The effect of Hofmeister anions on water structure at protein surfaces

To understand the effects of specific ions on protein-water interactions and the thermodynamic stability of proteins in salt solutions, we use a molecular dynamics (MD) simulation to examine the water structure, orientational distribution, and dynamics near the surface of ubiquitin. In particular, we consider NaCl, NaBF₄, NaSCN, and NaClO₄ salt solutions containing ubiquitin, where the anions of the latter three salts are well-known chaotropic ions in the Hofmiester anion series. The number of hydrogen bonds (H-bonds) per water molecule is found to decrease significantly at the ubiquitin-water interface, indicating a significant disruption of the water H-bonding network. The distribution of the water H-bond numbers near the protein surface is modulated by dissolved ions, and the extent of the ion effect on the H-bonding network structure follows the order of the Hofmeister anion series, while there are no specific ion effects on water properties at distances larger than 5 Å from the protein surface. From detailed analyses of the surface area, volume, and root-mean-square deviation (RMSD) of ubiquitin, we show that changes in the properties of the protein could originate from the disruption of the water H-bond network induced by ions with a higher affinity for the protein surface instead of direct protein residue-ion interactions. An interesting observation made here is that the orientational distribution of water molecules at the protein-water interface is close to random, but there is a slight preference for interfacial water molecules with a straddle structure within 2.5 Å of the protein surface, where one of the two OH groups points away from the protein surface and the other points toward the surface. In addition, comparing the MD simulation results for ubiquitin solutions with dissolved NaSCN and KSCN, we show that Na⁺ affects the water H-bonding structure at the protein surface more than K⁺. It is clear that the H-bonding network structure of water more than one water layer away from the protein surface is not distinguishably different from that of neat water. We thus anticipate that the present work will provide insights into the scale of specific ion effects on the H-bonding structure and orientational distribution of water in the vicinity of protein surfaces in aqueous solutions.