Differential stability of the triple helix of (Pro-Pro-Gly)10 in H2O and D2O:: Thermodynamic and structural explanations

(Pro-Pro-Gly)(10) [(PPG(10))], a collagen-like polypeptide, forms a triple-helical, polyproline-II structure in aqueous solution at temperatures somewhat lower than physiological, with a melting temperature of 24.5 degrees C. In this article, we present circular dichroism spectra that demonstrate an increase of the melting temperature with the addition of increasing amounts of D2O to an H2O solution of (PPG)(10), with the melting temperature reaching 40 degrees C in pure D2O. A thermodynamic analysis of the data demonstrates that this result is due to an increasing enthalphy of unfolding in D2O vs. H2O. To provide a theoretical explanation for this result, we have used a model for hydration of (PPG)(10) that we developed previously, in which inter-chain water bridges are formed between sterically crowded waters and peptide bond carbonyls. Energy minimizations were performed upon this model using hydrogen bond parameters for water, and altered hydrogen bond parameters that reproduced the differences in carbonyl oxygen-water oxygen distances found in small-molecule crystal structures containing oxygen-oxygen hydrogen bonds between organic molecules and H2O or D2O. It was found that using hydrogen bond parameters that reproduced the distance typical of hydrogen bonds to D2O resulted in a significant lowering of the potential energy of hydrated (PPG)(10). This lowering of the energy involved energetic terms that were only indirectly related to the altered hydrogen bond parameters, and were therefore not artifactual; the intra-(PPG(10)) energy, plus the water-(PPG(10)) van der Waals energy (not including hydrogen bond interactions), were lowered enough to qualitatively account for the lower enthalpy of the triple-helical conformation, relative to the unfolded state, in D2O vs. H2O. This result indicates that the geometry of the carbonyl-D2O hydrogen bonds allows formation of good hydrogen bonds without making as much of an energetic sacrifice from other factors as in the case of hydration by H2O.