The hydration of globular proteins as derived from volume and compressibility measurements: Cross correlating thermodynamic and structural data

We report the first thermodynamic characterization of protein hydration that does not depend on model compound data but rather is based exclusively on macroscopic (volumetric) and microscopic (X-ray) measurements on protein molecules themselves. By combining these macroscopic and microscopic characterizations, we describe a quantitative model that allows one for the first time to predict the partial specific volumes, v°, and the partial specific adiabatic compressibilities, k°(s), of globular proteins from the crystallographic coordinates of the constituent atoms, without using data derived from studies on low-molecular-mass model compounds. Specifically, we have used acoustic and densimetric techniques to determine v° and k°(s) for 15 globular proteins over a temperature range from 18 to 55°C. For the subset of the 12 proteins with known three-dimensional structures, we calculated the molecular volumes as well as the solvent-accessible surface areas of the constituent charged, polar and nonpolar atomic groups. By combining these measured and calculated properties and applying linear regression analysis, we determined, as a function of temperature, the average hydration contributions to v° and k°(s) of 1 Ų of the charged, polar, and nonpolar solvent-accessible protein surfaces. We compared these results with those derived from studies on low-molecular-mass compounds to assess the validity of existing models of protein hydration based on small molecule data. This comparison revealed the following features: the hydration contributions to v° and k°(s) of charged protein surface groups are similar to those of charged groups in small organic molecules. By contrast, the hydration contributions to v° and k°(s) of polar protein surface groups are qualitatively different from those of polar groups in low-molecular-mass compounds. We suggest that this disparity may reflect the presence of networks of water molecules adjacent to polar protein surface areas, with these networks involving waters from second and third coordination spheres. For nonpolar protein surface groups, we find the ability of low-molecular-mass compounds to model successfully protein properties depends on the temperature domain being examined. Specifically, at room temperatures and below, the hydration contribution to k°(s) of protein nonpolar surface atomic groups is close to that of nonpolar groups in small organic molecules. By contrast, at higher temperatures, the hydration contribution to k°(s) of protein nonpolar surface groups becomes more negative than that of nonpolar groups in small organic molecules. We suggest that this behaviour may reflect nonpolar groups on protein surfaces being hydrated independently at low temperatures, while at higher temperatures some of the solvating waters become influenced by neighboring polar groups. We discuss the implications of our aggregate results in terms of various approaches currently being used to describe the hydration properties of globular proteins, particularly focusing on the limitations of existing additive models based on small molecule data.