G. C. Goyal and D. A. Armstrong
A similar mechanism for the dissolution of ZnS has been proposed by Locker and de Bruyn.' In summary, the dissolution of FeS in dilute sulfuric acid is not controlled by the transport of the reactants or the products. The forward dissolution reaction is first order with respect to H+ and the reverse reaction is one-half order with respect to Fez+ and H2S. Further work with other iron sulfides is in progress. References a n d Notes (1) L. D. Locker and P. L. de Bruyn, J. Electrochem. SOC., 116, 1659 (1969). (2) T. Krokowski, Rocz. Chem., 13, 561 (1933). (3) H. A. Pohl, J. Am. Chem. Soc., 76, 2182 (1954). (4) D. A. Vermilyea, J. Electrochem. Soc., 116, 1179 (1969).
A. C . Riddiford, Adv. Electrochem. Electrochem. Eng., 4, 47 (1967). (a) D. D. MacDonald and D. Owen, Can. J. Chem., 49,3375 (1971); (b) V. G. Levich, "Physic0 Chemical Hydrodynamics", Prentice Hall, Englewood Cliffs. N.... - , 1962. --R. A'Berner, Am. J. Sci., 265, 773 (1967). I. M. Kolthoff, J. Phys. Chem., 35, 2711 (1931). , , I. M. Kolthoff and F. S.Griffith, J. Am. Chem. Soc., 60, 2038 (1938). (10) W. D. Treadwell and 0. Gubeli, Heiv. Chim. Acta, 24, 137 (1941). (11) D. D. Wagman, W. H. Evans, V. B. Parker, I. Halow, S.M. Bailey, and R. H. Schurn, Nati. Bur. Stand. Tech. Note, No. 270-4 (1969). (12) D. D. Wagman, W. H. Evans, V. B. Parker, I. Halow, S.M. Bailey, and R. H. Schum, Nati. Bur. Stand. Tech. Note, No. 270-3 (1968). (13) M. Randall and M. Frandsen, J. Am. Chem. SOC.,54,40 (1932). (14) J. W. Larson, P. Cerutti, H. K. Garber, and L. G. Hepler, J. Phys. Chem., 72, 2902 (1968). (15) T. Hurien, Acta Chem. Scand., 14, 1533 (1960). (16) L. G. Hepler and S.R. Rao, private communications. (17) H. Majima and E. Peters, Trans. Metal. SOC.AiM€, 236, 1409 (1966). (18) R. W. Gurney, "Ionic Processes in Solutions", Dover Publications, New York, N.Y., 1 9 5 3 , ~24.
y Radiolysis of Penicillamine in Deaerated 1 M Perchloric Acid Greesh Chand Goyai and David Anthony Armstrong"' Department of Chemistry, University of Calgary, Calgary, Alberta, Canada T2N IN4 (Received December 1 1, 1975) Publication costs assisted by the National Research Council of Canada
The products of radiolysis of penicillamine in 1M HC104 are reported and their yields discussed. The reactions of .OH and mCH2OH are qualitatively similar to those observed for cysteine and produce primarily RSradicals. However, the secondary reactions of RS. radicals give rise to a substantially higher yield of trisulfide (RSSSR) than is observed with cysteine. This appears to be due to the presence of weaker C-S bonds in the tertiary C-SH compound and to the fact that this bond can be broken as a result of interactions between RS. and RSH, possibly via the formation of a RSS(H)R intermediate. The magnitude of the rate constant ratio k3(k4 kg)-l a t 1f 1"C is 0.54 f O.lo. At 23 "C it was found to be 0.50 0.10 in good agreement with an earlier value of 0.44.l'
+
Introduction
RS.
P u l ~ e and ~ - ~y r a d i o l y s i ~ ~studies -~ have shown that the sulfhydryl-containing amino acid cysteine (HSCH2CH(NH3f)COOH)gis highly reactive toward eaq-, -OH, and .H radicals. Also product yields from and M solutions in the pH range 0-8 can be satisfactorily explained by the following reactions: eaq- + RSH
-
eaq-
+ H+
+ RSH .H + RSH R. + RSH *H+ RSH .SH + RSH *OH+ RSH RS. + RS. -H
(-2
Re + HS-
-+
+
-+
+H2S
He
(2)
RS.
(3)
H2S
+ R.
(4)
+ RS. -SH + RH H2S + RS. H2O + RS-
-+
(1)
H2
RH
-.+
)
-+
RSH + RS-
+ RSS-R
-.+
RSS-R RS-
+ RSSR
(11) (12)
The abbreviated formula RSH (R = CH2CH(NH3+)COOH) is used here, since the observed chemical changes involve only the sulfhydryl group. The related molecule penicillamine resembles cysteine in having a high reactivity toward eaq-, .OH, and .H.3,4J0J1 However, there is evidence that a t pH 5 and 8 the reactions of the secondary radicals formed from penicillamine differ in certain respects from those of the radicals occurring in cysteine.1°J2 Since there is no information relating to the products of .OHreactions at pH below 5 and relatively little for those of -H atoms, we have investigated the radiolysis of solutions of penicillamine in l M HC104.
(5) (6)
(7) (8)
RSSR
(9)
+ H+
(10)
The Journal of Physical Chemistry, Vol. 80, No. 17, 1976
RS.
+ RS-
Experimental Section Materials. All solutions for irradiation were prepared in fresh triply distilled nitrogen-purged water, to which the required amount of HC104 had already been added. D-penicillamine was obtained from the Sigma Chemical Co. This compound and the amino chromatography standards and reagents (D-valine from Mann Research Laboratories, grade
y Radiolysis of Penicillamine
A P-hydroxyvaline from Calbiochem, and D-penicillamine disulfide and 5,5'-dithiobis(Z-nitrobenzoic acid) from the Aldrich Chemical Co.) were all used without further purification. The penicillamine trisulfide was kindly supplied by Dr. J. W. Purdie of the Defence Research Board in Ottawa. Only free base forms of amino acids were used. All other chemicals were of reagent or analar grade. Procedures. After preparation the solutions for irradiation were divided into 5-ml aliquots. These were placed in irradiation cells and after degassing by several freeze-pumpthaw cycles had been completed they were sealed off while frozen under vacuum. Amino-containing products were determined with a Technicon amino acid analyzer using techniques and standardization procedures already described.6,7J2J3 Loss of penicillamine was determined both from its peak on the amino acid chromatogram and by SH titration with 5,5'-dithiobis( 2nitrobenzoic acid) after H2S had been removed from the irradiated solutions by purging with purified nitrogen.12 Hydrogen sulfide and hydrogen concentrations were measured as in ref 6. All other procedures were as described in ref 12 and 13. Irradiations. Irradiations in the Gamma Cell 220 (Atomic Energy of Canada) were a t 23 f 2 "C except for one or two experiments at 1 f 1"C, where the irradiation cells were kept in an ice bath. The average value of the dose rate during the course of the experiments was 5 X 10l6 eV ml-l min-l. The actual dose rates were measured on several occasions by ferrous sulfate dosimetry and were corrected for 6oCodecay.12
Results Examples of plots of product concentration vs. dose are shown in Figures 1 and 2a and 2b. The yields of products in molecules per 100 eV in Table I were calculated from the slopes of such plots in the 1to 1.7 X l0ls eV ml-l dose range for M penicillamine solutions and in the 2 to 7 X eV ml-l dose range for M solutions. For the latter solutions linearity of these plots for the major products valine and penicillamine disulfide extends to 10 X 1018 eV ml-1 (Figure 2a). In Table I and throughout the subsequent Discussion section we use the abbreviated formulae with R = C(CH&-, CH(NH3+)COOH in designating penicillamine (RSH), and the disulfide (RSSR), trisulfide (RSSSR), and valine (RH) products. Yields of analogous products from M cysteine solutions are given in parentheses. A blank indicates either that there is no analogous product (e.g., for the vinyl compound C H ~ = C ( C H ~ ) C H ( N H ~ + ) C O Z ) orHthat its yield is 50.1 molecules per 100 eV. Comparison of the 100-eVyields for total nitrogen and total sulfur in the products of penicillamine with the penicillamine loss (G(-RSH)) shows a satisfactory material balance. Before leaving this section one may note that with the conditions and procedures used here the oxidation of penicillamine by hydrogen peroxide was negligible. Discussion Formation of H2 and H2S. Calculationsbased on the known rate constants of reactions 1and 2 show that for 1M H+ and M penicillamine effectively all eaq- undergo reaction 2 (cf. ref 11).Reactions 4 and 6 then compete with reaction 3 and the sequences 4-5 and 6-7 both produce equivalent amounts of H2S and RH. The observed hydrogen is the combined yield of reaction 3 and the molecular hydrogen yield GH~M.Allowing for spur scavenging of -H,6J4we take G H , = ~ 0.30 in M penicillamine and therefore kz(h4 -t = (G(H2) -
1849
DOSE
(ev rn1-l)
x
Figure 1. Hydrogen and hydrogen sulfide concentratlon. Dose plots for 10-2 M penicillamine in 1 M HC104: e, HsS; A,Ha. 4.0 I
I
I
.
I
I
I
/
4
8
12
DOSE (ev r n P ) x
Flgure 2. Amino-containing product concentration. Dose plots for penicillamine in 1 M HC104:(a) M penicillamine: A,disulfide: B, trisulfide; e, valine; 0, vinyl compound; (b) M penicillamine: A , disulfide; 0 trisulfide; filled points are for solutions containing 1 M CH30H.
G H , ~ ) ( G ( H z S ) )=- ~0.50 f 0.10 at 23 f 2 "C from the data in Table I. This is in good agreement with Tung and Kuntz's value of 0.44 based on a determination of the hydrogen yield from M penicillamine and assuminggH = 3.62 per 100 eV, G H = 0.42 ~ ~per 100 eV and G(H2S) = g H - G(H2) + G H ~ ~ . ~ ~ In the present study experiments similar to those in Figure 1 were carried out at 1 f 1 "C and gave G(H2) = 1.33 and G(H2S) = 1.91 per 100 eV, from which k3(h4 + k g ) - l = 0.54 f The Journal of Physical Chemistry, Vol. 80,No. 17, 1976
1850
G. C. Goyal and D. A. Armstrong
TABLE I: Product Yields from Deaerated Solutions Containing 1 M HClO4 at 23 "C M penicillamine G (RSSR) G(RSSSR) G (RH) G (CH,=C(CH,)CH(NH,+)COOH) G(NH3) G (Hz) G (HzS) Total G(-N)b Total G(-S)' G(-RSH)
2.40 f 0.20~ 0.72 f 0.05 2.40 f 0.10 0.28 f 0.04 0.19 f 0.04 1.39 f 0.08 2.i8 f 0 . 2 ~ 9.2 f 0.7 9.2 f 0.8 10.6 f O.gd 9.6 f 1.2e
M cysteinelf
M penicillamine
+ 1M CH30H
1.73f 0.15a 0.40 f 0.04 1.5, f 0.10 0.41 f 0.04
1.93 f 0.20' 0.40 f 0.04 0.93 f 0.06 0.32 f 0.03
6.3 f 0.5
6.1 f 0.6
6.3 f O . j d
6.1 f 0.5d
(3.30 f 0.20) (0.87f 0.05)
(3.35 f 0.10) (0.78 f 0.05) (7.5 f 0.5) (7.4 f 0.5) (7.8 f 0.3)
M penicillamine
a Average deviations of four to six determinations in the range 2 to 7 X l0ls eV ml-l for IO-* M solutions and 0.7 to 2 x l0ls eV ml-1 for lo-, M solutions. G(-N) = Z(G value X no. of N atoms per molecule for each product); include trace (0.10 f 0.07)yields of ROH. G(-S) = B(G value X no. of S atoms per molecule for each product). Determined from amino acid chromatograms as for amino products. e Determined by SH titration with Ellman's reagent after purging the solution of H2S. f Data in parentheses in column three are yields of corresponding cysteine products from ref 6.
0.10. The absence of a significant temperature dependence and
small magnitude of this rate constant ratio is in contrast to cysteine for which the corresponding ratios calculated from H2 and H2S yields are 3.315 and 8.e6 at 23 and 1 "C, respectively. The difference in behavior of the two systems is probably due to a lower bond dissociation energy of the C-S bond in penicillamine. Dissociation energies of C-S bonds are normally lower when the C atom is tertiary.16 Formation of Trisulfide and Reactions of Secondary Radicals. The formation of trisulfide from penicillamine with a yield 0.72 in M and 0.40 in lo-, M solution represents a second important difference from cysteine, for which the yield of trisulfide is negligible at pH 0 (i.e., less than 0.1 in M solutions), and it would appear that the presence of this product must also be linked to the influence of the two methyl groups on the p carbon. If the penicillamine trisulfide were being formed by reactions of radicals with the disulfide produced in the radiolysis, its rate of production would be expected to increase with dose, since [RSSR] is initially near zero and grows steadily during radiolysis (see the filled triangles in Figure 2a). The fact that the concentration-dose plot for penicillamine trisulfide is linear to -8 X 10l8eV ml-l and bends downward beyond 10 X 10ls eV ml-l shows that this cannot be the case. To determine whether it was a specific product of .OH or -H reactions the amino-containing products from M penicillamine in 1M HC104 were measured in the presence and absence of 1 M CH30H. Using the following data for reactions 3,4,6, and 8 in penicillamine, and for
+
*H CH3OH
-
H2 + C H 2 0 H
(13)
and -OH
+ CH30H
+
HzO
+ eCH20H
+ +
+ RSH
(14)
+
CH30H
was slightly higher in the 1 M CHBOH solutions, as would be expected for increased scavenging of radicals in the spurs due to the added solute ~0ncentration.l~ However, the ratio G(RSSSR){G(RSSSR) G(RSSR)}-l was the same (0.18 f 0.01) with and without methanol. This is in keeping with the view that RSSSR is not a specific product of .H or .OH reactions. In fact the results are best explained if RS. is the only important product of reactions 3,5,7,8,and 15, and RSSSR arises from secondary reactions of RS.. The same conclusion has been drawn from results with a greater variety of radicals reacting with penicillamine a t pH 5.12 It is also true for cysteine.l3 The fact that G(RSSSR) increased from 0.40 to 0.72 molecules per 100 eV when the penicillamine concentration was to 10+ M (see Table I) suggests that the raised from mechanism of trisulfide formation includes an interaction between RS. and RSH. I t is qualitatively similar to the previously observed effect of penicillamine concentration a t pH S,1°where trisulfide production was attributed to the formation of RS.S-R in reaction 11 and the decomposition of a fraction of these inlo
+
RSS-R( +HzO)
-
RSS-
+ RH + OH-
(16)
The RSS. radicals then produce trisulfide via the radicalradical combinationlo RSS-
+ RS.
and also through reactions RSS.
k 3 h4 h6 = 1.9 x 109,11h8 = 4.7 x iO9,l'i h13 = 1.6 x 1 0 6 , l S and h14 = 8.4 X 108,19respectively, in M-l s-l, we estimated that in these solutions only 1.98 .H and 0.02 -OH radicals per 100 eV would react directly with the RSH compound. The remainder of these two primary radicals would undergo reactions 13 and 14 to form 4.60 CHzOH, which also react with penicillamine.20 Hence in the presence of 1 M CHsOH penicillamine would be attacked by 4.60 -CH20H, 1.98-H, and 0.02 .OH radicals per 100 eV, instead of the 2.95 .OH and 3.65 -H per 100 eV in the absence of CH30H.21 The Journal of Physical Chemistry, Vol. 80, No. 17, 1976
+ RS* (15) As shown by the results in Table I (G (RSSSR) + G (RSSR)} CH20H
-
+ RSS.
+ RSH RSSR + RSSH
RSSSSR
+
RSSSR
(17)
RSSSSR
+ RSSH RSSSR + RSH RSSSR
(19)l0 (20)12
Some contribution to the trisulfide yield at pH 5 is expected from these processes, but our work showed that G (RSSSR) from penicillamine a t that pH was too large to be explained quantitatively on the basis of reaction rate data available for reaction 16.12Because [RS-] is negligible at pH 0 (pK of S H of penicillamine = 8.09) and reaction 16 can be completely excluded in 1 M HC104, the present observation of RSSSR
y Radiolysis of Penicillamine
1851
formation confirms that there must be another means of producing it. Since the forward reaction
RS.
+ RSH + R&(H)R
(21, -21)
should also give rise to an increase in G(RSSSR) with penicillamine concentration, one ,must consider the possible participation of sulfenium (RSS(H)R) radicals. This type of radical has been postulated as an explanation of ESR spectra in solid and glassy matrices containing RSH,22 and an absorption spectrum attributed to it has been reported for the cyclic disulfide of lipoic acid in aqueous ~ o l u t i o nThe . ~ sulfenium radicals of noncyclic disulfides in aqueous solution are unstable, reaction -21 being the main decomposition channel.3,4However, if
RS'S(H)R
-
RSS.
+ RH
(22)
also occurs, a faster rate would be expected for compounds with the weaker tertiary C-S bondsl6 than for those with primary C-S bonds. This agrees with the trisulfide yield being much larger for penicillamine than for cysteine. Radical Balances. For the cysteine system a t pH 0 2G(RSSR) is equal to g H g o H , and it was concluded698 that all .OH, .H, and secondary radicals formed by them eventually produced RS.. If reactions 21 and 22 were the only source of RSS. and reactions 9 and 17 the only radical combination rea c t i o n ~then , ~ ~ in penicillamine solutions a t pH 0 2{G(RSSR) + G(RSSSR))should also be equal to gH g o H = 6.6 per 100 eV. This is borne out for the M solutions, for which 2{G(RSSR) G(RSSSR)) = 6.3 per 100 eV. For the 10-3 M solutions the magnitude of this quantity is too low (4.3 per 100 eV), but there are two logical reasons. In the the first place the fraction of primary radicals reacting with products will be appreciable as the present analytical sensitivity meant that yields had to be determined a t doses near to 15%decomposition, while those for M solutions were measured below 10%decomposition. A second factor is the occurrence of radical-radical reactions other than reactions 9 and 17. The product RSR was not formed in detectable amounts, possibly because a single S atom between the two /3 carbons is insufficient to overcome the steric hindrance of the four methyl groups on the combining R. and RS. radicals. However, the following reaction is not susceptible to this effect.
+
+
+
+
R* RS.
+
+
CH~=C(CH~)CH(NHS+)COOHRSH
(23)
Indeed the fact that G(CHZ=C(CH~)CH(NH~+)COOH) is smaller in lov2M than in M solutions without CH30H (see Table I) implies that reaction 23 occurs to a greater extent in the latter case, where the rate of reaction 5 will be much slower.24 If reactions 23 and 5 are the main reactions of R. radicals and reaction 7 the main reaction of .SH in 10-2 M solutions, then the H2S yield should be equal to (G(RH) G(CHZ=C(CH~)CH(NH,+)COOH) - G(RSSSR)J,which is 2.0 per 100 eV from the data in Table I and agrees with G(H2S) = 2.2 per 100 eV. Thus the reactions proposed are in general consistent with the observed product yields.
+
Conclusions An understanding of the radiation chemistry of penicillamine is of interest because it differs significantly from typical 1,2-aminothiols in possessing a relatively poor in vivo protective ability and in some systems is a sensitizer.25 The present study has shown that the reactions of .OHand -H with penicillamine a t p H -0 are qualitatively similar to those of cysteine. However, the much smaller value of k3(k4 + kG)-l
reported in ref 11and confirmed here, and the production of penicillamine trisulfide in a significant yield, are important quantitative differences. Both probably stem from a weakening of the C-S bond in the tertiary compound. Trisulfide formation is attributed to secondary reactions of RS. radicals and could be explained by C-S cleavage in sulfenium radical intermediates. While further work will be required to confirm this it may be noted that there is already evidence from pulse radiolysis pointing to a difference in the reactions of transitory RSS(H)R species when R is a tertiary group. Thus Hoffman and Hayon3 produced radicals having spectra characteristic of RS. by using
+
*H RSSR
-
RS-
+ RSH
(24)
a t pH 1 with R = CH&H(NH3+)COOH and other groups which are attached to S through primary C atoms. However, with R = C(CH~)ZCH(NH~+)COOH the absorption was very weak an$ did not exhibit the expected maximum a t 330 nm. Since RSS(H)R is a probable intermediate in reaction 24, this observation argues in favor of an alternative decomposition channel for the penicillamine sulfenium radical, such as reaction 22, or of its being long lived and having little absorption a t 330 nm. Clarification of the reason for the interesting observation of Hoffman and Hayon is therefore very relevant to the mechanism of trisulfide formation postulated here for strongly acid solutions.
Acknowledgment. This work was supported by the Canadian National Research Council and Defence Research Board through Grants No. A3571 and 957004, respectively. References and Notes (1) (2) (3) (4) (5)
Author to whom correspondence should be addressed. J. P. Barton and J. E. Packer, lnt. J. Radiat. Phys. Chem., 2, 159 (1970). M. 2. Hoffman and E. Hayon, J. Am. Chem. SOC.,94, 7950 (1972). M. Z. Hoffman and E. Hayon, J. Phys. Chem., 77, 990 (1973). A. El. Samahy, H. L. White, and C. N. Trumbore, J. Am. Chem. SOC.,86, 3177 (19641. (6) V. G. Wilkening, M. Lal, M. Arends, and D. A. Armstrong, Can. J. Chem., 45. 1209 11967). (7) V. G.Wilkening: M. Lal, M. Arends, and D. A. Armstrong, J. Phys. Chem., 72, 185 (1968). (8) A. AI-Thannon, R. M. Peterson, and C. N. Trumbore, J. Phys. Chem., 72, 2395 (1968). Note that there is good agreement between the product yields of this study and references 6 and 7. (9) Formulae given refer to pH 0. The carboxyl pKs are near 2 and the SH pKs near 8. See ref 10. (IO) J. W. Purdie, H. A. Gillis, and N. V. Klassen, Can. J. &em., 51, 3132 (1973). (11) T.-L. Tung and R. R. Kuntz. Radiat. Res., 55, 10 (1973). (12) G. C. Goyal and D. A. Armstrong, Can. J. Chem., in press. (13) G. C. Goyal and D. A. Armstrong, Can. J. Chem., 53, 1475 (1975). (14) 2. D. Draganit: and I. G. Draganic, J. Phys. Chem.. 76, 2733 (1972); 77, 765 (1973). (15) Average of 3.5 from ref 6 and 3.0 from ref 8. (16) H. Mackel, Tetrahedron, 19, 1159 (1963). (17) W. A. Armstrong and W. G. Humphreys, Can. J. Chem., 45, 2589 (1967). (18) P. Neta, R. W. Fessenden, and R. H. Schuler, J. Phys. Chem., 75, 1654 (197 1). (19) L. M. Dorfman and G. E. Adams, "Reactivity of Hydroxyl Radicals in Aqueous Solution", US. Department of Commerce, NBS No. 46, Washington, D.C., 1973. (20) Note that kls = 1.1 X IO8 M-ls-' from ref 10. (21) M. Anbar in "Fundamental Processes in Radiation Chemistry", P. Ausloos, Ed., Interscience, New York, N.Y., 1968, p 651. Note that differences in and radical yields in M penicillamine due to the different solute reactivities have been neglected. (22) M. C. R. Symons. J. Chem. SOC.,Perkin Trans. 2, 1619 (1975). (23) Reactions 18-20 lead to the same stoichiometry, but are probably negligible as few RS- aqtually form RSS. (G(RSSSR) = 0.4 or 0.72),and [RSS.] must be
