INTERPRETATION OF THE REDUCTION POTENTIAL OF 6,6'-DITHIOSUCROSE CYCLIC DISULFIDE BY COMPARISON OF THE CONFORMATIONS OF 6,6'-DITHIOSUCROSE CYCLIC DISULFIDE, 6,6'-DITHIOSUCROSE, AND SUCROSE IN AQUEOUS-SOLUTION

6,6'-Dithiosucrose (sucrose dithiol, Suc(SH)2) is a weakly reducing species. The equilibrium constant for reduction of mercaptoethanol disulfide, ME(ox), by sucrose dithiol in aqueous solution is K(eq) = 0.3 M = {[SucS2][Me(red)]2}/{[Suc(SH)2][ME(ox)]}, where SucS2 (6,6'-dithiosucrose cyclic disulfide, sucrose disulfide) is the cyclic disulfide formed from the oxidation of sucrose dithiol. Measurements of values of T1 and J using H-1 NMR spectroscopy indicate that sucrose and sucrose dithiol adopt indistinguishable conformations in water. The conformations of sucrose dithiol and sucrose disulfide are similar but distinguishable. Molecular mechanics calculations indicate that two of the possible structures of sucrose disulfide have relatively low energies. Comparison of coupling constants calculated for these structures with experimental coupling constants from H-1 NMR spectra indicated that one of the two structures was more probable; this more probable structure had the conformation more similar to that of sucrose. The reduction potential of the Suc(SH)2/SucS2 (an 11-membered ring) couple is similar to that of an n-alkane-1,n-dithiol/cyclic disulfide (n = 5 or 6) couple; the value of EC (effective concentration = K(eq) = 0.3 M) characterizing oxidation of the two thiols of Suc(SH)2 to a disulfide is similar to that for oxidation of the two thiols of hexane-1,6-dithiol (K(eq) = 0.2 M) and pentane- 1,5-dithiol (K(eq) = 3.6 M). The similarity of these values of EC suggests that SucS2 is a relatively strain-free structure and reflects the proximity of the two thiol groups in this carbohydrate.