Geometrical Embedding Governs a Dramatic Variation of Electron Paramagnetic Resonance Hyperfine Coupling Constants of Disulfide Radical Anions

Abstract

Among the large variety of experimental techniques amenable to probe disulfide radical anions, electron paramagnetic resonance (EPR) spectroscopy provides the most definitive assignment of these versatile transient intermediates in biochemistry [Stubbe et al. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 8979-84; J. Am. Chem. Soc. 2009, 131, 200-211]. EPR parameters along both a series of 12 aliphatic 1,2-dithia-cycloalkane radical anions and a representative set of 18 short-loop peptides are investigated by means of density functional theory. While the g-tensor remains quasi-isotropic (with diagonal terms very close to 2.0, as expected for a σ* singly occupied orbital), we evidence a dramatic conformational dependence of isotropic sulfur hyperfine coupling constants (hcc). Potential energy surface exploration of the prototypical dimethyldisulfide rationalizes their 3-4-fold amplitude, with values ranging between 10 and 29 G for aliphatic moieties. Sulfur hcc's are readily decomposed into three geometrical components: intersulfur distance, dihedral, and valence angles, with the latter being predominant. Increasing (respectively decreasing) contribution of sulfur atomic s orbital to the σ* molecular orbital, with a concomitant higher (respectively weaker) density around the sulfur nuclei, can be monitored on Walsh diagrams along each degree of motion. In peptidic disulfide radical anionic systems, sulfur hcc's are dissymmetrized and span an even larger range of values, from 14 to 40 G. Again, dependence is governed by the mechanical embedding of the -CH(2)-S∴S-CH(2)- motif, this time with a noticeable contribution from the hemibond lenghtening and some punctual short-range additive electrostatic contributions. This analysis comes within the scope of a unified picture of both spectroscopy and reactivity of the mechanochemistry of disulfide hemibonds.