A new angle on heat capacity changes in hydrophobic solvation.
The differential solubility of polar and apolar groups in water is important for the self-assembly of globular proteins, lipid membranes, nucleic acids, and other specific biological structures through hydrophobic and hydrophilic effects. The increase in water's heat capacity upon hydration of apolar compounds is one signature of the hydrophobic effect and differentiates it from the hydration of polar compounds, which cause a decrease in heat capacity. Water structuring around apolar and polar groups is an important factor in their differential solubility and heat capacity effects. Here, it is shown that joint radial/angular distribution functions of water obtained from simulations reveal quite different hydration structures around polar and apolar groups: polar and apolar groups have a deficit or excess, respectively, of "low angle hydrogen bonds". Low angle hydrogen bonds have a larger energy fluctuation than high angle bonds, and analysis of these differences provides a physical reason for the opposite changes in heat capacity and new insight into water structure around solutes and the hydrophobic effect.
Gallagher
Kelly R
KR
Johnson Research Foundation and Department of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA.
Sharp
Kim A
KA
D003198:Computer Simulation; D006860:Hydrogen Bonding; D057927:Hydrophobic and Hydrophilic Interactions; D008563:Membrane Lipids / Q000737:chemistry; D009696:Nucleic Acids / Q000737:chemistry; D011506:Proteins / Q000737:chemistry; D012995:Solubility; D013816:Thermodynamics; D014867:Water / Q000737:chemistry*
D008563:Membrane Lipids; D009696:Nucleic Acids; D011506:Proteins; D014867:Water