Local order and mobility of water molecules around ambivalent helices

Water on a protein surface plays a key role in determining the structure and dynamics of proteins. Compared to the properties of bulk water, many aspects of the structure and dynamics of the water surrounding the proteins are less understood. It is interesting therefore to explore how the properties of the water within the solvation shell around the peptide molecule depend on its specific secondary structure. In this work we investigate the orientational order and residence times of the water molecules to characterize the structure, energetics, and dynamics of the hydration shell water around ambivalent peptides. Ambivalent sequences are identical sequences which display multiple secondary structures in different proteins. Molecular dynamics simulations of representative proteins containing variable helix, variable nonhelix, and conserved helix are also used to explore the local structure and mobility of water molecules in their vicinity. The results, for the first time, depict a different water distribution pattern around the conserved and variable helices. The water molecules surrounding the helical segments in variable helices are found to possess a less locally ordered structure compared to those around their corresponding nonhelical counterparts and conserved helices. The long conserved helices exhibit extremely high local residence times compared to the helical conformations of the variable helices, whereas the residence times of the nonhelical conformations of the variable helices are comparable to those of the short conserved helices. This differential pattern of the structure and dynamics of water molecules in the vicinity of conserved/variable helices may lend valuable insights for understanding the role of solvent effects in determining sequence ambivalency and help in improving the accuracy of water models used in the simulations of proteins.