. J. Chem. 62, 1672 (1984).The kinetics and equilibria of the oxidation of penicillamine by glutathione disulfide to form, in the first step, penicillamine-glutathione mixed disulfide and glutathione and, in the second step, penicillamine disulfide and glutathione have been studied over the pH range 4-9 by 'H nmr. The reactive species are found to be penicillamine with its amino group protonated and its thiol group deprotonated and glutathione disulfide and penicillamine-glutathione mixed disulfide with their two amino groups protonated. The rate and equilibrium constants for the first step are much larger than those for the second step, indicating a small tendency for penicillamine to form its symmetrical disulfide by thiol/disulfide exchange reactions. This and the smaller reducing power of penicillamine as compared to glutathione are attributed to steric hindrance from the methyl groups adjacent to the sulfur. The kinetics and equilibria of the oxidation of N-acetylpenicillarnine by glutathione disulfide were studied at neutral pH. Conditional equilibrium and rate constants for the oxidation of penicillamine by glutathione disulfide at pH 7.4 are presented and discussed in terms of the metabolism of penicillamine.

. Can. J. Chem. 63, 33 (1985). The oxidation of captopril (CpSH, I-(D-3-mercapto-2-methylpropanoy1)-I-proline) by glutathione disulfide (GSSG) via thiol/disulfide exchange to form, in the first step, CpSSG and GSH and, in the second step, CpSSCp and GSH, has been studied in aqueous solution by 'H nmr. Due to slow rotation around the amide bond(s) of CpSH and CpSSCp and of the captopril part of CpSSG, separate resonances are observed for the cis and trans conformations across these bonds. Conformational equilibrium constants were estimated as a function of pH for CpSH, CpSSCp, and CpSSG from the intensities of resonances for the cis and trans isomers. These equilibrium constants were used in the determination of equilibrium constants for the two steps in the oxidation of CpSH by GSSG. The results suggest that CpSH has a greater tendency to reduce disulfide bonds by thiol/disulfide exchange at physiological pH, and thus form mixed disulfides, than do the thiol groups in amino acids. Also, the conformational equilibrium constants indicate that, at physiological pH, approximately two thirds of the captopril, either free or in a disulfide form, has the trans conformation. Les constantes d'Cquilibres conformationnels indiquent Cgalement, a pH physiologique, qu'approximativement les deux tiers du captoprile, tant sous la forme libre que sous la forme de disulfure, existe sous une conformation trans.[Traduit par le journal]

. Can. J. Chem. 62, 13 12 (1984). The acid-base chemistry of the ammonium groups of penicillamine-glutathione mixed disulfide and cysteine-glutathione mixed disulfide has been characterized by I3C nmr and that for the ammonium groups of penicillamine-cysteine mixed disulfide by 'H nmr. ' The mixed disulfides were formed by thiol/disulfide exchange. Chemical shift titration data were obtained simultaneously for the mixed disulfides, the thiols, and the symmetrical disulfides in the mixture. Since the fractional titration of individual ammonium groups can be obtained directly from chemical shift data, it was not necessary to isolate the mixed disulfides. The chemical shift data indicate that the two ammonium groups of each of the mixed disulfides are titrated over the same pH range, with the acidity of the ammonium group of the penicillamine and cysteine parts of the penicillamineglutathione and cysteine-glutathione mixed disulfides 4.4 and 3.9 times as acidic as those of the glutathione part. In the penicillamine-cysteine mixed disulfide, the penicillamine ammonium group is 1.7 times as acidic as that of the cysteine part. Macroscopic and microscopic acid dissociation constants are reported for each of the mixed disulfides. The advantages of nmr as a method for characterizing the acid-base chemistry of mixed disulfides are discussed.YVON THERIAULT, BRUCE V. CHEESMAN, ALAN P. ARNOLD et DALLAS L. RABENSTEIN. Can. J. Chem. 62, 13 12 (1984). On a caractCrisC la chimie acide-base des groupes ammonium des disulfures mixtes pCnicillamine-glutathion et cystCine-glutathion en faisant appel 21 la rmn du I3C et celle des groupes ammonium du disulfure mixte penicillamine-cysttine par la rmn du 'H. Les disulfures mixtes ont CtC obtenus via un tchange thiol/disulfure. On a obtenu simultankment les donnCes de titrage du diplacement chimique pour les disulfures mixtes, les thiols et les disulfures symktriques du melange. I1 n'est pas nicessaire d'isoler les disulfures mixtes puisqu'on peut obtenir directement le titrage fractionnel des groupes ammonium individuels B partir des donnCes de dtplacement chimique. Ces donnCes indiquent que les deux groupes ammonium de chaque disulfure sont titrCs dans le m&me domaine de pH. Les aciditis des groupes ammonium des portions pCnicillamine et cystkine des disulfures mixtes pinicillamine-glutathion et cystkine-glutathion sont respectivement 4,4 et 3,9 fois plus tlevies que celle de la partie glutathion. Dans le disulfure mixte p6nicillamine-cystkine 1'aciditC du groupe ammonium de la ptnicillamine est de 1,7 fois plus ClevCe que celle de la cystiine. On rapporte les constantes macroscopique et microscopique de dissociation acide pour chaque disulfure mixte. On discute des avantages de la rmn en tant que mtthode de caracttrisation de la chimie acide-base des disulfures mixtes.[Traduit par le journal] Introduction Thiols are oxidized by disulfides in two steps:

The thiol/disulfide exchange reactions of penicillamine (PSH) with cystine and several related disulfides (RSSR) have been studied by 1H nmr. The reactions take place in two steps:[Formula: see text]The equilibria and kinetics of the reactions of PSH with cystine were characterized over the pH range 5–8, while the reactions with the disulfides of cysteamine, homocysteine, 2-mercaptoethanol, mercaptoacetic acid, 3-mercaptopropionic acid, and mercaptosuccinic acid were studied at neutral pH. From the pH dependence of the rate of the reaction of PSH with cystine, the reactive species are identified as penicillamine with its amino group protonated and its thiol group deprotonated and cystine and penicillamine–cysteine mixed disulfide with their amino groups protonated. For all the disulfides studied, the extent to which the first reaction occurs is within a factor of 2–3 of that predicted by a random distribution, while the extent to which the second reaction occurs is considerably less than for a random distribution. This is attributed to steric effects due to the two methyl groups next to the sulfur of penicillamine.

Proton and 31P magnetic resonance experiments were employed in the study of the weak, stoichiometric, one-to-one complex that is formed between coenzyme A and manganese-(II). The distances in solution between the three phosphorus atoms and the manganese(II) ion are obtained at 287 K and at 298 K. The experiments are carried out over a pH range of 6.3 to 7.7. The correlation time τc of the water molecules bound to the Mn(II) ion in the CoA–Mn(II) complex was calculated using the Solomon–Bloembergen equation from the T1M value obtained for the water protons for solutions where most of the Mn(II) ions are complexed. Paramagnetic spin-lattice relaxation rates for all three phosphorus atoms in the CoA molecule were obtained using proton decoupled Fourier transform 31P nmr techniques. For most probable values of two or three water molecules in the CoA–Mn(II) complex, the distances between the manganese (II) ion and the three phosphorus nuclei are 4.0 ± 0.4 Å (1 Å = 0.1 nm) or 3.7 ± 0.4 Å respectively for 0.062 M CoA solutions. For experiments carried out on 0.010 M CoA, a correlation time which has been determined previously for propionyl CoA was used for the distance calculations and the distances remain unchanged within experimental error. These results indicate that the Mn(II) ion coordinates equally with the oxygens of the three phosphate groups in the CoA–Mn(II) complex.