Abstract

Vibrational relaxation, mode coupling and peptide structure were investigated by femtosecond two-dimensional infrared spectroscopy of an acetylproline-NH₂ model dipeptide. Three high frequency vibrational modes were examined, two amide-I modes (1693 and 1633 cm⁻¹) and an amide-II amine end (denoted as (N-)) mode (1580 cm⁻¹). Efficient vibrational energy transfer between amide-I and amide-II bands located on the same amide has been clearly observed (τ~2.8 ps). The change in the nature of the cross peaks from off-diagonal to diagonal coupling types, caused by vibrational energy transfer (EnT), has been analyzed for the first time. The energy transfer process from the amide-I band acetyl end (C-) to the amide-II band has been also detected (~10 ps). No clear EnT between two amide-I bands was seen, giving an estimate of the upper limit of 20 ps for the EnT. An evolution of the diagonal spectrum was clearly demonstrated, giving a reason for the nonexponential decay kinetics observed in conventional pump-probe experiments. A long lived cross-peak was detected in each of the amide-I bands when the other amide-I band was excited, indicating the existence of at least one vibration in the molecule which remains excited for much longer than the average lifetime of the three vibrations being studied. Its decay time (~10 ps) represents the vibrational cooling of the dipeptide by energy transfer to the solvent. The coupling strength between the three vibrational modes was determined. The angles between the transition dipoles of these modes given by the polarization measurements were close to those of a C₇ conformation.

© 2002 Optical Society of America