Anisotropic waters in atomic resolution protein crystal structures

Abstract

In protein crystal structures at extremely high resolution, B-factors of water oxygen atoms can be refined anisotropically. Here, some properties and trends associated with the anisotropy of the water oxygen B-factors are presented and commented. Anisotropy, defined as the ratio of the highest and the smallest eigenvalue of the anisotropic B-factor, is very variable and its distribution can be described by a Gumpel function, which slightly depends on the equivalent isotropic B-factor. Moreover, water oxygen atom anisotropies are very similar in the first and in the second hydration layers; are nearly independent of the number of hydrogen bonds that are formed by the water molecule; and are weekly correlated with protein atom anisotropy. This suggests that hydration water molecules might be in a physicochemical state intermediate between the liquid state, which is present and abundant in the crystal, and the solid state, which is assumed by protein. Further studies and analyses are apparently necessary to investigate this hypothesis.