Disulfide linkage Raman markers: a reconsideration attempt

During the last decades, Raman spectroscopy has been routinely used for probing the conformational features of disulfide linkages in peptides and proteins. However, the interpretation of disulfide Raman markers is currently performed by a simple rule derived from the earliest observations on dialkyl disulfides. More precisely, this rule consists of the following: (1) in analyzing the Raman bands in the 550-500 cm(-1) region ascribed to disulfide bond stretch motion, namely, nu(S-S), and (2) assigning the three types of Raman markers observed at similar to 500, similar to 520, and similar to 540 cm(-1) to three families of rotamers defined along the three successive bonds of the -C-S-S-C-moiety, referred to as ggg, ggt, and tgt. In this report, we attempt to show that an accurate analysis of disulfide vibrational features needs the use of the five torsion angles (chi(1), chi(2), chi(3), chi(2)', and chi(1)') along the five successive bonds joining the two alpha-carbon atoms in the cystine (Cys-Cys) unit. The present work is inspired by the disulfide conformational investigations performed by a statistical scan of numerous protein crystal and nuclear magnetic resonance data, taking into account the handedness (right and left) of a disulfide bridge, its spatial shape (Staple, Hook, and Spiral), as well as the signs of the two extreme torsion angles chi(1) and chi(1)'. It appears that the combined use of the old and recent conformational notations allows a more accurate structural and vibrational analysis of disulfide linkage. Copyright (C) 2014 John Wiley & Sons, Ltd.