Hydration and hydrogen bonding of carbonyls in dimyristoyl-phosphatidylcholine bilayer

We combine two-color ultrafast infrared spectroscopy and molecular dynamics simulation to investigate the hydration of carbonyl moieties in a dimyristoyl-phosphatidylcholine bilayer. Excitation with femtosecond infrared pulses of the OD stretching mode of heavy water produces a time dependent change of the absorption band of the phospholipid carbonyl groups. This intermolecular vibrational coupling affects the entire C=O band, thus suggesting that the optical inhomogeneity of the infrared response of carbonyl in phospholipid membranes cannot be attributed to the variance in hydration. Both the experimental and the theoretical results demonstrate that sn-1 carbonyl has a higher propensity to form hydrogen bonds with water in comparison to sn-2. The time-resolved experiment allows following the evolution of the system from a nonequilibrium localization of energy in the OD stretching mode to a thermally equilibrated condition and provides the characteristic time constants of the process. The approach opens a new opportunity for investigation of intermolecular structural relations in complex systems, like membranes, polymers, proteins, and glasses.