title: On the behavior of water at subfreezing temperatures in a protein crystal: evidence of higher mobility than in bulk water.
abstract: NMR experiments have shown that water molecules in the crystal of the protein Crh are still mobile at temperatures well below 273 K. In order to investigate this water anomaly, a molecular dynamics (MD) simulation study of crystalline Crh was carried out to determine the mobility of water in this crystal. The simulations were carried out at three temperatures, 150, 200, and 291 K. Simulations of bulk water at these temperatures were also done to obtain the properties of the simple point charge (SPC) water model used at these temperatures and to allow a comparison of the properties of water in the Crh crystal with those of bulk water at the same temperatures. According to the simulations, water is immobilized at 150 K both in crystal and in bulk water. As expected, at 291 K it diffuses and rotates more slowly in the protein crystal than in bulk water. However, at 200 K, the translational and rotational mobility of the water molecules is larger in the crystal than in bulk water. The enhancement of water mobility in the crystal at 200 K was further investigated by MD simulations in which the backbone or all protein atoms were positionally restrained, and in which additionally the electrostatic protein-water interactions were removed. Of these changes in the environment of the water molecules, rigidifying the protein backbones slightly enhanced water diffusion, while it slowed down rotation. In contrast, removal of electrostatic protein-water interactions did not change water diffusion but enhanced rotational motion significantly. Further investigations are required to delineate particular features of the protein crystal that induce the anomalous behavior of water at 200 K.
authors.affiliation: Laboratory of Physical Chemistry, ETH Zürich , CH-8093 Zürich, Switzerland.
authors.forename: Dongqi
authors.initials: D
authors.lastname: Wang
authors.forename: Anja
authors.initials: A
authors.lastname: Böckmann
authors.forename: Jožica
authors.initials: J
authors.lastname: Dolenc
authors.forename: Beat H
authors.initials: BH
authors.lastname: Meier
authors.forename: Wilfred F
authors.initials: WF
authors.lastname: van Gunsteren
mesh_terms: D000465:Algorithms; D001426:Bacterial Proteins / Q000737:chemistry*; D003080:Cold Temperature; D018360:Crystallography, X-Ray; D004058:Diffusion; D005615:Freezing; D057927:Hydrophobic and Hydrophilic Interactions; D056004:Molecular Dynamics Simulation; D010100:Oxygen / Q000737:chemistry; D010750:Phosphoproteins / Q000737:chemistry*; D011312:Pressure; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D017382:Reactive Oxygen Species / Q000737:chemistry; D012399:Rotation; D055672:Static Electricity; D013696:Temperature; D055471:Thermal Diffusion; D013816:Thermodynamics; D013997:Time Factors; D014867:Water / Q000737:chemistry*
chemical_list: D001426:Bacterial Proteins; C118042:Crh protein, Bacillus subtilis; D010750:Phosphoproteins; D017382:Reactive Oxygen Species; D014867:Water; D010100:Oxygen