Gough C A, Bhatnagar R S
Laboratory of Connective Tissue Biochemistry, University of California, San Francisco 94143-0424, USA.
J Biomol Struct Dyn. 1999 Dec;17(3):481-91. doi: 10.1080/07391102.1999.10508379.
(Pro-Pro-Gly)10 [(PPG10)], 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 enthalpy 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-(PPG10) energy, plus the water-(PPG10) 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.
(脯氨酸-脯氨酸-甘氨酸)10 [(PPG10)]，一种类胶原蛋白多肽，在略低于生理温度的水溶液中形成三螺旋的聚脯氨酸II结构，其解链温度为24.5摄氏度。在本文中，我们展示了圆二色光谱，该光谱表明在(PPG)10的H2O溶液中加入越来越多的D2O时，解链温度会升高，在纯D2O中解链温度达到40摄氏度。对数据的热力学分析表明，这一结果是由于在D2O中与H2O相比，解链焓增加。为了对这一结果提供理论解释，我们使用了之前开发的(PPG)10水合模型，其中在空间拥挤的水分子和肽键羰基之间形成链间水桥。使用水的氢键参数以及改变后的氢键参数对该模型进行能量最小化，改变后的氢键参数再现了在含有有机分子与H2O或D2O之间氧-氧氢键的小分子晶体结构中发现的羰基氧-水氧距离的差异。结果发现，使用再现与D2O典型氢键距离的氢键参数会导致水合(PPG)10的势能显著降低。这种能量降低涉及的能量项仅与改变后的氢键参数间接相关，因此不是人为造成的；(PPG10)内部能量加上水-(PPG10)范德华能量(不包括氢键相互作用)降低到足以定性解释相对于未折叠状态，D2O与H2O中三螺旋构象较低的焓。这一结果表明，羰基-D2O氢键的几何结构允许形成良好的氢键，而不会像H2O水合那样在能量上因其他因素而有很大牺牲。
