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Structure-Reactivity Relations for Thiol-Disulfide Interchange Janette Houk and George M

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Structure-Reactivity Relations for Thiol-Disulfide Interchange Janette Houk and George M J. Am. Chem. SOC.1987, 109, 6825-6836 6825 ‘H NMR (22 OC, benzene-d6): 6 3.26 (s, OSCMe), 3.03 (s, 2JwH= A systematic search of a limited hemisphere of reciprocal space lo- 8.54 Hz, CH2CMe3next to S), 2.85 (s, ’JWH= 9.98 Hz, CH2CMe3next cated a set of diffraction maxima with systematic absences corresponding to Oh 2.32 (s. O,CMe), 2.26 (s,.- 0,CMe trans to OSCMe) 1.49, 1.46 (s, to the unique monoclinic space group P2,/a. Subsequent solution and CH2CMe3): refinement confirmed this choice. IR: 324 (m), 333 (m), 375 (w), 450 (w), 620 (w), 627 (w), 668 (ms), The structure was solved by a combination of direct methods 680 (m). 907 (mw), 932 (w). 1018 (w), 1040 (w), 1088 (vs), 1175 (ms), (MULTAN~S)and Fourier techniques and refined by full-matrix least 1202(mw), 1236 (ms), 1349 (m), 1361 (ms), 1374 (ms), 1438 (vs), 1460 squares. Many of the hydrogen atom positions were visible in a difference (s), 1492 (m) cm-I. Fourier phased on the non-hydrogen parameters. The positions of all W2(np)2(02CNMe2)4.In a Schlenk flask W2(np)2(NMe2)4(0.30 g, hydrogens were calculated and placed in fixed idealized positions (d(C- 0.44 mmol) was dissolved in CH2C12and then frozen in liquid N,. A H) = 0.95 A) for the final cycles. The hydrogen atoms were assigned large excess (>4 equiv) of C02was condensed into the flask. The so- a thermal parameter of 1 + Bise of the carbon atom to which they were lution was then slowly warmed to room temperature while the flask was bound. The data were corrected for absorption for the final cycles of the connected to a Hg bubbler. The solvent was removed in vacuo to yield refinement. a yellow powder (0.35 g, 92%). A final difference Fourier was essentially featureless, with the largest ‘H NMR (22 OC, benzene-d6): 6 2.66 (s, bridge 02CNMe2),2.59 (s, peak being 0.50 e/A3. chelate 02CNMe2),2.46 (s, CH2CMe,), 1.23 (s, CH2CMe3). IR: 212 (mw), 256 (m), 421 (m), 455 (m), 600 (mw), 615 (m), 651 Acknowledgment. We thank Dr. D. M. Hoffman of Harvard (s), 669 (m), 696 (vw),730 (ms), 748 (m), 769 (ms), 773 (ms), 778 (ms), University for helpful discussions and the National Science 845 (mw), 872 (m), 995 (w), 1042 (m), 1060 (mw), 1091 (w), 1108 (vw), Foundation and the Wrubel Computing Center for support. 1150 (vw), 1227 (m), 1268 (s, br), 1359 (m), 1410 (vs), 1500 (vs), 1570 D.L.C. gratefully acknowledges the support of a General Electric (vw), 1610 (vs) cm-I. Foundation fellowship for 1985-1986. W,Bz,(O,CNMe,)d. In a similar procedure a large excess of C02was reacted with W,Bz;(NMe,),. Registry No. W2(np),(02CMe),, 108603-67-4; W,(II~)~(O~CM~),- ‘H NMR (22 OC. benzene-dr): 6 3.79 (s, CH,Ph), 2.67, 2.28 S, (S2CNEt2)2,110097-42-2; W2(np),(O2CMe),(0SCMe), 110116-42-2; 02CNMe2). w,(r~p)~(NMe~)~,72286-69-2; W2(np),(O2CNMe2),, 72286-53-4; Crystallographic Studies. General operating procedures and listings W2B~2(NMe2)4r82555-52-0; W2Bz2(02CNMe2),,84913-56-4; W2(C- of programs have been previously published.23 Crystal data for W2- H,),(O,CH),, 91549-49-4; W2(CH3)2(02CNH2)4, 110097-43-3; (np)2(02CMe)2(S2CNEt2)2are given in Table VI. A suitable crystal NaS2CNEt2, 148-18-5; thioacetic acid, 507-09-5. was transferred to the goniostat by standard inert-atmosphere handling techniques employed by the IUMSC and cooled to -160 OC with a Supplementary Material Available: Anisotropic thermal pa- gas-flow cooling system. rameters and a complete listing of bond distances and bond angles for the Wz(np)2(02CMe)z(SzCNEtz)zmolecule (4 pages); Fo and (23) Chisholm, M. H.; Folting, K.; Huffman, J. C.; Kirkpatrick, C. C. F, values for the same compound (10 pages). Ordering infor- Inorg. Chem. 1984, 23, 1021. mation is given on any current masthead page. Structure-Reactivity Relations for Thiol-Disulfide Interchange Janette Houk and George M. Whitesides* Contribution from the Departments of Chemistry, Harvard University, Cambridge, Massachusetts 021 38, and Massachusetts Institute of Technology, Cambridge, Massachusetts 021 39. Received February 5, 1987 Abstract: Equilibrium constants were determined for thiol-disulfide interchange between 36 di- and trithiols and the disulfides derived from either 2-mercaptoethanol or dithiothreitol. Reactions were conducted in methanol-d,/aqueous buffer (pH 7) or methanold, at 25 “C,using NMR spectroscopy to follow the reactions. These data were used to rank the dithiols in terms of reduction potential and to infer the structure of the disulfides formed from them on oxidation. There is a general correlation between the reducing ability of the dithiol and the size of the disulfide-containing ring formed on oxidation: dithiols that form six-membered rings are most strongly reducing (K = 103-105 M with respect to oxidized 2-mercaptoethanol); five- and seven-membered rings are approximately 1 order of magnitude less reducing. Compounds resembling 1,2-ethanedithiol form cyclic bis(disu1fide) dimers in relatively dilute solutions (- 1 mM) but polymerize at higher concentrations. Other classes of dithiols form polymers on oxidation. The importance of the thiol-disulfide interchange reaction to ization of the thiol to thiolate anion, nucleophilic attack of the biochemistryz-8 and the remarkable ability of this reaction to effect thiolate anion on a sulfur atom of the disulfide moiety, and the reversible cleavage and formation of strong, covalent S-S bonds at room temperature in aqueous soluti~n~-~have prompted many (1) Supported by the National Institutes of Health, Grant GM 34411. studies of the physical-organic chemistry of this reaction.’&17 (2) Friedman, M. The Chemistry and Biochemistry of the Sulfhydryl These studies have established the reaction to be mechanistically Group in Amino Acids, Peptides and Proteins, 1st ed.; Pergamon: New York, simple. Interchange involves three steps (eq la-c): initial ion- 1973. (3) Jocelyn, P. C. Biochemistry ofrhe SH Group; Academic: New York, PX. 1972... - RSH + RS- (la) (4) Torchinskii, Yu. M. Sulfhydryl and Disuljide Groups of Proteins; Plenum: New York, 1974. RS->SrL\CR” RS-S + -SR“ (1b) (5) Fluharty, A. L. In The Chemistry of the Thiol Group; Patai, S., Ed.; Wiley: New York, 1974; p 589. R‘ R’ (6) Arias, I. M.; Jacoby, W. G. Glutathione, Metabolism and Function; Raven: New York, 1976.- (7) Kosower, N. S.;Kosower, E. M. In Free Radicals in Biology; Pryor, W. A,, Ed.; Academic: New York, 1976; Vol. 11, Chapter 2. (8) Lin, T.-Y. In The Proteim, 3rd ed.; Neurath, H., Ed.; Academic: New *To whom correspondence should be addressed at Harvard University. York, 1977; Vol. 111. Gilbert, H. F. Methods Enzymol. 1984, 107, 330. 0002-7863/87/1509-6825$01.50/0 0 1987 American Chemical Society 6826 J. Am. Chem. SOC.,Vol. 109, No. 22, 1987 Houk and Whitesides oligomeric disulfide formation becomes competitive with cyclic monomer formation. The reducing potentials of dithiols that form oligomeric products are similar to those for monothiols. The observation that a solution of BAL (2,3-dimercapto-1- propanol) is much more strongly reducing than a solution of a monothiol containing an equal concentration of thiol groups has led to the conclusion that BAL forms a cyclic bis(disulfide)." The initial formation of the half-oxidized dimer of BAL requires an intermolecular step (eq 3), but the second disulfide bond is formed R'SH + A'SS-R-SSR' mixed disulfide (3) in a process that forms an eight-membered ring by an intramo- lr lecular process. This reaction is concentration sensitive: At low HS-R-SH { S-R-S cyclic monomer (2) concentration, the dimer forms; at high concentration the product W is polymeric." Beyond these correlations of equilibrium constant with ring size for cyclic monomeric dithiols, the factors that influence the re- S-R-s cyclic dimer duction potentials of dithiols are not known. We have studied these factors and report our results in this paper. We consider this study in part a problem in molecular design: Can we design dithiols that will form cyclic monomers, dimers, or polymers I exclusively on oxidation under equilibrating conditions? What ~s-R-s+,,II oligomers structural features must be built into a dithiol to make it strongly reducing, and can a structural parameter or parameters be identified that will enable us to predict the reducing ability of a nature of R and the concentrations of the dithiol and disulfide. dithiol based on its structure? We are most interested in reactions Cyclic mmomeric disulfides are the major products when the thiol that form thermodynamically stable cyclic bis(disulfides), because groups are separated by three to six atoms. These thermody- these materials could form the basis for reversible coupling agents namically stable cyclic monomers are strongly reducing relative for use in biochemical systems and because they pose a more to monothiols, reflecting the high effective concentration22-26of demanding problem for molecular design than do monomeric or thiol groups in the intramolecular thiol-disulfide interchange step.27 polymeric disulfides. Qualitatively, we expect to be able to favor Their relative reducing potential is sensitive to the spacing between formation of cyclic bis(disu1fides) by constraining the starting the thiols: 1,4-alkanedithiols that form strain-free six-membered dithiols to geometries resembling those of the bis(disu1fides) and 1,2-dithianes on oxidation are most strongly reducing. Rings larger by eliminating as many rotational degrees of freedom in the dithiols than six members are less favored, presumably primarily for as possible.
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