Osmolyte-induced perturbations of hydrogen bonding between hydration layer waters: Correlation with protein conformational changes

Gadolinium vibronic sideband luminescence spectroscopy (GVSBLS) is used to probe osmolyte-induced changes in the hydrogen bond strength between first and second shell waters on the surface of free Gd³⁺ and Gd³⁺ coordinated to EDTA and to structured calcium binding peptides in solution. In parallel, Raman is used to probe the corresponding impact of the same set of osmolytes on hydrogen bonding among waters in the bulk phase. Increasing concentration of added urea is observed to progressively weaken the hydrogen bonding within the hydration layer but has minimal observed impact on bulk water. In contrast, polyols are observed to enhance hydrogen bonding in both the hydration layer and the bulk with the amplitude being polyol dependent with trehalose being more effective than sucrose, glucose, or glycerol. The observed patterns indicate that the size and properties of the osmolyte as well as the local architecture of the specific surface site of hydration impact preferential exclusion effects and local hydrogen bond strength. Correlation of the vibronic spectra with CD measurements on the peptides as a function of added osmolytes shows an increase in secondary structure with added polyols and mat the progressive weakening of the hydrogen bonding upon addition of urea first increases water occupancy within the peptide and only subsequently does the peptide unfold. The results support models in which the initial steps in the unfolding process involve osmolyte-induced enhancement of water occupancy within the interior of the protein.