Ultrafast Vibrational Dynamics of Membrane-Bound Peptides at the Lipid Bilayer/Water Interface.

Vibrational energy transfer (VET) of proteins at cell membrane plays critical roles in controlling the protein functionalities, but its detection is very challenging. By using a surface-sensitive femtosecond time-resolved sum-frequency generation vibrational spectroscopy with infrared pump, the detection of the ultrafast VET in proteins at cell membrane has finally become possible. The vibrational relaxation time of the N-H groups is determined to be 1.70(±0.05) ps for the α-helix located in the hydrophobic core of the lipid bilayer and 0.9(±0.05) ps for the membrane-bound β-sheet structure. The N-H groups with strong hydrogen bonding gain faster relaxation time. By pumping the amide A band and probing amide I band, the vibrational relaxation from N-H mode to C=O mode through two pathways (direct coupling and through intermediate states) is revealed. The ratio of the pathways depends on the NH⋅⋅⋅O=C hydrogen-bonding strength. Strong hydrogen bonding favors the coupling through intermediate states.