Dynamical Crossover of Interfacial Water upon Melting of a Lipid Bilayer: Influence of Different Parts of the Headgroups.

All-atom molecular dynamics simulations of a 1,2-dimyristoyl- sn -glycero-3-phosphocholine bilayer in contact with liquid water were performed at different temperatures ranging from 285 to 320 K. We have investigated the heterogeneity and dynamical transitions in interfacial water as the lipid bilayer undergoes a melting transition. Results are obtained for water at the outer surface of the bilayer and for those buried more deeply in the lipid chains of the bilayer. It is found that lipid bilayer melting influences both the structure and dynamics of interfacial water. The number of interfacial water molecules shows a jump in the melting of the bilayer. The temperature dependence of the diffusivity and orientational relaxation of interfacial water molecules exhibits a dynamical crossover upon melting of the bilayer. The extent of dynamical crossover is found to be rather strong with significant changes in activation barriers for interfacial water around the carbonyl groups, which are deeply buried toward the lipid chains of the bilayer. The dynamical crossover gradually decreases as one moves further away from the outer surface, and it essentially vanishes for water in the region of 5-10 Å from the outer surface. It is found that the lipid melting-induced dynamical crossover of interfacial water is significant only for water that is in close proximity to the bilayer surface or deeply buried into it. The current results reveal that water molecules in different parts of the interface respond differently on melting of the bilayer. The current study also shows that the carbonyl-bound water molecules can play an important role in the phase transition of the bilayer as the temperature is raised through its melting point.