Ultrafast energy flow in model biological membranes
We report on the energy flow dynamics in model membranes, investigated by surface-specific time-resolved (femtosecond) sum frequency generation spectroscopy. This recently developed technique allows us to probe energy dynamics selectively at the water/lipid interface. We report vibrational relaxation dynamics of CÂ–H stretch modes in the lipid alkyl chains, and reveal that incoherent energy transfer occurs from the excited CH2 groups to the terminal CH3 groups. We also find evidence for strong anharmonic coupling between different CH2 and CH3 modes. Relaxation and the energy transfer processes within the lipid alkyl chain occur on (sub-)picosecond timescales. Studies of the dynamics on different lipid phases (gel or liquid crystalline phase) reveal a marked independence of the dynamics on the precise molecular conformation of the lipids. In addition, we report the energy transfer dynamics between membrane-bound water and lipids, and find that the transfer of heat between water and lipids occurs remarkably fast: heat is transferred across the monolayer, from the polar head group region of the lipid to the end of the alkyl chain, within 1 ps. These results demonstrate the potential of using ultrafast surface-specific spectroscopies to elucidate biomolecular dynamics at membrane surfaces.