We have investigated the factors contributing to the stability in aqueous solution of the triple-helical conformation of (Pro-Pro-Gly)10 [(PPG)10], a simplified mimic of some features of the collagen triple helix. An understanding of the factors stabilizing this molecule may contribute to the design of intelligent polymers and biomaterials. This polypeptide forms a triple-stranded conformation similar to that of collagen. Unlike collagen, however, it has no hydroxyproline residues to form inter- or intra-chain bridging hydrogen bonds involving bound waters. However, we have constructed a model for bound waters in (PPG)10, in which bridging hydrogen bonds are formed between each bound water and a pair of backbone carbonyls on different polypeptide chains. These waters are considered bound, rather than waters of bulk solvation, because they are in sterically restricted regions which can only be accessed by waters with limited mobility and not by bulk solvent. We have also shown that the role of these sterically crowded bound waters in structure stabilization qualitatively explains the greater stability and higher melting point of (PPG)10 in deuterium oxide than in water This occurs due to slight changes in the inter-heavy atom distances in hydrogen bonds to bound D2O relative to bound H2O, which allow the structure to adopt a geometry somewhat more favorable for inter-proline van der Waals interactions while still maintaining optimal hydrogen-bonding geometry. MidasPlus and the facilities of the UCSF Computer Graphics Laboratory have been used for graphical modeling and visualization of the molecular geometry, for preparing (PPG)10 in the starting conformation for the theoretical calculations, for monitoring the structures after these calculations, and for preparing images for publication and poster presentation.
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