Observation of Vibrational Energy Exchange in a Type-III Antifreeze Protein

We performed time- and polarization-resolved pump–probe and two-dimensional infrared (2D-IR) experiments to study the dynamics of the amide I vibration of a 7 kDa type-III antifreeze protein. In the pump–probe experiments, we used femtosecond mid-infrared pulses to investigate the vibrational relaxation dynamics of the amide mode. The transient spectra show the presence of two spectral components that decay with different lifetimes, indicative of the presence of two distinct amide subbands. The 2D-IR experiments reveal the coupling between the two bands in the form of cross-peaks. On the basis of previous work by Demirdöven et al. ( J. Am. Chem. Soc. 2004, 126, 7981−7990), we assign the observed bands to the two infrared-active modes α(−) and α(+) found in protein β-sheets. The amplitudes of the cross-peak were found to increase with delay time, indicating that the cross-peaks originate from population transfer between the coupled amide oscillators. The time constant of the energy transfer was found to be 6–7 ps.