMBAow (1-2 X lo" dm3 IllOl-'). HeKOTOpbIeCKOpOCTHbleKOHCTaHTbIpeaK~~e~paAIiKaJIOB OHu aTOMOB H 6b1,IH H3MepeHbI TOXCC OTHOCBTeAbHO KLiHeTHKU 06pa30BaHAx pC3yJIbTupyIou(lfX IIepeXOAHbIX CIICKTpOB IIOl-JIOIAeHUR. 3HaVeHWI k(OH+S)OKa3amiCb 3aBUCUMbIMU OT COCTOIIHWi IIpOTOHH3aUHH PaCTBOpeHHOrO BeIIJeCTBa. nepCXOAHble CIlCKTpbI IIOI-JIOIAeHHff pC3yJIbTHpyFOIAW 113 peaKA&% &-, paAHKaJIOB OH a B HeKOTOpbIX ClIy¶aSIXaTOMOB H C 3TIIMH COeAUHeHUSlMLi 6bIna perHCTpHpOBaHb1 H COIIOCTaBJICHbIC peaKWiBMH donee KOMlUIeKCHbIX AepSiBaTOB [HaIIp. (HuKOT~aMuA~eHwHn~Hy~COT~A)+]. EbIJIO Ii aTOMb H J'BCAWIliBaIOT IIHpHAWHOBbIi L@DUl, IIpH'feM 3aKJIH)YCHO, YTO pWWKaJIb1 f&c,- OH COCTORHHC IIpOTOH5i3aIWiaAAyKTOB3aBWZHMO OT pH paCTBOpOB.
Zusammenfassung-Es wird aufgewiesen, dass die Geschwindigkeitskonstanten der Reaktionen von hydratisierten Elektronen mit Karboxy-, Karbamidound Pyridyl-Derivaten des Pyridins diffusionsbeschrankt sind (1-2 x lOlo dms mol-l s-l). Einige Geschwindigkeitskonstanten der Reaktionen von OH Radikalen und H Atomen wurden such aus der Entstehungskinetik der entstprechendren transienten Absorptionsspektren ermessen und es wurde festgestellt, dass die Werte von k(OH + S) vom Grad der Protonisierung der gelosten Substanz abhlngen. Die transienten Absorptionsspektren die aus Reaktionen von e&a-, OH Radikalen und in manchen Fallen von H Atomen mit diesen Verbindungen stammen, wurden aufgenommen, und mit Reaktionen mit komplizierteren Derivaten [z.B. (Nikotinamid-Adenin-Dinukleotid)+] verglichen. Man schliesst, dass e,,-, OH Radikale und H Atome zum Pyridinring addiert werden, wobei der Protonisierungsgrad der Addukte vom pH der Losung abhlngt.
Vol. 3, pp. 259-272. Pergamon Press. Printed in Great Britain
PULSE RADIOLYSIS OF AQUEOUS SOLUTIONS OF CARBOXY, CARBAMIDO AND PYRIDYL DERIVATIVES OF PYRIDINE M. SIMIC * and M. EBERT Paterson Laboratories, Christie Hospital and Holt Radium Institute, Manchester M20 9BX, England (Received 21 December 1970)
Abstract-Rate constants of the reactions of the hydrated electrons with carboxy, carbamido and pyridyl derivatives of pyridine were found to be diffusion controlled (1-2 x lV” dm8 mol-l). Some rate constants of the reactions of OH radicals and H atoms were measured also from the formation kinetics of the resulting transient absorption spectra. The k(OH+S) values were found to be dependent on the state of protonation of the solute. The transient absorption spectra resulting from the reactions of e.,,-, OH radicals and in some instances H atoms with these. compounds were recorded and compared to reactions of more complex derivatives [e.g. (nicotinamide adenine dinucleotide)+]. It was concluded that e,,-, OH radicals and H atoms add to the pyridine ring, the state of protonation of the adducts being dependent on pH of the solutions.
INTRODUCTION ONE ELECTRON reaction with nicotinamide adenine dinucleotide (NAD+) and N-methylnicotinamide(l) positive ions has been studied by pulse radiolysis in aqueous
solutions and a correlation of the observed transient absorption spectra to the possible free radicals(2) proposed.
(0 esq- (or*co3-)
+
Q-
_ QCONH?
h
B
In this work the free radicals obtained through the reactions of the primary radicals produced on radiolysis of water, the OH radicals, and in particular hydrated electrons and H atoms with the simpler basic units of NAD+ such as nicotinic acid and nicotinamide have been studied over a wider pH range to elucidate the role of these units in the redox steps of the respiratory chain. In addition, the reactions with the other corresponding isomers of the pyridine derivatives substituted in the 2 and 4 position
6,
6
0
t3
f2
have been investigated in an attempt to discriminate between the electrophilic properties of these isomers and spectral properties of the resulting isomeric free radicals. * Laboratory of Radiation Biology, University of Texas, Austin, Texas 78712, U.S.A. 259
M. SIMIC and M. EBERT
260
Beside the general interest in the reactions of hydrated electrons with the carboxy and amino derivatives of pyridine, we were also interested in other pyridine derivatives which form complexes with various metal ions. The reactions of the primary radicals of water with these complexes are greatly dependent on the reactivities of the ligands and the properties of the ligand free radicals. Some of the dipyridyl derivatives were investigated from this particular point of view. EXPERIMENTAL
All chemicals used were of first grade supplied by Koch-Light. Water was Dissolved air was removed by bubbling argon redistilled from alkaline permanganate. containing < 2 x low6 of 0,. N,O was of medical grade containing negligible amounts of 0,. The pulse radiolysis equipment used has been described by Keenec3). Electron pulses of 0.1-l ps duration were used and the solutions were irradiated in cells with either 2.5 or 5 cm optical path length. Two Bausch 8z Lomb monochromators were coupled to eliminate the scattered light. A band width of 5 nm was used unless otherwise stated. At low wavelengths and in the regions where the parent compound absorbed strongly a pulsed 250 W xenon-lamp was used as the analysing light. The absorbed dose per pulse was monitored with a secondary emission chamber calibrated with solutions of KCNS, using E[(CNS)~-] = 7600 dm3 mol cm -I (760 m2/mol) at 500 nmt4) and G(OH) = 2.8. RESULTS
The optical absorption spectra of the transient species were produced radiolysis by the reaction of the solutes with the primary species of water:
on pulse
HZ0 ------yuh- OH (2.8), H (0.6), eaqm (2.8), H,, H,O,. The G-values, i.e. the number of species produced per 100 eV absorbed, used in calculations, are indicated in the brackets. When argon-bubbled solutions are pulsed the overall contribution of the eaq , OH and H reactions are recorded. The OH contribution was recorded in the presence of N,O. Under these conditions the eaq- were converted almost quantitatively into OH radicals :
(2)
t-w
e aq-
-----•,
OH+OH-+N,.
The H atom contribution is in general relatively small (< 12 per cent) and could be either ignored or corrected. In most of the systems studied the spectra of the transients resulting from the reactions of the eaq- were resolved by the difference method [total absorption minus the (OH + H) contribution]. In some of the systems the OH radicals were removed with excess l-methyl 2-propanol (~-BuOH)‘~). Both the difference and the t-BuOH method yielded the same spectra for the transients produced in the reactions of eaq -. The H atom reactions were studied in acid solutions at pH = 1 adjusted with perchloric acid in the presence of excess t-BuOH (usually 1 moldm-3). Under these conditions, provided the solute concentration does not exceed 1O-2 moldm-3, all the e,,- are converted into H atoms and the OH radicals removed by ~-BuOH(~J.
Pulse radiolysis of aqueous solutions of derivatives of pyridine
261
In the spectral regions where the extinction coefficient of the parent compound was comparable (> 5 per cent) to that of the resulting transient a correction was applied for the increased transmission of light due to the destruction of the parent compound using G(radica1) = G(solute) = 6.2. The rate constants for the reaction of the hydrated electrons were obtained by following the rates of disappearance of the hydrated electron absorption at 650 nm in solutions containing 1O-5 to lOa moldm-3 of the particular solute. Neither the eaq- nor the OH reactions with the investigated solutes resulted in an absorption big enough to interfere with these measurements. The pH of the solutions was adjusted with NaOH and was either self-buffering or buffered with borax. The results of these measurements are presented in Table 1. TABLE 1. RATE CONSTANTS OF THE REACTIONS OF THE HYDRATED ELECTRON WITH SOME PYRIDINE DERIVATIVES AND THE pK VALUES OPTHEPARENT COhiPOUNDSINAQUEOIJSSOLUTIONS
PK, S
_.__~_~
J&q-- + S) (dma mol - s-3
PH
~~~ _ _
Pyridine
7
Picolinic acid Nicotinic acid Isonicotinic acid Nicotinamide Nicotinuric acid Isonicotinamide 2,2’-Dipyridyl 4,4’-Dipyridyl Dipyridylamine
1.0 x 3.1 x 1.1 x 1.0 x 2.4 x 2.4 x 2.1 x 3.2 x 2.5 x 3.3 x 1.4 x
9.1 10.5 10.5 7.5 9.2 9.0 9.2 9.3 9.1
Pyridine ring
100 10s 1010 l@O lOlo lV” 1010 1O’O 10’0 1O’O l@O
5.23 5.32 4.81 4.86 3.40 3.61 444 4.82 -
Substituent 1.01 2.07 l-84 3=17 -
The rate constants for the reactions of the H atoms and OH radicals with some of these compounds were obtained from the formation kinetics of the resulting transients at the maxima of absorption (Table 2). CONSTANTS OF THE REACTIONS OF OH RADICALS AND H ATOMS WITHSOMEPYRIDINEDERIVATWES
TABLE 2. RATE
S
____ Pyridine Nicotinic acid Nicotinamide Nicotinuric acid 2,2’-Dipyridyl 4,4’-Dipyridyl
PH
k(OH + s) (dma md - s-l)
7.0 2.0 9.1
1.8 x 10s 2x10’ 2.3 x log 2.6~108 1.5x1og 1~1x10~ 6.2 x lo* 5.3 x 10s
;:; 7.5 9.3 9.3
PH
k(H+S) (dma mol s-3
1
1.7 x 108 5x108 1.5 x 10s 2x108
-
1 1 1
The transient spectra produced from the reaction of eaq- and OH radicals with 2-, 3- and 4-carboxyl pyridine are presented in Figs. 1, 2 and 3. At about pH 7 these
M. SIMIC and M. EBERT
262
0 200
0 300
Xnm
400
500
FIG. 1. Pulse radiolysis of 2.5 x 1O-4 mol dm3 picolinic acid (Zcarboxy pyridine) in aqueous solutions, pH 7, 1.3 krad/pulse. Transient spectra resulting from the reactions of e,,- (0) and OH (0) (in the presence of N,O, normalized to E scale).
4000
.03
5 7 z E "E -0 u
g .02 2000
.Ol
0 200
300
Xnrn
400
500
0
FIG. 2. Pulse radiolysis of 2.5 x 10d4 mol dm3 nicotinic acid (3-carboxy pyridine) in aqueous solutions, pH 7, 1.3 krad-pulse. Transient spectra resulting from the reactions of e,,- (0) and OH (0) (in the presence of N20, normalized to 8 scale).
Pulse radiolysis
of aqueous solutions
of derivatives
of pyridine
263
compounds exist in the C,NH,COO- form. In the case of nicotinic acid, the biologically relevant isomer (3-carboxy pyridine), the absorption spectra were investigated at different pH’s (Fig. 4). An increase of pH as high as 12.7 did not result in an observable change of the spectra of the transients formed in a reaction with the e W-. On the other hand a decrease of the pH with a consequent conversion of the e,,- to H atoms produc+d a drastic change. The H atoms react quite efficiently with nicotinic acid in the CBNH,COOH form (the pK, values for the carboxy group and the nitrogen in the ring are 2.07 and 4.81 for nicotinic acid, Table 1). In addition to
-a
0.0
0
d
-4
I
200
300
400
500
-c
Xnm FIG. 3. Pulse radiolysis of isonicotinic acid (4-carboxy pyridine) in aqueous solutions, pH 65,1.3 krad/pulse. Transient spectra resulting from the reactions of e*q- (a) and OH (0) (in the presence of N,O, normalized to E scale).
these relatively short-lived transients a strongly absorbing long-lived species (t> 10 ms) or product resulted from the interaction of the e,,- and H atom with nicotinic acid from pH 1 to 12.2 with a X,,, = 340 nm. These species were observed in the presence and absence of excess t-BuOH in argon-purged solutions. But they were absent in N,O-saturated solutions where the reactions of OH radicals were present only. In 1 mol dm-3 CH,OH+ 1O-s mol dm-3 nicotinic acid saturated with N,O at pH = 7 only l H,OH radicals are produced. The methanol radicals do not react with the nicotinic acid to yield transients equivalent to those obtained from the H atom or eaq - reactions. They react with each other in a second-order process. Some amides of carboxy pyridine derivatives were investigated: nicotinamide (Fig. 5), isonicotinamide (Fig. 6) and nicotinuric acid (Fig. 7) which is a peptide (nicotinyl-glycine) but falls into the same class as nicotinamide when the e,,- reaction is considered. These compounds were investigated only in neutral solutions due to their instability at low and high pH’s.
M. SIMI(: and M. EBERT
264
400
300
Xnm
PIG. 4. Pulse radiolysis of 1 mol dm-3 r-BuOH+2 acid in aqueous solutions, 5 krad/puIse. Absorption and the resulting products (read after 1 ms) of eaqinitial; A, product; and H adduct at pH 1; 0,
x 10e3 mol dm-a nicotinic spectra of initial transients adduct at pH 5-12-7; Lz, initial; 0, product.
Xnm
FIG. 5, Pulse radiolysis of S x 10m4 mol drne3 nicotinamide in aqueous solutions, pH 7,1.3 kradlpulse. Transient spectra resulting from the reactions of cap- (e) and OH (0) (in the presence of N,O, normalized to the F scale).
Pulse radiolysis of aqueous solutions of derivatives
of pyridine
265
loo0
iOO0
Anm
FIG. 6. Pulse radiolysis of 5 x lo-‘mol dm-s isonicotinamide in aqueous solutions, pH 7.5, l-3 krad/pulse. Transient spectra resulting from the reactions of e*,- (0) and OH (0) (in the presence of N,O normalized to the E scale).
FIG. 7. Pulse radiolysis of 5 x IO4 mol dm-8 nicotinuric acid (nicotinyl glycine) in aqueous solutions, pH 8.5, 1.3 krad/pulse. Transient spectra resulting from the reactions of e,,- (0) and OH (0) (in the presence of NeO, normalized to
266
M. SIMICand M. EBERT
FIG. 8. Pulse radiolysis of IO-3 mol dm-3 dipyridyls in aqueous solutions in the presence of 2.5 x 1O-2 mol dmA3 N,O, 0.5 krad/pulse. Transient spectra resulting from the reactions of OH radicals with 2,2’-dipyridyl (0) at pH 9.3 and 4,4’-dipyridyl (0) at pH 7.
FIG. 9. Pulse radiolysis of 2.5 x 10e3 mol dm-” 2,2’-dipyridyl t- 1 mol dme3 t-BuOH in aqueous solutions, 0.55 krad/pulse. Transient spectra resulting from the reactions of e,,- at pH 7 (O), pH 12.7 (o) and H atoms at pH 1 (0).
Pulse radiolysis
of aqueous solutions of derivatives
of pyridine
267
D
6
0
300
400
500
600
700
xnm FIG. 10. Pulse radiolysis of 2.5 x lo-* mol dm-s 4,4’-dipyridyl+ 1 mol dm-s GBuOH in aqueous solutions, 055 krad/pulse. Transient spectra resulting from the reactions of e,,- at pH 3-7 ( x ), pH 7 (O), pH 12.7 (o) and H atoms at PH l(O).
The reactions of the primary species (H, OH and eaq-) were investigated only with the symmetrical 2,2’ and 4,4’ dipyridyls. The spectra of the OH adducts of these compounds in the presence of N,O are shown in Fig. 8. The transients resulting from the reactions of the eaq- and H atoms were studied over a wide pH range in the presence of 1 mol dm-3 t-BuOH and are given in Figs. 9 and 10. The spectral change for the electron adduct to the 4,4’-dipyridyl as a function of pH was investigated (Fig. 10) and pK, = 10.5 found. DISCUSSION
We shall discuss separately the results of the carboxy and carbamido derivatives of dipyridyls which are important biological molecules and the dipyridyls which are of interest in the chemistry of metal complexes. The pyridyl group, -C5NH4, will be designated as Py. Carboxy and carbamido pyridines
The state of protonation of the nitrogen in the pyridine ring as well as the state of protonation of the substituents have to be considered in the study of reactions with H atoms, OH radicals and hydrated electrons. In near-neutral solutions the carboxy derivatives are present in the PyCOO- form while at pH 1 in the H+PyCOOH form. In the intermediate pH’s they exist as the zwitter ions H+PyCOO-. The reaction of the OH radicals with the pyridine derivatives is expected to lead to addition@) to the ring as in the case of other aromatic systems@). In near-neutral solution, therefore, *Py(OH)COOOH + PyCOO- B 19
M. SIP.& and M. EBERT
268
Besides this obvious reaction it was argued (s) that the OH radicals in aqueous solutions of pyridine could add to the nitrogen in the ring giving C,H,N-OH radicals which absorb at lower wavelengths than the adducts to the ring. This reaction may also take place with the carboxy derivatives. The spectra show beside the easily recorded maximum in the 320-340 nm region an additional band at much lower wavelengths (A<260 nm). The evidence for the addition of OH radicals to the nitrogen in the ring is still lacking. The other absorption maxima could be due to the formation of isomeric ring adducts. The distribution of the isomers probably depends on the state of protonation of the parent molecules. The amides of the 3- and 4-carboxy derivatives gave very similar spectra when attacked by OH radicals in near-neutral solutions. In the case of isonicotinamide (Fig. 5), for instance, the 340 nm peak was more pronounced than the equivalent one for isonicotinic acid (Fig. 3) indicating a possible enhancement of one of the isomers due to the effect of the substituent. The transient spectrum resulting from the attack on the OH radicals of nicotinuric acid seemed at first somewhat puzzling. Kinetic data suggest that OH radicals also attack the side-chain : /.Py(OH)CONHCH,COO-
@a) OH + PyCONHCH,COO-
-PyCONHCHCOO-.
(3b) The overall rate for the OH reaction
is 1.1 x log dm3 mol-l
s-l while the rate with the
side-chain is about 4 x lo* dm3 mol-l s-l. The RCONHCHCOOradical has two absorption bands with A,,, at x 260 and z 340 nm, and E z 1000 m2/mol and 300 m2/mol, respectively(lO). Superposition of the spectra of an OH adduct which should absorb about 320 nm as in the case of nicotinamide and of the sidechain (peptide) radical could lead to the overall observed spectrum. The reaction rates of the OH radical with the deprotonated pyridine ring of various pyridine derivatives reported here is of the order of 10s dm3 mol-l s-l compared to k = 1.8 x log dm3 mol-l s-r for deprotonated pyridine@). On the other hand, the rates of reaction of OH with the protonated ring is at least an order of magnitude less for nicotinic acid (pH 3.15) and about two orders of magnitude less for pyridine (pH 2). The higher OH rate for the protonated carboxy derivative could be due to the activation of the ring through the effect of the carboxy group, i.e. the change of electron densities at various positions in the ring. Other carboxy derivatives of pyridine are expected to behave very similarly. The reactions of esq- with the carboxy derivatives give pyridine ring adducts which are quickly protonated in solutions at pH 4-12.7. (4)
e,,-+
PyCOO-
11f
____f
PyHCOO-.
The reaction of eaq- is diffusion controlled (1 - 2 x 1O1Odm3 mol-’ s-l) and is about twenty times faster than for pyridine. This considerable increase in rate is probably due to the perturbation of the r system of the ring by COO- and CONH,. This is analogous to the electron reactivity of the benzene ring in benzene and its carboxy derivatives (Z 10’ and 3 x 10” dm3 mol-r s-r, respectively)“). The similarities are extended further to the spectra of these compounds. The electron adducts of the
Pulse radiolysis of aqueous solutions of derivatives of pyridine
269
carboxy derivatives of both benzene(ll* 12)and pyridine have two bands, the stronger at around 300 nm and the weaker at around 400 nm. While the electron adduct of benzoic acid shows spectral transition in highly alkaline solutions (pK, = 12) attributed to the appearance of the non-protonated electron adduct’12), the electron adduct of nicotinic acid does not show any marked difference up to pH 12.7. This is probably due to a higher charge concentration in the electron adduct to pyridine around nitrogen in the ring, while the charge in the benzene ring is more evenly distributed, resulting in lower pK, value for the benzene derivatives. Radicals similar to those described here have been prepared previously’2) by reduction of the corresponding pyridinium iodide with Zn or Mg and were found to be relatively stable in non-aqueous solutions. The spectra of these radicals with maxima at 300 and 400 nm strongly suggest that the reaction of eaq- with the carboxy derivatives of pyridine leads to similar transients, e.g.
o- l
(5) eaq- + I
’
N’
coo-
\
H+
coo-
1 /
Q
The interaction of the unpaired electron with the carboxy group in the 2-, 3- or 4position could lead to different optical transitions. The spectra of the electron adducts of these three derivatives are characteristic, i.e. the separation of the two bands and the ratio of extinction coefficient (E) of the bands being different. It is interesting to note that the spectra of the corresponding amides (Figs. 5 and 6) are almost identical except for a somewhat higher E value. The disappearance of the electron adducts through a second-order reaction is followed by the formation of a long-lived species with h,,, = 340 nm and another band at lower wavelengths which was not investigated. A similar product was observed in the steady-state and pulse radiolysis of N-methylnicotinamide and NAD+(l). It has been suggested that the product is a dimer of a radical formed through one electron reduction of these solutes(13). The original notion that the product might be NADH which was based on purely spectral considerations had to be discarded since the radiation product lacked enzymic activity(14). We may conclude now that in a one-electron process nicotinic acid, nicotinamide, N-methylnicotinamide and NAD+ behave almost identically. Furthermore the other two isomers, the 2- and the 4-carboxy derivatives, are in that respect very much the same. If this is so, what is the role of adenine in NADf and why are the 2- and 4-derivatives not equally efficient? Obviously the charge, the steric hindrance and the electric potentials play an important role. The reaction rates of H atoms with the pyridine derivatives are of the order of 10s dm3 mol-l s-l and are considerably lower than those of electrons. They are very similar to the reaction rates of H atoms with the heterocyclic systems containing two N atoms in the ring, e.g. the pyrimidine derivatives’15). The transient spectrum resulting from the reactions of H atoms with nicotinic acid is very different from the spectrum of the isomeric protonated electron adduct. The H atoms are likely to be adding to the ring since abstraction reactions have in general low rates which, in an aromatic system, could be even further reduced. Assuming spectroscopic features of the H atom adducts to be similar to those of the electron adducts, two absorption
270
M.
SIMIC
and
M.
EBERT
bands could be expected for these transients. The apparent presence of three bands at 410, 340 and < 290 nm perhaps indicates more than one transient formed. It is possible that the H atoms are less selective in the site of attack than the conjoint adding of an electron and proton. Here again the long-lived species (or product) which is formed from the reaction of the transients absorbs at 340 nm, although the absorption is only half of that in neutral or alkaline solutions. A higher state of protonation of this long-lived species at pH 1 could lead to a lower extinction coefficient. On the other hand, only one of the isomers might give this particular product while the products from the other isomers lack an absorption in this region. Dipyridyls The reactions of the OH radicals with the dipyridyls are expected to proceed at different rates depending on the state of protonation of the two rings. For the fully deprotonated dipyridyls the rates are close to those for deprotonated pyridine and its derivatives when the two-ring structure is taken into consideration. Here again the OH radicals are expected to add to the ring. While the 4,4’-dimer has only one major absorption band in the 280-700 nm region with &,,, = 365 nm, the 2,2’-dimer has two bands with X,,, = 365 and 305 nm. It is probable that the OH radicals add at two different positions in the latter case. The extinction coefficients of the dipyridyl OH adducts are considerably higher than for the compounds with a single pyridine ring. The reactions of the electrons with the dipyridyls are also diffusion controlled of the order of lOlo dm3 mol-l ssi. The spectrum of the electron adduct to the 2,2’dimer is pH independent between 4 and 12.7, with h,,, = 365 nm and another weak band at 460 nm. The spectrum of the electron adduct to the 4,4’-derivatives is pH independent only between 3.7 and 7 with h,_, = 385 and 570 nm (Fig. 10). The spectrum of the electron adduct to the 4,4’ dipyridyl is very similar to the spectrum of methylviologen cation radicalt16)
which was found to have two major bands at 400 and 600 nm and Ed,,,,= lo4 dm3 mol-l cm-l (103 m2/mol) in acetonitrile solutions. We would like to suggest therefore that the reaction of hydrated electrons with dipyridyls leads to transients with the same structure, e.g.
The small shift of x 20 nm in X,,, probably arises from the difference in solvents and slightly different structures of the radicals. The disappearance of the methylviologen cation radical in the presence of watero6) is in keeping with the transient nature of the dipyridyl electron adduct in aqueous solutions. The products of the decay reactions are unknown for both radicals.
Pulse radiolysis of aqueous solutions of derivatives
of pyridine
271
The electron adduct to 4,4’-dipyridyl has at pH 12.7 a noticeably different spectrum with both bands shifted towards lower wavelengths (pK, = 10.5) (Fig. 10). It is our belief that this spectrum at pH 12.7 belongs to the unprotonated electron adduct of the 4,4’-dipyridyl. The absence of the unprotonated electron adduct to 2,2’-dipyridyl at the same pH could be explained on the basis of a higher resonance stability of the para-para form. In these, as in some other dimeric systems, such as diphenyl where a highly resonating structure of the electron adduct is present, the molar extinction coefficients are very high ( > 104 dm3 mol-1 cm-l (= 10s m2/mol), and the pK, values for the protonation of the negative electron adduct are considerably lowered. It is interesting to note that the reaction of electrons with the CO(P~Z)~+ complex did not lead to the formation of the electron adduct to the ligand in spite of relatively very high electron rates for Py2, but rather to reduction of ColI1 and an intermediate assigned to a low-spin complex with one unpaired electronu7). It is of interest to find out which substituents of Py, could retain the electron in the ligand as in the case of the [Co(NH,),-benzoato] complex where the NO, group was found to be in that respect quite efficientus). The H atom adducts to the dipyridyls which are formed at pH 1 have lower extinction coefficients than the electron adducts, although much higher than, for instance, the H atom adduct of nicotinic acid. This behaviour of H atom adducts follows a general trend observed for many other systems(lv).
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2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.
E. M. KOSOWERand E. J. POZIOMEK,J. Am. them. Sot. 1964,86,5515. J. P. KEENE, J. sci. Instrum. 1964, 41, 493. J. H. BAXENDALE,P. L. T. BEVANand D. A. STOIC, Trans. Faraday Sot. 1968, 64, 2389. M. &MI&, P. NETA and E. HAYON, J. phys. Chem. 1969, 73, 3794. M. SIMIC and M. Z. HOFFMAN. J. Am. them. Sot. 1970.92.6096. M. ANBAR and P. NETA, Znt. i appl. Radiat. Isotopes, i96$ 16, 227. B. CER~EK and M. EBERT, Trans:karaday Sot. 1967, 63, 1687. L. M. DORFMAN.R. E. BWLER and I. A. TAUB. J. them. Phvs. 1962.36. 3051. M. SIMIC, P. NE;A and E. HAYON, J. Am. them: Sot. 1970,92,4763: ’ D. F. SANGSTER,J. phys. Chem. 1966,70, 1712. M. Z. HOFFMANand M. SIMIC, Fourth International Congress of Radiation Research, Evian, 1970, Abstract No. 383. G. STEIN and A. J. SWALLOW,J. them. Sot. 1958, 306. A. J. SWALLOW.Biochem. J. 1955. 61. 197. G. SCHOLESand M. SIMIC, Biochik. biophys. Acta 1968, 166, 255. E. M. KOSOWERand J. L. COTTER,J. Am. them. Sot. 1964,86, 5524. J. H. BAXENDALE,Ric. Sci. 1970, 68, 43. M. Z. HOFFMANand M. SIMIC,J. Am. them. Sot. 1970,92,5533. M. SIMIC and M. Z. HOFFMAN,to be published.
Resume---On montre que les vitesses des reactions des electrons hydrates avec les derives carboxyliques, carbamidiques et pyridyliques de la pyridine sont limit& par la diffusion (k(e,,-+S)a l-2 x 1Orodm* mol-l s-l). On a aussi mesure quelques constantes de vitesse des radicaux OH et des atomes H a partir de la cinetique de croissance des spectres d’absorption transitoires correspondants; les valeurs de k(OH + S) dependent du degre de protonation du solute. On enregistre les spectres d’absorption transitoires resultant des reactions de e,,-, des radicaux OH et, dam quelques cas, des atomes H avec ces composts, et on les compare avec les reactions des derives plus complexes [par exemple (nicotinamide dinuclCotide adtnine)+]. On conclut que e,-, OH et atomes H s’ajoutent sur l’anneau pyridinique, le degrt de la protonation des radicaux form& dtpendant du pH des solutions.
212
M.
SIMIC:and
M.
EBERT
Pe3lOMe -
CKOpOCTHbIe KOHCTaHTbI peaKL@iti NApaTH3HpOBaHHbIX 3JIeKTpOHOB C Kap6OKCK; mipqqun-nepnaaraMu nkipnAnrisa 06HapyxRHbI KaK YITpaBAReMbI AlW&$y3liek aMBAow (1-2 X lo" dm3 IllOl-'). HeKOTOpbIeCKOpOCTHbleKOHCTaHTbIpeaK~~e~paAIiKaJIOB OHu aTOMOB H 6b1,IH H3MepeHbI TOXCC OTHOCBTeAbHO KLiHeTHKU 06pa30BaHAx pC3yJIbTupyIou(lfX IIepeXOAHbIX CIICKTpOB IIOl-JIOIAeHUR. 3HaVeHWI k(OH+S)OKa3amiCb 3aBUCUMbIMU OT COCTOIIHWi IIpOTOHH3aUHH PaCTBOpeHHOrO BeIIJeCTBa. nepCXOAHble CIlCKTpbI IIOI-JIOIAeHHff pC3yJIbTHpyFOIAW 113 peaKA&% &-, paAHKaJIOB OH a B HeKOTOpbIX ClIy¶aSIXaTOMOB H C 3TIIMH COeAUHeHUSlMLi 6bIna perHCTpHpOBaHb1 H COIIOCTaBJICHbIC peaKWiBMH donee KOMlUIeKCHbIX AepSiBaTOB [HaIIp. (HuKOT~aMuA~eHwHn~Hy~COT~A)+]. EbIJIO Ii aTOMb H J'BCAWIliBaIOT IIHpHAWHOBbIi L@DUl, IIpH'feM 3aKJIH)YCHO, YTO pWWKaJIb1 f&c,- OH COCTORHHC IIpOTOH5i3aIWiaAAyKTOB3aBWZHMO OT pH paCTBOpOB.
Zusammenfassung-Es wird aufgewiesen, dass die Geschwindigkeitskonstanten der Reaktionen von hydratisierten Elektronen mit Karboxy-, Karbamidound Pyridyl-Derivaten des Pyridins diffusionsbeschrankt sind (1-2 x lOlo dms mol-l s-l). Einige Geschwindigkeitskonstanten der Reaktionen von OH Radikalen und H Atomen wurden such aus der Entstehungskinetik der entstprechendren transienten Absorptionsspektren ermessen und es wurde festgestellt, dass die Werte von k(OH + S) vom Grad der Protonisierung der gelosten Substanz abhlngen. Die transienten Absorptionsspektren die aus Reaktionen von e&a-, OH Radikalen und in manchen Fallen von H Atomen mit diesen Verbindungen stammen, wurden aufgenommen, und mit Reaktionen mit komplizierteren Derivaten [z.B. (Nikotinamid-Adenin-Dinukleotid)+] verglichen. Man schliesst, dass e,,-, OH Radikale und H Atome zum Pyridinring addiert werden, wobei der Protonisierungsgrad der Addukte vom pH der Losung abhlngt.