Stable conformations of aliphatic disulfides: influence of 1,4 interactions involving sulfur atoms.

Abstract

The present state of knowledge of the stable conformations of the XCCSSCCX fragment (in which X is any saturated group) of aliphatic disulfides is examined. In particular, the evidence for the existence of attractive 1,4 interactions between CH groups and S atoms across C-S bonds and their influence on the potential function for rotation about C-S bonds is discussed. The effects of similar attractive interactions between NH groups and S atoms on the potential function for rotation about C-C bonds also are considered. It is concluded that weak attractive interactions between CH groups and S atoms are capable of stabilizing rotamers with unusually low (about 30°) values of the SS-CC dihedral angle.In a recent series of papers (1-8) from this laboratory, various experimental and theoretical techniques were used to study the conformations of aliphatic disulfides structurally related to cystine. On the basis of these studies, it was suggested that there exist weak attractive interactions between CH groups and S atoms that are separated by three intervening single bonds. These were referred to as 1,4 carbon-sulfur interactions, and were thought to be responsible for the presence of rotamers about the C-S bonds of aliphatic disulfides with unusually low values (about 300) of the SS-CC dihedral angle. Such rotamers were referred to as A conformations, and were thought to comprise about 20% of all rotamers about C-S bonds. The existence of A conformations is unexpected on the basis of "classical" conformational analyses of saturated molecules in which heavy groups/atoms generally would be considered to be only gauche or trans (SS-CC dihedral angles of +600 and 1800, respectively) to each other. While the available experimental and theoretical evidence strongly supports the existence of A conformations, a direct proof of this hypothesis has been difficult to establish because of inherent experimental and theoretical limitations. We, therefore, thought it useful to examine briefly what is known about the conformation of aliphatic disulfides in general, devoting special attention to the evidence regarding the existence of 1,4 carbon-sulfur interactions and A conformations. The possible existence of attractive 1,4 interactions between S atoms and NH groups also will be considered. RESULTS AND DISCUSSIONThe conformation of the XCCSSCCX fragment will be considered, where X is any saturated group such as the amino group of cystine. The stable conformations of this unit, arising from rotations about the S-S, C-S, and C-C bonds, will be considered separately. and -0 are mirror images of one another and, hence, isoenergetic. Therefore, symmetric disulfides with values of X(CS-SC) of +85°and -85°correspond to different screw-senses of the same conformation (i.e., they are enantiomers). Rotation about S-S bondsRing closure or other structural constraints can cause the S-S bond to adopt values of Ix(CS-SC)I in the range of 0-60°or 110-180°. As Ix(CS-SC)I is reduced from near 850 to near 00,

Conformational Analysis of a Nitroxide Side Chain in an α-Helix with Density Functional Theory

“…A “1,4 interaction” between sulfur and CH group that is less directional than the C-H⋯S hydrogen bonds discussed by Weiss et al 67 but comparable to it in strength (1–2 kcal/mol) were previously reported. 66,68 …”

Structural determinants of nitroxide motion in spin‐labeled proteins: Tertiary contact and solvent‐inaccessible sites in helix G of T4 lysozyme

“…Other evidence for S δ ···HC α interactions exists in the literature. Van Wart and Scheraga (1977) first noted a potential S δ ···HC α interaction in aliphatic disulfides. Recent surveys of the protein database reveal a preponderance of the { m , m } configuration for the half cysteines of the disulfide in which S δ is close to the HC α (Dani et al 2003; Bhattacharyya et al 2004).…”

Vibrational and electronic optical activity of the chiral disulphide group: Implications for disulphide bridge conformation

“…17 Some correlation between the stretching vibrational frequency m(SÀ ÀS) and size of the dihedral angle CSSC was shown with various organic model compounds. [18][19][20][21] Correlation of stretching vibrations m(SÀ ÀS) and m(CÀ ÀS) protein/peptide conformation (F, w torsion angles) was studied theoretically by Krimm and coworkers (calculations were performed for the basic model of peptide/protein and for the torsion angle CSSC (v 3 ) 5908 ( Fig. 1) and are summarized in the literature.…”

“…using Raman spectroscopy by Scheraga and coworkers in the 1970s. 18,19,38 Our calculations place the SÀ ÀS stretching vibration at 457-473 cm 21 for H 2 S 2 (473 cm 21 for v SÀ ÀS 5 908, 458 cm 21 for v SÀ ÀS 5 08, and 464 cm 21 for v SÀ ÀS 5 1808) and at 428-467 cm 21 for (CH 3 ) 2 S 2 (467 cm 21 for v SÀ ÀS 5908, 428 cm 21 for v SÀ ÀS 5 08, and 471 cm 21 for v SÀ ÀS 51808) (see Table 2). However, the calculated trends in the spectra of H 2 S 2 and (CH 3 ) 2 S 2 are not internally consistent.…”