Probing the replacement of water by dimethyl sulfoxide in the hydration shell of N-methylacetamide by FTIR-spectroscopy

The influence of dimethyl sulfoxide (DMSO) on the hydration of a peptide group has been explored by measuring the FTIR spectrum of N-methylacetamide in different mixtures of DMSO and D2O. An analysis of the amide I' profile indicates two processes. For DMSO mole fractions between 0 and 0.5, the hydration shell on the side of the carbonyl group is systematically perturbed, which leads to a destabilization and eventual elimination of the hydrogen bonding between the carbonyl group and D2O. Upon further increasing the mole fraction of DMSO, water on the amide group side of the peptide is completely replaced by DMSO and the NH (sic) ND exchange is significantly impeded. In mixtures with a very high fraction of DMSO, NMA with and without or weak hydrogen bonding to a DMSO molecule coexist with the latter configuration being clearly dominant in pure DMSO. Our results corroborate the recently debated notion that DMSO functions as a chaotropic cosolvent even at low fractions in that it exhibits preferential binding of water to the solute and by perturbing the stability of the peptide-water entity. They also reveal a rather fast exchange between different peptide-solvent complexes at low DMSO concentrations. (C) 2017 Elsevier B.V. All rights reserved.