Vibrational Dynamics, Mode Coupling, and Structural Constraints for Acetylproline-NH2

Abstract

Vibrational relaxation, mode coupling, and peptide structural constraints were obtained by femtosecond two-dimensional vibrational spectroscopy of an acetylproline-NH2 model dipeptide. Three high-frequency vibrational modes were examined, two amide-I modes (1693 and 1633 cm-1) and the amide-II amino end 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 ∼ 3.1 ps), which accounts for ∼36% of all deactivation pathways of the excited amide-I mode. The evolution of the transient spectral shape (cross-peak) as a result of vibrational energy transfer has been observed for the first time. The ET from the amide-I (acetyl end) band to the amide-II (amino end) band was detected (∼10 ps). However, no clear energy transfer between two amide-I bands was seen, yielding an upper limit for the rate constant of (20 ps)-1. A switch of the transient signal from a transition of the pumped state to an overtone (diagonal peak) to one involving a combination band (off-diagonal peak) was clearly demonstrated. This process provides an explanation for the nonexponential decay of the transient spectrum of each of the oscillators. A long-lived signal (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. The decay time (∼10 ps) of this signal represents one signature of the vibrational cooling of the dipeptide by energy transfer to the solvent. The angles between the transition dipoles of these modes given by the polarization measurements were close to those of a C7 conformation. The magnitudes of the coupling between the three vibrational modes were determined from a transition charge model of the off diagonal anharmonicity. The values obtained for the amide-I/amide-I and the amide-I/amide-II interactions are about twice those calculated from a dipole−dipole interaction potential based on a C7 conformation.