Water in the Half Shell: Structure of Water, Focusing on Angular Structure and Solvation

摘要

Water is a highly polar molecule, consisting of a verynelectronegative atom, oxygen, bonded to two weaklynelectropositive hydrogen atoms with two lone pairs of elec-ntrons. These features give water remarkable physical prop-nerties, some of which are anomalous, such as its lowerndensity in the solid phase compared with the liquid phase.nIts ability to serve as both a hydrogen bond donor andnhydrogen bond acceptor governs its role as a solvent, a rolenthat is of central interest for biological chemists.nIn this Account, we focus on water’s properties as a sol-nvent. Water dissolves a vast range of solutes with solubili-nties that range over 10 orders of magnitude. Differences innsolubility define the fundamental dichotomy between polar,nor hydrophilic, solutes and apolar, or hydrophobic, solutes.nThis important distinction plays a large part in the struc-nture, stability, and function of biological macromolecules. Thenstrength of hydrogen bonding depends on the H O···OnH-bond angle, and the angular distribution is bimodal. Changes in the width and frequency of infrared spectral lines andnin the heat capacity of the solution provide a measure of the changes in the strength and distribution of angles of the hydro-ngen bonds. Polar solutes and inorganic ions increase the population of bent hydrogen bonds at the expense of the morenlinear population, while apolar solutes or groups have the opposite effect.nWe examine how protein denaturants might alter the solvation behavior of water. Urea has very little effect on water’snhydrogen bond network, while guanidinium ions promote more linear hydrogen bonds. These results point to fundamen-ntal differences in the protein denaturation mechanisms of these molecules. We also suggest a mechanism of action for anti-nfreeze (or thermal hysteresis) proteins: ordering of water around the surface of these proteins prior to freezing appears toninterfere with ice formation.