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Degradative oxidation of d -ribono-1, 4-lactone by CrVI in perchloric acid
Polyhedron Vol. 16, No. 4, pp. 701 706, 1997
~
Pergamon PII
: S0277-5387(96)00282-3
Copyright ~ 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0277 5387/97 $17.01)+0.00
Degradative oxidation of o-ribono-l,4-1actone by C r vI in perchloric acid S. Signorella, S. Garcia and L. F. Sala* Area lnorgfinica, Facultad de Ciencias Bioquimicas y Farmac6uticas (UNR), Suipacha 531, 2000 Rosario, Argentina
(Received 16 January 1996; accepted 5 June 1996)
Abstract--The oxidation of D-ribono-1,4-1actone by Cr w yielded o-erythronic acid, erythrose, carbon dioxide and Cr m as final products when a 15-fold or higher excess of sugar over Cr vl was used. The kinetics of the redox processes involving the CrVl/Cr m couple were determined and a mechanism is proposed. The complete rate law for the Cr vl oxidation reaction is expressed by: -d[CrVl]/dt = {a[S]+b[S][H +] +c[H+] 3} [CrVl]r, where a = 3.09 × 10 _3 M -~ s -~, b = 4.98 × 10 -2 M -2 s -~, c = 1.07 × 10 -~ M -3 s -~ and S refers to the total reductant concentration, at 60°C. Crv is formed in a fast step by reaction of the CO~- radical and Cr vt, and Crv reacts with the organic substrate faster than Cr w does. The EPR spectra show that the intermediate Crv complex (g = 1.978) is formed and decays by a first-order process. Copyright © 1996 Elsevier Science Ltd
Keywords: ribonolactone ; chromium ; oxidation ; kinetics ; mechanism ; Crv ; Cr v~.
Hexavalent chromium, in many different compounds, is a well established carcinogen and mutagen [1~,]. There is, therefore, appreciable interest in determining the mode of action of chromium species derived from initial uptake of Cr v~ compounds [5]. Ligands that possess two oxygen atoms able to form five-membered rings about the metal atom, such as 1,2-diols and ~hydroxyacids are reported to stabilize Crv intermediates after further interaction with biological reductants [6-10]. For this reason, some sugars, or their derivatives, may play an important role in the chemistry of Cr w. Aldonic acids (or their lactones) have been found as metabolic intermediates in animals, plants and microorganisms, as products of the action of microorganisms on carbohydrates and as constituents of acidic polysaccharides [11]. As part of a general project dealing with the interaction of carbohydrates and transition metal ions, we have studied the reaction of Cr vl with low molecular weight model reductants [12-20]. Chromic oxidation of ~-hydroxyacids has been extensively studied [12,21-26]. These oxidations may yield products coming from either C - - C [12, 23-26] or C - - H , [22,26] depending on the particular molecular characteristics, through mechanisms involving either
* Author to whom correspondence should be addressed.
two-electron or three-electron initial oxidation of substrate by Cr vl. Polyhydroxyacids offer additional binding sites to form the chromic ester precursor of the slow electron-transfer steps and might help to elucidate stereoelectronic effects affecting the reaction course [26]. In this paper we report the results obtained in our investigation of the chromic oxidation of D-ribono-1,4-1actone (RL) (Scheme I) in perchloric acid.
EXPERIMENTAL Materials o-Ribono-l,4-1actone (sigma grade), potassium dichromate (Cicarelli c.a), perchloric acid (P.A. Merck), acrylonitrile (Aldrich grade) and sodium perchlorate (Sigma grade) were used without further purification.
Spectropholometric measurements Kinetic measurements were made at 350 nm by monitoring the absorbance changes on a Guilford Response I1 spectrophotometer with fully thermostated cell compartments. Pseudo-first-order rate constants, determined from the slope of ln(Abs) vs
701
702
S. Signorella et al. COOH
CHO --OH
HOH2C
O
CO2H
2 CrVI
--OH
2 CO2 + 2 CrIII+ OH
OH
--OH OH
CH2OH
OH
CH2OH
CH20H RL
RA
Scheme I.
time, were deduced from multiple determinations and were within + 5% of each other. In most experiments the concentration of Cr v~ was kept constant at 7.75 10 -4 M while the RL was varied from 0.0116 to 0.062 M. Mixtures of sodium perchlorate and perchloric acid were used to maintain a constant ionic strength of 1.0 M. Disappearance of Cr vI was followed until at least 80% conversion. Reactant solutions were previously thermostated and transferred into a cell of 1 cm path length immediately after mixing. Experiments were performed at 60°C unless otherwise mentioned. The acid : lactone proportion of the reductant in the starting solution was calculated by optical rotation measurements at given hydrogen ion concentrations [27]. Table 1 shows the composition of the substrate solution at different perchloric acid concentrations. Ribonic acid (RA) is the major form in solution over the whole acid concentration range (0.2-1.0 M HC104), and as expected, the lactone proportion increases with increasing [HCIO4]. In order to confirm that no cooxidation process occurs at the primary hydroxy group, RL was methylated at C-5OH to yield the 5-O-methyl-D-ribono-l,4-1actone (MRL) [29]. The chromic oxidation of MRL and RL affords the same kobsvalues, under the same reaction conditions.
Product analysis Under conditions employed in the kinetic measurements (ratios of RL over Cr vI from 15/1 to 80/1) qualitative identification of D-erythronic acid (EA) and erythrose as the reaction products was carried out by paper chromatography. EA and erythrose were identified against authentic samples using n-butanol/acetic acid/water (4 : 1 : 5) as eluent. Paper chromatograms were visualized by two kinds of development reagents: a three-stage dip of silver nitrate, sodium hydroxide and sodium thiosulfate [30] and p-anisidine reagent [31]. Under these conditions, carbon dioxide was detected and the quantitative determination of this gas afforded 1 mole of CO2 per mole of Cr vl.
Test for free radicals A solution of potassium dichromate (1.16× 10 3 M) in 1 cm 3 of 0.5 M HC104 was added to a mixture of 1.16x 10 -3 M RL in 0.5 M HCIO 4 (2 cm 3) and acrylonitrile (0.5 cm 3) at 20°C. After 20 min a white precipitate of a polymer slowly appeared. Blank experiments in the absence of organic substrate or
Table 1. Concentration of ribonolactone and ribonic acid in the starting solution" [HCIO4]/M
Crb/M
R"
0.2-0.3 0.44).5 0.6 0.75 0.9-1
0.1623 0.1360 0.1317 0.1370 0.14
+0.200 +0.189 +0.200 - 0.222 +0.234
102[RL]/M 10[RA]/M 105[R-I/M 0.476 1.710 2.68 3.640 4.16
1.575 1.190 1.049 1.006 0.984
7.75 2.00 2.10 1.74 1.34
"Values used in calculations: molar rotations: + 5.65 (RL), +2.37 (RA), +0.95 (R-); K [28] = [H+][R-]/([RL] + [RA]) = 9.55 x 10 5. bCr = [RA]+ [RL] + [R-I, where R refersto ribonate. cOptical rotation.
703
Chromic oxidation of o-ribono- 1,4-1actone oxidant agent did not show formation of any such precipitate.
14
,
.
~
(a)
12 EPR measurements
E P R spectra were recorded with a Bruker spectrometer model ER-200. Spectra were acquired at a frequency of ca 9.8 G H z with a center field of 3500 G, a swept width of 100-1000 G and variable modulation amplitude. The cavity and sample tube were kept at 2 5 C . Initial Cr w concentration in all the E P R measurements was 7.75 x 10 -4 M.
E! 10 ~° -o .~
8
RESULTS AND DISCUSSION Over the whole range of perchloric acid concentrations used in the kinetic measurements, spectrophotometric studies showed that the reaction of R L and Cr w resulted in an absorbance band at 350 nm and a shoulder at 420-500 nm characteristic of the C r 2 0 ~ - ion. It is known that Crv species, usually formed in these oxidation reactions, absorb at 350 nm and may superimpose Cr v~ absorbance yielding the wrong interpretation of spectrophotometric absorbance decay values [22]. However, if Crv reacts faster than Cr vl and exists in solution in a sufficiently small concentration, changes in absorbance at 350 nm essentially reflect changes in Cr v~ concentration. We observed deviations from first-order decay over very short time periods and we calculated pseudo-firstorder rate constants from the slopes of the linear part of the ln(Abs) vs time curves ; a good criterion only if Crv decays faster than Cr w. The relative values of the Cr w vs Crv oxidation rates constants obtained from the Crv EPR signal intensities during the reaction course (discussed below) demonstrate this is the case for the present reaction. Unlike that observed for the chromic oxidation of gluconic acid [16], in the present case, Crv was not detected spectrophotometrically at 750 nm under conditions used in the kinetic experiments. Fortunately, it could be easily detected by E P R measurements. The reaction of Cr v~ with excess R L generates an E P R spectrum dominated by a single detectable sharp signal at g,~o = 1.978 (Fig. 1 inset (a)), typical of fivecoordinate oxochromate(V) complexes [32,33]. The Crv signal grows and decays with time to yield the broad Cr m signal (9~so = 1.981) as the ultimate fate of c h r o m i u m in this reaction (Fig. 1 inset (b)). Figure 1 shows a curve for peak-to-peak heights of Crv E P R signal t,s time obtained on a reacting solution. Values of the rates of disappearance of Cr w (k6) and Crv (k~), respectively, may be calculated by eq. (1) [12,16], which fits well the experimentally observed Crv intermediate growth and decay curve in Fig. 1. h = Ak6{exp(-kst)-exp(-2k6t)}/(2k6-ks).
(1)
It has been shown that any rate profiles dealing with the build up and decay of an intermediate by first
4
i 0
20
i
i
40 60 t (min)
i
80
__ 100
Fig. I. Peak-to-peak heights of Crv EPR signals ~;s time IS] = 0.039 M; [CrV~]0= 7.75 x 10-4 M; [HCIO4] =: 1 M: T = 25C. Inset : (a) Crv EPR signal recorded at t = 5 rain ; (b) superimposition of Crv and Cr ul EPR signals as Cr "~ builds up during the run (t = 35 min).
order processes can be described by a pair of solutions having the same numerical values for the rate constants, but with the order of the assignment reversed [34]. For conditions stated in Fig. 1, k 6 = 8.35 x 10-Ss ~andk5 = 0.00768 s t and thetime of maximum Crv concentration, T~a x -~ 8.5 min. We have chosen this assignment for the rate constant values and not the reverse one, for several reasons. In the E P R measurements, the temperature is lower than that used in the spectrophotometric experiments. Since the redox rates increase when the temperature rises, k6 obtained from E P R data (at 25"C) should be lower than values obtained from the spectrophotometric measurements at 60'~C (a value of 0.00768 exceeds all the kob~values in Table 3). Besides, we have evaluated kob~(2k6) spectrophotometrically at 25°C under conditions used in the EPR experiments, and we have reproduced the kob~(2k6) value (the value obtained by this technique, for conditions in Fig. 1, is: 1.84x 10 -4 s ~). Thus, the fact that k5 is 91 times higher than k~ justifies our initial assumption that changes in absorbance at 350 nm essentially reflect changes in Cr w concentration. In order to verify the dependence of rate upon Cr vj, pseudo-first order rate constants were calculated at various [CrW]0 (4-26 x 10 -4 M) but at constant temperature, [RL], [H +] and ionic strength (Table 2). The rate constant was found to increase with increasing [CrW]0. However, values of kobs[CrVl]o/[Cr20 ~ ] at various [CrV~]0 were found to be constant, meaning that the dimeric species, viz. Cr:O7 , is the effective oxidant. The equilibrium H C r O 4 / C r 2 0 ~ (K = 76) [35] was used to calculate [Cr20~ ], neglecting the
704
S. Signorella et al.
Table 2. Effect of oxidant on the oxidation of ribonolactone. [RL] = 0.08 M ; [ H C 1 0 4 ] = 1 M ; T = 35°C 104[CrVll0/M
Table 3. Observed pseudo-first-order rate constants (kobs) for different concentrations of H f l O 4 and organic substrate. T=60°C;[CrW]0=7.75×10 4 M ; I = 1M
103kobs[CrVI]o/[Cr20~ 7 ]
4 8 14 20 26
14.7 13.0 12.9 14.1 13.7
[S]/M
subsidiary equilibrium HCrO4/CrO42- because of the high [HC104] employed. Table 3 summarizes values of kobs for various concentrations of R L at fixed concentrations of perchloric acid. Plots of kob, vS [ R L + R A ] ([S]) (Fig. 2) gave straight lines with a positive intercept from which values ofk~ and k2 were determined (Table 4). Dependence of kl and k2 with [HCIO4] is shown in Fig. 3 and may be expressed by : k, = a + b [ H + ] ,
(2)
~2 = c[H+] 3,
(3)
wherea=3.09×10-3M I s-l,b=4.98×10 s 1, c = 1.07×10 3 M - 3 s I. The complete rate law is then given by :
2M-2
0.0116 0.0155 0.019 0.023 0.027 0.031 0.035 0.039 0.043 0.047 0.054 0.062
[HC104]/M [10'kob~(s-')] 0.2 0.4 0.5 0.6 3.47
4.10 4,80 5.24 5,95 6.94 7.38
4.85 5.77 7.16 7.78
4.54 5.70 6.63 7.77 8.23 9.57
10.2 11.8 13.2
0.75
0.9
6.90 8.41
9.51 9.99 14.2
12.7 13.6 17.0 18.2
12,8 13.9
16.3 17.0 19.3 20.1
16.4
25.2
Table 4. Values ofkL and k2 for different [HC104] [HC104]/M
0.2
0.4
0.5
0.6
lOZkl/M ~1 s t 105k2/s ~
1.14 4.9
2.25 5.80
3 . 0 3 3.49 1 1 . 6 30.0
0.75
0.9
3 . 9 7 4.65 5 0 . 9 72.8
- d[CrVZ]/dt = kobs[C rw] r = (k~ [S] + k2 )[Cr vl] T = {(a+b[H+])[S]+c[H+]3}[frW]T,
(4)
where [S] refers to the total organic substrate concentration : R L + RA.
According to all experimental kinetic data, oxidation of the substrate by Cr vl should occur through three parallel slow steps leading to the redox products. A mechanism considering this fact and fitting the
0.00400.00350.00300.0025-
2
H [ ~=05.
0.0020-
v
r,
+
0.0015-
M
+ ;
0.0010~ 0
.
0
0
0
5
~
0.0000 0.01
0.02
0.03
0.04
0.05
0.06
[S] (M) Fig. 2. kob s a s a ~nction of the total reductant concentration at di~rent acidities.
0.07
705
Chromic oxidation of D-ribono- 1,4-1actone Kh
RL+H20~-RA KI
K2,H +
RA + Cr207 = ~- A = .
kl
K1.H +
" AH-
" AH 2
k2
k3 H +
RA
--4~p4 where P = °CO£ + C H 2 O H ( C H O H ) 2 C H O + + C r m i
H20 -*CH2OH(CHOH)CO,H
decompose to the products in the presence of a second R A molecule, whereas the diprotonated form (AH2) yields the products through an acid catalyzed step. We propose here that redox paths involving either the anionic or neutral complexes, are three-electron steps, just as observed for the chromic oxidation of mandelic acid [23], and Cr m is formed with the oxidation products. If A: , AH and AH2 are formed in rapid equilibria, the rate law corresponding to this mechanism may be expressed as : v = -d[CrW]/dt = {k, K, [RA] 2 +k2K2K, [RA]2[H -]
Scheme II.
].
(5)
+K, K2[RA][H+]+K,K~K~[H+]2[RA]I.
(6)
+k~K3K2K,[H+]S[RA]}[Cr~O~ experimental kinetic law well, is proposed in Scheme I1. In this mechanism we have assumed that the open carboxylic acid is the reactive form of the organic substrate. This assumption is based on previous findings showing the enhanced ability of hydroxyacids to bind Cr v~ and Crv [21-26]. In any case, a possible mechanism involving the lactone may not be discarded. However, if the lactone was the reactive form of the organic reductant, coordination of Cr w should be favored at the pair of syn hydroxyl groups of the molecule, C ( 2 ) - - O H and C ( 3 ) - - O H [36]. The intermediate complex formed in this way might be present in the equilibrium mixture but should not be a direct precursor of the slow redox steps. Opening of the lactone ring and rearrangement should be required for the slow electron-transfer steps to take place within a complex with the carboxylate bound to Cr w. Thus, the first step of the proposed mechanism may be interpreted as the formation o f a monochelate with RA acting as a bidendate ligand bound to Cr w at C ( I ) - - O and C(2)--O to yield the anionic species [(RA)(O)2(OH~)CrOCrO3] 2 ( A - ' ) . This intermediate, or its monoprotonated form ( A H ) , may
0.06
....
,
,
,
,
Since [Crv'] T = [Cr207 1{ 1 + 1/Kd + K, [RA]
Replacing [Cr20~ ] in eq. (6) z' = {k,K,[RA] 2 +k2K2K ~[RA]2[H ~] + k3K, K2K~ [H+]3[RA][CrVI]r] × [ 1 + 1/K,~+ K~ [RA] + Kt K2 [RA] [H " ] + K , K2K.~[H ~]2[RA]I ~'
(7)
If K~[RA] is the largest term in the denominator (a reasonable assumption since K~ corresponds to a complex formation constant while K2 and K~ are protonation ones), eq. (7) will be simplified ~0 the experimentally observed form : z, = {/<~[RA] + k2 K2 [RA] [H + ] +k~K~K2[H~13}[CrV'].,.
(8)
Equation (8) may be expressed as a function of the total organic substrate concentration : v = {(k, [S] +k~K2[S][H+I)K~/(I + Kh)
•
+ksKsK2[H+]s}[CrV']7. 0.05
0.0006
0.04
7m o.ooo4 m v
"T 0.03
(9)
Polymerization after addition of acrylonitrib supports the formation of "CO_;- radicals. These radicals formed in the slow steps may react with Cr w to yield Crv and carbon dioxide : fasl
"CO~ + C r vx ~ CO2 + C r v .
Z 0.02 0.0002 0.01
0.00 0.0
0.2
0.4 [HCI04]
0.6
0.8
0.0000 .0
(M)
Fig. 3. Effect of acidity on k~ and k>
(10)
Finally, Crv may react with excess substrate to yield erythrose, carbon dioxide and Cr m in a two-electron step : C r V + R A ~ C H 2 O H ( C H O H ) 2 C H O + C O 2 4 Cr m.
(il)
706
S. Signorella et al. CONCLUSIONS
The present results indicate that the oxidation of RA occurs by a CrVr/Cr m three-electron path yielding erythronic acid and the "CO~ radical; followed by a CrVl/CrV/CrH~ one-electron/two-electron path, leading to erythrose and carbon dioxide. This mechanistic proposal fits the experimental kinetics well and explains the products detected after the reaction completion, when an excess of sugar acid over Cr w is used. The present results are consistent with those obtained for a number of reported ~-hydroxy acids chromic oxidations [12,23-26]. However, a reactant structure/ cleaved bond type relation may not be easily rationalized upon comparison with other polyhydroxy acids. In the case of gluconic acid, C 2 - - H break occurs through a two-electron transfer process [16] and the c~-keto-acid is the only reaction product. On the other hand, galactonic acid yields lyxonic acid and carbon dioxide as products [37]. It seems reasonable to suggest that the relative C2, C3, C4 configuration influences the reaction course. Thus, the C2, C3, C4 ribo configuration behaves as the arbino (in galactonic acid) but different from the xylo (in gluconic acid). Even when any conclusive explanation may not be given at present, we think that after studying the chromic oxidation of a number of sugar acids of the same carbon-chain length it will be possible to find correlations between the structure of the reactants and the type of bond cleavage. Acknowledgements--We thank the National Research Council of Argentina (CONICET), the International Science Foundation (IFS), the Third World Academy of Science (TWAS) and the National University of Rosario, Argentina, for financial support. REFERENCES
1. P. O'Brien and G. Wang, Polyhedron 1993, 12, 1409. 2. V. Ramana and K. Sastry, J. lnor#. Biochem. 1994, 56, 87. 3. J. Alexander and J. Aaseth, Analyst 1995, 120, 931. 4. S. Katz and H. Salem, The Biological and Environmental Chemistry of Chromium. VCH Publishers, Inc., New York (1994). 5. A. Fan and L. Harden-Barlow, Adv. Mod. Environ. Toxicol. 1987, 11, 87. 6. K. Wetterhahn, J. Am. Chem. Soc. 1982, 104, 874. 7. S. Rossi, N. Grorman and K. Wetterhahn, Chem. Res. Toxicol. 1988, 1, 101. 8. S. Brauer and K. Wetterhahn, J. Am. Chem. Soc. 1991, 113, 3001. 9. D. Goodgame, P. Hayman and D. Hathway, Polyhedron 1992, 1,497.
10. D. Goodgame and M. Joy, J. Inory. Biochem. 1986, 26, 219. 11. W. Pigman and D. Horton, The Carbohydrates. Chemistry and Biochemistry, Vol. 1B, p. 1014. Academic Press, Inc., New York (1980). 12. S. Signorella, S. Garcfa and L. Sala, Polyhedron 1992, 11, 1391. 13. L. Sala, S. Signorella, M. Rizzotto, M. Frascaroli and F. Gandolfo, Can. J. Chem. 1992, 70, 2046. 14. L. Sala, C. Palopoli, V. Alba and S. Signorella, Polyhedron 1993, 12, 2227. 15. S. Garcia, S. Signorella, S. Acebal, E. Piagio and L. Sala, Oxidation Commun. 1993, 16, 313. 16. S. Signorella, M. Santoro, M. Mulero and L. Sala, Can. J. Chem. 1994, 72, 398. 17. M. Rizzotto, S. Signorella, M. Frascaroli, V. Daier and L. Sala, J. Carbohyd. Chem. 1995, 14, 45. 18. S. Signorella, C. Palopoli and L. Sala, Polyhedron 1995, 14, 1725. 19. M. Rizzotto, M. Frascaroli, S. Signorella and L. Sala, Polyhedron 1996, 15, 1517. 20. S. Signorella, M. Rizzotto, A. Boussecksou, C. Palopoli and L. Sala, J. Chem. Soc., Dalton Trans. 1996, 1607. 21. M. Mitewa and P. Bontchev, Coord. Chem. Rev. 1985, 61,241. 22. G. Haight, G. Jwisich, M. Kelso and P. Merril, lnor9. Chem. 1985, 24, 2740. 23. D. Ip and J. Rocek, J. Am. Chem. Soc. 1979, 101, 6311. 24. M. Krumpolk and J. Rocek, J. Am. Chem. Soc. 1977, 99, 137. 25. S. Ramesh, S. Mahapatro, J. Lui and J. Rocek, J. Am. Chem. Soc. 1981, 103, 5772. 26. K. Srinivasan and J. Rocek, J. Am. Chem. Soc. 1978, 100, 2789. 27. D. Sawyer and J. Bagger, J. Am. Chem. Soc. 1959, 81, 5302. 28. G. Escandar, Ph.D, Thesis, National University of Rosario (1992). 29. L. Hough, J. Jones and D. Mitchell, Can. J. Chem. 1958, 36, 1720. 30. W. Travelyan, D. Proctor and J. Harrison, Nature 1950, 166, 44. 31. L. Haugh, J. Jones and N. Wadman, J. Chem. Soc. 1950, 1702. 32. R. Bramley, J. Ji, R. Rudd and P. Lay, Inorg. Chem. 1990, 29, 3089. 33. G. Barr-David, M. Charara, R. Codd, R. Farrell, J. Irwin, R. Bramley, S. Brumby, J. Ji, G. Hanson and P. Lay, J. Chem. Soc., Faraday Trans. 1995, 91, 1207. 34. J. Moore and R. Pearson, Kinetics and Mechanism, p. 295, 3rd. edn. Wiley, New York (1981). 35. H. Spenson and R. Kinney, Inorg. Chem. 1971, 10, 1376. 36. M. Branca, A. Dessi, H. Kozlowski, G. Micera and J. Swiatek, J. Inor9. Biochem. 1990, 39, 217. 37. S. Signorella, M. Santoro and L. Sala, unpublished work.
~
Pergamon PII
: S0277-5387(96)00282-3
Copyright ~ 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0277 5387/97 $17.01)+0.00
Degradative oxidation of o-ribono-l,4-1actone by C r vI in perchloric acid S. Signorella, S. Garcia and L. F. Sala* Area lnorgfinica, Facultad de Ciencias Bioquimicas y Farmac6uticas (UNR), Suipacha 531, 2000 Rosario, Argentina
(Received 16 January 1996; accepted 5 June 1996)
Abstract--The oxidation of D-ribono-1,4-1actone by Cr w yielded o-erythronic acid, erythrose, carbon dioxide and Cr m as final products when a 15-fold or higher excess of sugar over Cr vl was used. The kinetics of the redox processes involving the CrVl/Cr m couple were determined and a mechanism is proposed. The complete rate law for the Cr vl oxidation reaction is expressed by: -d[CrVl]/dt = {a[S]+b[S][H +] +c[H+] 3} [CrVl]r, where a = 3.09 × 10 _3 M -~ s -~, b = 4.98 × 10 -2 M -2 s -~, c = 1.07 × 10 -~ M -3 s -~ and S refers to the total reductant concentration, at 60°C. Crv is formed in a fast step by reaction of the CO~- radical and Cr vt, and Crv reacts with the organic substrate faster than Cr w does. The EPR spectra show that the intermediate Crv complex (g = 1.978) is formed and decays by a first-order process. Copyright © 1996 Elsevier Science Ltd
Keywords: ribonolactone ; chromium ; oxidation ; kinetics ; mechanism ; Crv ; Cr v~.
Hexavalent chromium, in many different compounds, is a well established carcinogen and mutagen [1~,]. There is, therefore, appreciable interest in determining the mode of action of chromium species derived from initial uptake of Cr v~ compounds [5]. Ligands that possess two oxygen atoms able to form five-membered rings about the metal atom, such as 1,2-diols and ~hydroxyacids are reported to stabilize Crv intermediates after further interaction with biological reductants [6-10]. For this reason, some sugars, or their derivatives, may play an important role in the chemistry of Cr w. Aldonic acids (or their lactones) have been found as metabolic intermediates in animals, plants and microorganisms, as products of the action of microorganisms on carbohydrates and as constituents of acidic polysaccharides [11]. As part of a general project dealing with the interaction of carbohydrates and transition metal ions, we have studied the reaction of Cr vl with low molecular weight model reductants [12-20]. Chromic oxidation of ~-hydroxyacids has been extensively studied [12,21-26]. These oxidations may yield products coming from either C - - C [12, 23-26] or C - - H , [22,26] depending on the particular molecular characteristics, through mechanisms involving either
* Author to whom correspondence should be addressed.
two-electron or three-electron initial oxidation of substrate by Cr vl. Polyhydroxyacids offer additional binding sites to form the chromic ester precursor of the slow electron-transfer steps and might help to elucidate stereoelectronic effects affecting the reaction course [26]. In this paper we report the results obtained in our investigation of the chromic oxidation of D-ribono-1,4-1actone (RL) (Scheme I) in perchloric acid.
EXPERIMENTAL Materials o-Ribono-l,4-1actone (sigma grade), potassium dichromate (Cicarelli c.a), perchloric acid (P.A. Merck), acrylonitrile (Aldrich grade) and sodium perchlorate (Sigma grade) were used without further purification.
Spectropholometric measurements Kinetic measurements were made at 350 nm by monitoring the absorbance changes on a Guilford Response I1 spectrophotometer with fully thermostated cell compartments. Pseudo-first-order rate constants, determined from the slope of ln(Abs) vs
701
702
S. Signorella et al. COOH
CHO --OH
HOH2C
O
CO2H
2 CrVI
--OH
2 CO2 + 2 CrIII+ OH
OH
--OH OH
CH2OH
OH
CH2OH
CH20H RL
RA
Scheme I.
time, were deduced from multiple determinations and were within + 5% of each other. In most experiments the concentration of Cr v~ was kept constant at 7.75 10 -4 M while the RL was varied from 0.0116 to 0.062 M. Mixtures of sodium perchlorate and perchloric acid were used to maintain a constant ionic strength of 1.0 M. Disappearance of Cr vI was followed until at least 80% conversion. Reactant solutions were previously thermostated and transferred into a cell of 1 cm path length immediately after mixing. Experiments were performed at 60°C unless otherwise mentioned. The acid : lactone proportion of the reductant in the starting solution was calculated by optical rotation measurements at given hydrogen ion concentrations [27]. Table 1 shows the composition of the substrate solution at different perchloric acid concentrations. Ribonic acid (RA) is the major form in solution over the whole acid concentration range (0.2-1.0 M HC104), and as expected, the lactone proportion increases with increasing [HCIO4]. In order to confirm that no cooxidation process occurs at the primary hydroxy group, RL was methylated at C-5OH to yield the 5-O-methyl-D-ribono-l,4-1actone (MRL) [29]. The chromic oxidation of MRL and RL affords the same kobsvalues, under the same reaction conditions.
Product analysis Under conditions employed in the kinetic measurements (ratios of RL over Cr vI from 15/1 to 80/1) qualitative identification of D-erythronic acid (EA) and erythrose as the reaction products was carried out by paper chromatography. EA and erythrose were identified against authentic samples using n-butanol/acetic acid/water (4 : 1 : 5) as eluent. Paper chromatograms were visualized by two kinds of development reagents: a three-stage dip of silver nitrate, sodium hydroxide and sodium thiosulfate [30] and p-anisidine reagent [31]. Under these conditions, carbon dioxide was detected and the quantitative determination of this gas afforded 1 mole of CO2 per mole of Cr vl.
Test for free radicals A solution of potassium dichromate (1.16× 10 3 M) in 1 cm 3 of 0.5 M HC104 was added to a mixture of 1.16x 10 -3 M RL in 0.5 M HCIO 4 (2 cm 3) and acrylonitrile (0.5 cm 3) at 20°C. After 20 min a white precipitate of a polymer slowly appeared. Blank experiments in the absence of organic substrate or
Table 1. Concentration of ribonolactone and ribonic acid in the starting solution" [HCIO4]/M
Crb/M
R"
0.2-0.3 0.44).5 0.6 0.75 0.9-1
0.1623 0.1360 0.1317 0.1370 0.14
+0.200 +0.189 +0.200 - 0.222 +0.234
102[RL]/M 10[RA]/M 105[R-I/M 0.476 1.710 2.68 3.640 4.16
1.575 1.190 1.049 1.006 0.984
7.75 2.00 2.10 1.74 1.34
"Values used in calculations: molar rotations: + 5.65 (RL), +2.37 (RA), +0.95 (R-); K [28] = [H+][R-]/([RL] + [RA]) = 9.55 x 10 5. bCr = [RA]+ [RL] + [R-I, where R refersto ribonate. cOptical rotation.
703
Chromic oxidation of o-ribono- 1,4-1actone oxidant agent did not show formation of any such precipitate.
14
,
.
~
(a)
12 EPR measurements
E P R spectra were recorded with a Bruker spectrometer model ER-200. Spectra were acquired at a frequency of ca 9.8 G H z with a center field of 3500 G, a swept width of 100-1000 G and variable modulation amplitude. The cavity and sample tube were kept at 2 5 C . Initial Cr w concentration in all the E P R measurements was 7.75 x 10 -4 M.
E! 10 ~° -o .~
8
RESULTS AND DISCUSSION Over the whole range of perchloric acid concentrations used in the kinetic measurements, spectrophotometric studies showed that the reaction of R L and Cr w resulted in an absorbance band at 350 nm and a shoulder at 420-500 nm characteristic of the C r 2 0 ~ - ion. It is known that Crv species, usually formed in these oxidation reactions, absorb at 350 nm and may superimpose Cr v~ absorbance yielding the wrong interpretation of spectrophotometric absorbance decay values [22]. However, if Crv reacts faster than Cr vl and exists in solution in a sufficiently small concentration, changes in absorbance at 350 nm essentially reflect changes in Cr v~ concentration. We observed deviations from first-order decay over very short time periods and we calculated pseudo-firstorder rate constants from the slopes of the linear part of the ln(Abs) vs time curves ; a good criterion only if Crv decays faster than Cr w. The relative values of the Cr w vs Crv oxidation rates constants obtained from the Crv EPR signal intensities during the reaction course (discussed below) demonstrate this is the case for the present reaction. Unlike that observed for the chromic oxidation of gluconic acid [16], in the present case, Crv was not detected spectrophotometrically at 750 nm under conditions used in the kinetic experiments. Fortunately, it could be easily detected by E P R measurements. The reaction of Cr v~ with excess R L generates an E P R spectrum dominated by a single detectable sharp signal at g,~o = 1.978 (Fig. 1 inset (a)), typical of fivecoordinate oxochromate(V) complexes [32,33]. The Crv signal grows and decays with time to yield the broad Cr m signal (9~so = 1.981) as the ultimate fate of c h r o m i u m in this reaction (Fig. 1 inset (b)). Figure 1 shows a curve for peak-to-peak heights of Crv E P R signal t,s time obtained on a reacting solution. Values of the rates of disappearance of Cr w (k6) and Crv (k~), respectively, may be calculated by eq. (1) [12,16], which fits well the experimentally observed Crv intermediate growth and decay curve in Fig. 1. h = Ak6{exp(-kst)-exp(-2k6t)}/(2k6-ks).
(1)
It has been shown that any rate profiles dealing with the build up and decay of an intermediate by first
4
i 0
20
i
i
40 60 t (min)
i
80
__ 100
Fig. I. Peak-to-peak heights of Crv EPR signals ~;s time IS] = 0.039 M; [CrV~]0= 7.75 x 10-4 M; [HCIO4] =: 1 M: T = 25C. Inset : (a) Crv EPR signal recorded at t = 5 rain ; (b) superimposition of Crv and Cr ul EPR signals as Cr "~ builds up during the run (t = 35 min).
order processes can be described by a pair of solutions having the same numerical values for the rate constants, but with the order of the assignment reversed [34]. For conditions stated in Fig. 1, k 6 = 8.35 x 10-Ss ~andk5 = 0.00768 s t and thetime of maximum Crv concentration, T~a x -~ 8.5 min. We have chosen this assignment for the rate constant values and not the reverse one, for several reasons. In the E P R measurements, the temperature is lower than that used in the spectrophotometric experiments. Since the redox rates increase when the temperature rises, k6 obtained from E P R data (at 25"C) should be lower than values obtained from the spectrophotometric measurements at 60'~C (a value of 0.00768 exceeds all the kob~values in Table 3). Besides, we have evaluated kob~(2k6) spectrophotometrically at 25°C under conditions used in the EPR experiments, and we have reproduced the kob~(2k6) value (the value obtained by this technique, for conditions in Fig. 1, is: 1.84x 10 -4 s ~). Thus, the fact that k5 is 91 times higher than k~ justifies our initial assumption that changes in absorbance at 350 nm essentially reflect changes in Cr w concentration. In order to verify the dependence of rate upon Cr vj, pseudo-first order rate constants were calculated at various [CrW]0 (4-26 x 10 -4 M) but at constant temperature, [RL], [H +] and ionic strength (Table 2). The rate constant was found to increase with increasing [CrW]0. However, values of kobs[CrVl]o/[Cr20 ~ ] at various [CrV~]0 were found to be constant, meaning that the dimeric species, viz. Cr:O7 , is the effective oxidant. The equilibrium H C r O 4 / C r 2 0 ~ (K = 76) [35] was used to calculate [Cr20~ ], neglecting the
704
S. Signorella et al.
Table 2. Effect of oxidant on the oxidation of ribonolactone. [RL] = 0.08 M ; [ H C 1 0 4 ] = 1 M ; T = 35°C 104[CrVll0/M
Table 3. Observed pseudo-first-order rate constants (kobs) for different concentrations of H f l O 4 and organic substrate. T=60°C;[CrW]0=7.75×10 4 M ; I = 1M
103kobs[CrVI]o/[Cr20~ 7 ]
4 8 14 20 26
14.7 13.0 12.9 14.1 13.7
[S]/M
subsidiary equilibrium HCrO4/CrO42- because of the high [HC104] employed. Table 3 summarizes values of kobs for various concentrations of R L at fixed concentrations of perchloric acid. Plots of kob, vS [ R L + R A ] ([S]) (Fig. 2) gave straight lines with a positive intercept from which values ofk~ and k2 were determined (Table 4). Dependence of kl and k2 with [HCIO4] is shown in Fig. 3 and may be expressed by : k, = a + b [ H + ] ,
(2)
~2 = c[H+] 3,
(3)
wherea=3.09×10-3M I s-l,b=4.98×10 s 1, c = 1.07×10 3 M - 3 s I. The complete rate law is then given by :
2M-2
0.0116 0.0155 0.019 0.023 0.027 0.031 0.035 0.039 0.043 0.047 0.054 0.062
[HC104]/M [10'kob~(s-')] 0.2 0.4 0.5 0.6 3.47
4.10 4,80 5.24 5,95 6.94 7.38
4.85 5.77 7.16 7.78
4.54 5.70 6.63 7.77 8.23 9.57
10.2 11.8 13.2
0.75
0.9
6.90 8.41
9.51 9.99 14.2
12.7 13.6 17.0 18.2
12,8 13.9
16.3 17.0 19.3 20.1
16.4
25.2
Table 4. Values ofkL and k2 for different [HC104] [HC104]/M
0.2
0.4
0.5
0.6
lOZkl/M ~1 s t 105k2/s ~
1.14 4.9
2.25 5.80
3 . 0 3 3.49 1 1 . 6 30.0
0.75
0.9
3 . 9 7 4.65 5 0 . 9 72.8
- d[CrVZ]/dt = kobs[C rw] r = (k~ [S] + k2 )[Cr vl] T = {(a+b[H+])[S]+c[H+]3}[frW]T,
(4)
where [S] refers to the total organic substrate concentration : R L + RA.
According to all experimental kinetic data, oxidation of the substrate by Cr vl should occur through three parallel slow steps leading to the redox products. A mechanism considering this fact and fitting the
0.00400.00350.00300.0025-
2
H [ ~=05.
0.0020-
v
r,
+
0.0015-
M
+ ;
0.0010~ 0
.
0
0
0
5
~
0.0000 0.01
0.02
0.03
0.04
0.05
0.06
[S] (M) Fig. 2. kob s a s a ~nction of the total reductant concentration at di~rent acidities.
0.07
705
Chromic oxidation of D-ribono- 1,4-1actone Kh
RL+H20~-RA KI
K2,H +
RA + Cr207 = ~- A = .
kl
K1.H +
" AH-
" AH 2
k2
k3 H +
RA
--4~p4 where P = °CO£ + C H 2 O H ( C H O H ) 2 C H O + + C r m i
H20 -*CH2OH(CHOH)CO,H
decompose to the products in the presence of a second R A molecule, whereas the diprotonated form (AH2) yields the products through an acid catalyzed step. We propose here that redox paths involving either the anionic or neutral complexes, are three-electron steps, just as observed for the chromic oxidation of mandelic acid [23], and Cr m is formed with the oxidation products. If A: , AH and AH2 are formed in rapid equilibria, the rate law corresponding to this mechanism may be expressed as : v = -d[CrW]/dt = {k, K, [RA] 2 +k2K2K, [RA]2[H -]
Scheme II.
].
(5)
+K, K2[RA][H+]+K,K~K~[H+]2[RA]I.
(6)
+k~K3K2K,[H+]S[RA]}[Cr~O~ experimental kinetic law well, is proposed in Scheme I1. In this mechanism we have assumed that the open carboxylic acid is the reactive form of the organic substrate. This assumption is based on previous findings showing the enhanced ability of hydroxyacids to bind Cr v~ and Crv [21-26]. In any case, a possible mechanism involving the lactone may not be discarded. However, if the lactone was the reactive form of the organic reductant, coordination of Cr w should be favored at the pair of syn hydroxyl groups of the molecule, C ( 2 ) - - O H and C ( 3 ) - - O H [36]. The intermediate complex formed in this way might be present in the equilibrium mixture but should not be a direct precursor of the slow redox steps. Opening of the lactone ring and rearrangement should be required for the slow electron-transfer steps to take place within a complex with the carboxylate bound to Cr w. Thus, the first step of the proposed mechanism may be interpreted as the formation o f a monochelate with RA acting as a bidendate ligand bound to Cr w at C ( I ) - - O and C(2)--O to yield the anionic species [(RA)(O)2(OH~)CrOCrO3] 2 ( A - ' ) . This intermediate, or its monoprotonated form ( A H ) , may
0.06
....
,
,
,
,
Since [Crv'] T = [Cr207 1{ 1 + 1/Kd + K, [RA]
Replacing [Cr20~ ] in eq. (6) z' = {k,K,[RA] 2 +k2K2K ~[RA]2[H ~] + k3K, K2K~ [H+]3[RA][CrVI]r] × [ 1 + 1/K,~+ K~ [RA] + Kt K2 [RA] [H " ] + K , K2K.~[H ~]2[RA]I ~'
(7)
If K~[RA] is the largest term in the denominator (a reasonable assumption since K~ corresponds to a complex formation constant while K2 and K~ are protonation ones), eq. (7) will be simplified ~0 the experimentally observed form : z, = {/<~[RA] + k2 K2 [RA] [H + ] +k~K~K2[H~13}[CrV'].,.
(8)
Equation (8) may be expressed as a function of the total organic substrate concentration : v = {(k, [S] +k~K2[S][H+I)K~/(I + Kh)
•
+ksKsK2[H+]s}[CrV']7. 0.05
0.0006
0.04
7m o.ooo4 m v
"T 0.03
(9)
Polymerization after addition of acrylonitrib supports the formation of "CO_;- radicals. These radicals formed in the slow steps may react with Cr w to yield Crv and carbon dioxide : fasl
"CO~ + C r vx ~ CO2 + C r v .
Z 0.02 0.0002 0.01
0.00 0.0
0.2
0.4 [HCI04]
0.6
0.8
0.0000 .0
(M)
Fig. 3. Effect of acidity on k~ and k>
(10)
Finally, Crv may react with excess substrate to yield erythrose, carbon dioxide and Cr m in a two-electron step : C r V + R A ~ C H 2 O H ( C H O H ) 2 C H O + C O 2 4 Cr m.
(il)
706
S. Signorella et al. CONCLUSIONS
The present results indicate that the oxidation of RA occurs by a CrVr/Cr m three-electron path yielding erythronic acid and the "CO~ radical; followed by a CrVl/CrV/CrH~ one-electron/two-electron path, leading to erythrose and carbon dioxide. This mechanistic proposal fits the experimental kinetics well and explains the products detected after the reaction completion, when an excess of sugar acid over Cr w is used. The present results are consistent with those obtained for a number of reported ~-hydroxy acids chromic oxidations [12,23-26]. However, a reactant structure/ cleaved bond type relation may not be easily rationalized upon comparison with other polyhydroxy acids. In the case of gluconic acid, C 2 - - H break occurs through a two-electron transfer process [16] and the c~-keto-acid is the only reaction product. On the other hand, galactonic acid yields lyxonic acid and carbon dioxide as products [37]. It seems reasonable to suggest that the relative C2, C3, C4 configuration influences the reaction course. Thus, the C2, C3, C4 ribo configuration behaves as the arbino (in galactonic acid) but different from the xylo (in gluconic acid). Even when any conclusive explanation may not be given at present, we think that after studying the chromic oxidation of a number of sugar acids of the same carbon-chain length it will be possible to find correlations between the structure of the reactants and the type of bond cleavage. Acknowledgements--We thank the National Research Council of Argentina (CONICET), the International Science Foundation (IFS), the Third World Academy of Science (TWAS) and the National University of Rosario, Argentina, for financial support. REFERENCES
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