Electrochemical behaviour ofN-methylnicotinamide in acidic media

163

f. Electroanal.

Chem,

251 (1988) 163-172

Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

E~~~~hernic~ behaviour of ~OmethyInie~tin~ide in acidic media R. Marh GaIvin and J.M. Rodriguez Mellado * ~epart~me~to de Quimtca Fisrca y Te~~n~tcu Sun A/b&o Mugno s/n, 14UO4-Ckdoba (Spatnj

Aphcada,

~ntt~erstd~

de C&d&q

F. Garcia Bhco C.S.I.C., Institute de Quimtca Fisica, RocasoIano, 28006-Madrid

{Sparn)

(Received 20 January 1988; in revised form 30 March 1988)

ABSTRACT N-Methylnicotinamide (MNA) was studied by dc and DP polarography and linear sweep cyclic voltammetry in acidic media (pH < 3). Tafel slopes and reaction orders were obtained from I-E curves traced at potentials corresponding to the foot of the polarographic wave. The results show that in the pH range studied the species in solution is the non-protonat~ one. In a strong acidic medium the overall process is irreversible. The reduction pathway consists of the reversible transfer of one electron and two Hi ions followed by an irreversible one-electron transfer, which is the rate-determining step. At 1~ pH < 3. this process is in competition with the electrodimerization of MNA.

N-Methylmcotinamide (N-methyI-3-carbamidopyridinium ion) is an excretion product of niacine found mainly in the liver of mammals [l]. It was selected a long time ago as a model compound of the pyridine codehydrases NMN, NAD and NADP 127. These coenzymes play vital roles in enzymatic redox reactions, the nicotina~de ring being the part of the molecule responsible for their redox properties. Enzymes such as lactate, pyruvate and malate dehydrogenases are able to transfer one electron and a hydrogen atom stereospecifically from the reduced substrate to the nicotinamide ring of the coenzyme. Nevertheless, it is still in question whether the reaction involves the transfer of an H- ion or, on the contrary, whether there are mono-electronic transfers involving free radicals 131. An electrochemical approach could clarify this point. * To whom correspondence ~22-0728/88/$03.50

should be addressed.

0 1988 Elsevier Sequoia S.A.

164

The electrochemistry of the pyridine coenzymes is not yet well known. A good review of this subject is given in ref. 4. On the other hand, the use of model compounds of increasing complexity can aid in the comprehension of the electrochemical behaviour of the coenzymes. N-Methylnicotinamide (MNA) has been investigated polarographically and by cyclic voltammetry [5-71, but only recently Thevenot and Buvet [8] reached conclusions about its electroreduction process. These authors stated that, in the pH range 3-10, the first polarographic wave corresponds to an electrodimerization process taking part in the reaction layer. Nevertheless, the process occurring at pH -=z3 appears too complex to give a complete interpretation of the experimental results, especially those corresponding to the influence of the drop time, the reactant concentration and the pH on the polarographic wave. The aim of this study was thus to clarify the electr~he~cal reduction of MNA in strong acid media, as a first stage in the knowledge of the ele~troche~st~ of the pyridine coenzymes in these media. EXPERIMENTAL

All reagents used were of Merck p.a. grade, with the exception of MNA which was from Sigma. A buffer solution consisting of 0.1 M acetic and phosphoric acids was used as the supporting electrolyte with the exception of measurements made at pH < 1.2 in which perchloric or sulphuric acid solutions were used. The pH was adjusted with solid NaOH. The working concentration of MNA was 5 x lop4 M, except in experiments in which the influence of this variable was studied, the range of concentration employed in this case being 2 X 10e5-1 X 10e3 M. All potentials were measured against a saturated calomel electrode. Solutions were purged with purified nitrogen and the temperature was kept at 25 If O.l”C. DC measurements were carried out with an AMEL 465 polarograph and a dropping mercury electrode with m = 0.946 mg s-‘, t = 6.18 s (open circuit) at pH 1.94 in HClO,, h = 60 cm. DP and voltammetric measurements were made on an INELECSA assembly equipped with a potentiostat-generator and A/D and D/A converters attached to an 8-bit microprocessor. For voltammetric measurements a Metrohm EA-290 hanging-drop mercury electrode was used, the area of the drop being 0.022 cm2. An APPLE II PLUS microcomputer was used for data control acquisition and treatment. ‘UV measurements were made on a Cary model 219 spectrophotometer with 1 cm quartz cuvettes at 25.O”C. 0.1 M sulphuric acid solution was employed at pH > 1. RESULTS

Polarograpkic bdmuiour In dc polaro~aphy, variations of the listing 1.

MNA exhibits a single wave in the pH range studied. The current, jr_, and half-wave potential, +. are given in Fig.

1.0

I

-2

HC2

-2

0

2

0

2

PH

Fig. 1. (a) Yanation of the limitmg currentwith the pH. (0) Experrmental data; fc) data corrected by using the Stokes-Einstein law and the viscosity values found in ref. 9. fbj Variation of half-wave potential with the pH. The values of the acidity function UO were obtained from the HzSO, concentrations [IO].

As can be seen, zi_ varies from a maximum value ~o~espondi~g ti3 a two-elt3ctron process (pH < 1) [S] to a vaIue corresponding to a one-eiectron process (pH > 3). The slopes of the log i, vs. log t plots at pH < 1 and pH > 3 are close to 0.2. The slope of the El,z vs. pH plot is -80 mV/decade for pH < 1.8. In strong acidic solutions, Elj2 is independent of the concentration of the reactant and varies linearly with the drop time with a slope of 22 mV/decade. Figure 2a shows the logarithmic analyses for the entire pH range studied and for a MNA ~~centration of 10m4 ZM.The loga~th~~ plots are linear at high acidity with slopes of -41 mV/decade. When the pH increases, these plots show deviations from linearity. Differential pulse polarography confirms these rest&s with only one peak appearing in the entire pH range studied, The width of the wave increases with increasing pH values above 1.5, Figure 2b shows the compa~son between the expe~menta~ and theoretical data, for a perchloric acid concentration of 1 M, using the equation 1111 r=41&@

fL)?

with L = exp I--(E - E,)/b] and by applying a curve-fitting method described earlier [11.12]. From these calculations we obtain a b value of ~40 mV in very strong acidic media, which agrees with the value of the slope of the loga~thmi~ plots in these media. The influence of the concentration of MNA on the wave was studied at pH 1.5. The logarithmic plots are linear at very low concentrations with slopes of -40 mV/decade and show strong distortions when the MNA concentration is increased. Qn the other hand, the form of the DP po~arograms varies with the concentration of MNA at this pH. Thus, the peak is symmetric at low concentrations and becomes increasingly asymmetric when the concentration increases. Moreover, the peak

2

1

3

a 4

~

-900

-950

-1000

E/w?

1

-800

Fig. 2. (a) Loga~t~c

1

,/

L

zUldySeS.

-900 CMNA = 1 X 10 -4

ElmV

-1000 M.

(1)

&,

=

-0.86:

(2) Ho = -0.56;

(3) a=&, = -0.30;

(4) pi = 0.31; (5) pJ-J= 1.00; (6) pH = 2.41. (b) DP polarogram of MNA at Ho = - 1.02. (a) Expefimen-

tat values; (. . . . .) calculated values (b = - 41 mV).

-3.5

-3.0

-800

Fig. 3. (a) Tafel plots in 1.5 M H,SO,. low4 MNA concentration/moI I-‘: (I) 10.00; (2) 5.00; (3) 2.50; (4) 1.25. (b) El~tr~he~c~ order with respect to the MNA con~ntration. E = - 860 mV.

potentia1 is virtually independent of the concentration for c c 5 X fOW5 N and shifts strongly towards more negative values for c > 2 x 10W4M. Tafel curves and reaction orders

In a strong acidic medium (pH -Cl), the Tafel slope is independent of both the pH and the MNA concentration, having an average value of -40 mV/decade. Figure 3 shows the Tafel plots in 1.5 M sulphuric acid and at several concentrations of the reactant. The reaction order with respect to the HC ion concentration is 2.1. The reaction order with respect to the MNA concentration in a strong acid medium is one (Fig. 3b) and is independent of the potential at which it is measured.

In linear sweep cyclic voltammetry, MNA yields one reduction wave but an oxidation wave is not observed until a scan rate of 200 V s-l is used, as is shown in Fig. 4. ihA -200

-600

-800

-1

-1000

0

Rg. 4. {a) Linear sweep cyclic voltammogram in 2 M HCQ log 0.

logwvs-1) at 200 V s-l, (b) plot of peak potential vs.

168

The variation of the peak intensity with G’I/~is roughly linear for the scan rate range of 0.05-2 V S-I. The peak potential Ep varies with the scan rate, as shown in Fig. 4b. The sfope of the Ep vs. log u plot is close to - 20 mvfdecade. The value of E p,z - Ep is about 39 mV. Spectrophotometric

measurements

A spectrophotomet~c study was carried out in strong acid media. The spectrum shows a main band and a shoulder at wavelengths of 264 and about nm, respectively. The absorbance is independent of the acidity of the medium no spectral changes were observed until a sulphuric acid concentration of 5 M used, the molar extinction coefficient being close to 4 X lo3 M-’ cm-‘.

UV 272 and was

DISfUSSION

The results clearly indicate that in strong acidic media the process is diffusioncontrolled at potentials co~esponding to the limiting current of the wave. This fact indicates that there are no chemical reactions preceding the electron transfer, or that these reactions are faster than the diffusion. Moreover, the non-occurrence of spectral changes when the acidity is increased indicates that the amido group of MNA is not protonated even at a sulphuric acid concentration of 5 M. Lund [13f studied the reduction of analogous compounds in mineral acid solutions and showed that the quatemary isonicotinic amide l-ethyl-4-carbamidopyridinium ion is reduced to l-ethyl-4-formylpyridinium ion. Replacement of the hydrogen atoms of the amido group by alkyl or aryl groups does not alter this reduction route. Moreover, the simple py~~ne-c~rboxy~des (nicotinamide, picolinamide and iso~cotina~de) are reduced to the corresponding aldehydes in HCI solutions. Thus, in a very acidic medium the overall reaction can be written as *

On the other hand, the values of both logarithmic analysis slopes and the b parameter in DP polarography indicate that the second electron transfer is irreversible, this step being the rate-determining step (rds). This is also confirmed by the

* Underwood [St Identified l-methyl-3-carbarmdo-I,4-dlhydropyridme as the reductmn product m neutral and basic media. This would lead to the belief that this is the end product of the reduction in acidic media. Nevertheless, Lund also found that the pyridine ring IS reduced at pH > 7, but m acidic solutmns the amido group IS reduced ta aldehyde.

169

voltammetric results and the value of the Tafel slope. Studies by pulse radiolysis in strong acidic media [14,15] show that there is evidence of the existence of the radical OH

./ C 'NH2

CT

(MNAH ‘+I

N I +

Since the reaction orders with respect to the MNA and H+ ion concentrations are one and two, respectively, we propose the foollowing reduction scheme for pH < 1:

./ OH

P

C

C

‘W ++,+ + e-

'NH2

G======

I+

I +

CH3

CH3

.P” ‘NH2+H+

(2)

C

(3)

_

(4)

Reaction (4) involves the formation of a gem-hydroxylamine, as is proposed in the literature for other pyridinic amides 25,131.This unstable intermediate forms the aldehyde which is hydrated in these media [l&17]. The i-E-trelations~p for a process of this type reads

E = El/t + aRTIn-

i

i,-

(5)

i

where E m=W-

$& In 7012t

2RT

-t x

ln cH

(6)

The logarithmic analysis (done in the form of E vs.log[i/(i, - i)] plots) should = 1.5); the yield a straight line with a slope of -39.4 mV (assuming that OLD

170

predicted values of i+E/ll log t and aE/apH are 19.6 and -79 respectively. The experimental values agree with these predictions. On the basis of the above scheme, the i-E relationship for potentials ing to the foot of the wave can be expressed as i = 2FK,K3K’k4cMNAc$

exp[ -(l

+ P)FE/RT]

mV/decade, correspond-

(7)

where K, and K, are the equilibrium constants of reactions (2) and (3), respectively; k, is the rate constant of reaction (4) at E = 0; and K’ = exp[(l + j3)FA#r,f/RT], where A#r,,, is the potential of the reference electrode. Assuming that B = 0.5, the theoretical values of the Tafel slope (- 39 mV/decade) and the reaction orders (1 and 2, respectively) agree with the experimental ones. Above pH 3, the wave corresponds to a dimerization process [S], E,,, and E, being independent of the PH. The transition between the two-electron and the one-electron waves appears to be continuous and no second reduction wave is observed. The anomalous peak widths, the logarithmic analyses and the slopes of the EI,,z vs. log t plots observed at 0.8 I pH s 3 are indicative of the existence of two ill-separated waves. These facts can be explained taking into account that the electrodimerization process occurring at pH > 3 can be formulated as [S]

(8)

The potential at which this reaction occurs is pH-independent, whereas the reduction potential of the two-electron process varies by - 80 mV/decade with the pH. Moreover, the reduction process involves protonated species, while the predominant species in this pH zone is the non-protonated one: in addition, the dissociation pK of the radical MNAH’+ should be about 1.3 [15]. In this case, the current of the two-electron reduction decreases in the shape of a dissociation curve, and is replaced by the current due to the electrodimerization process. By using the procedure described in ref. 18, it can be shown that log

fL - ‘“2 - C-pH IDI

-

‘L

(9)

are the diffusion currents of the first- and second-order where i,, and i,, processes, respectively and C is a constant term related to the protonation reaction of MNA. The plot of log[(i, - i,,)/( i,, - iL)] vs. pH yields a straight line (Fig. 5) with a slope of -0.96, in agreement with eqn. (9). The effect of the MNA concentration on the E,,,2, E, and half-width values, and on the logarithmic analyses, could be indicative of the presence of adsorption phenomena. Moreover, these effects are complicated by the different dependences

171

I,og

‘1,-1L).: i

lll-lI.

-2 1.5

Fig. 5. Plot of log [(rr - zDt)/(iDI

2.0

2’.5

--IL)] vs. PH. c,,,=5x10-4

of the concentration of the polarogarphic electrochemical processes 1191.

5’. 0

pti

M

parameters of the first- and second-order

ACKNOWLEDGEMENT

This work was supported by a grant from Junta de Andalucia 07/CLM/MDM 85-87. REFERENCES A. White, P. Handler and E.L. Smith, Principles of Biochermstry, 4th ed., McGraw-Hill. New York. 1968. S.J. Leach. J.H. Baxendale and M.G. Evans, Aust. J. Chem., 6 (1953) 395. J.P. Klinman, J. Biol. Chem., 247 (1972) 7977. D. Thivenot and R. Buvet, J. Electroanal. Chem., 39 (1972) 429. J.N. Burnett and A.L. Underwood. J. Org. Chem., 30 (1965) 1154. A.J. Cunningham and A.L. Underwood, Biochemistry, 6 (1967) 266. C.O. Schmakel, K.S.V. Santhanam and P.J. Elving, J. Electrochem. Sot., 121 (1974) 345. D. Thbenot and R. Buvet, J. Electroanal. Chem., 39 (1972) 447.

172 9 R.C. Weast (Ed.), CRC Handbook of Chemrstry and Physics, 55th ed.. CRC Press, Cleveland, 1974. 10 R.G. Bates, Determination of pH. Theory and Practice, Wiley, New York. 1973, pp. 194-199. 11 J.M. Rodriguez Mellado, M. Blazquez, M. Dominguez and J.J. Ruiz. J. Electroanal. Chem., 195 (1985) 263 12 J.M. Rodriguez Mellado. M. Bliizquez and M. Dominguez, Comput. Chem.. 12 (1988) 257. 13 H. Lund. Acta Chem. &and., 17 (1963) 2325. 14 EM. Kosower, A. Teuerstem and A.J. Swallow, J Am. Chem. Sot., 95 (1973) 6127, 15 P. Neta and L.K. Patterson, J. Phys. Chem., 78 (1974) 2211. 16 J Tnoufflet and E. Lavnon, C.R. Acad. SCI., 247 (1958) 217. 17 J.F. Rusling and P. Zuman. J. Electroanal. Chem . 143 (1983) 291. 18 E. Mufioz, L Camacho, J.L Avila, A.M. Heras and J.J. Rutz. Bull. Sot. Chum. Belg.. 96 (1987) 255. 19 (a) S G. Mananowskn, Catalytic and Kinettc Waves in Polarography, Plenum Press, New York, 1968; (b) L. Camacho, M. Bltiquez, M. Jtmenez and M. Dominguez. J. Electroanal. Chem., 172 (1984) 173; (c) M. Blarquez, L. Camacho, M. JtmCnez and M. Dominguez, tbtd., 189 (1985) 195; (d) M. Blazquez, J.M. Rodriguez Mellado and J.J Ruiz, Electrochim. Acta, 30 (1985) 1527.