Vibrational Coupling between Amide-I and Amide-A Modes Revealed by Femtosecond Two Color Infrared Spectroscopy

Abstract

Two color femtosecond infrared pump/probe spectroscopy has been used to study the vibrational dynamics and vibrational mode coupling of amide-A and amide-I/II modes within the same amide unit and those connected by a hydrogen bond for several model dipeptides: AcAla(H)OMe, AcAla(D)OMe, and AcProNHMe. Three spectral ranges were explored as follows: around 3 μm (amide-A(H) mode), 4 μm (amide-A(D) mode), and 6−8 μm (amide-I/II modes). The lifetime of the excited amide-A mode in nonhydrogen-bonded amide is found to be strongly dependent upon deuteration: 4.0 ps in AcAla(H)OMe and 0.58 ps in AcAla(D)OMe. The diagonal anharmonicities are found to be ΔN-D = 110 cm-1 and ΔN-H = 144 cm-1, respectively, both much larger than the ca. 15 cm-1 for the amide-I mode. By pumping the amide-A band and probing the amide-I/II mode region, direct coupling between the amide-A and the amide-I/II modes has been observed. The off-diagonal anharmonicity between amide-A(D) and amide-I within the same amide unit has been measured to be 1.6 cm-1, and this anharmonicity is larger (4.6 cm-1) when a Fermi resonance component in the amide-I mode is involved. A mixed mode coupling with an apparent time-dependent off-diagonal anharmonicity is observed as a result of the relaxation of the amide-A mode to lower frequency modes, which are in turn coupled to the amide-I mode. This is found to be significant within the first half picosecond after the pump in the case of the deuterated AcAlaOMe since the T1 of the amide-A(D) vibration in that example is short. The amide-A/amide-I coupling across the hydrogen bond in the self-hydrogen-bonded C7 conformation of AcProNHMe is 1.4 cm-1. The alignment of the transition dipoles (amide-I/II modes) relative to the known directions for N−H and N−D was measured and used to obtain structural information on the chemical structure of peptides. The direction of the amide-I transition moment in the molecular frame changes upon deuteration of the amide and upon hydrogen bonding of the N−H group of the amide. The angle between the amide-I transition moment and N−H in the same amide is determined to be 23 ± 3° for AcAla(H)OMe where the N−H group of the amide is weakly hydrogen-bonded in the C5 conformation. This angle changes to 13 ± 4° for the deuterated compound and to 34.5 ± 3° when the N−H group is involved in strong intramolecular hydrogen bonding in the C7 conformation (AcProNHMe). Two different conformers were observed for AcAla(H)OMe based on cross-peak anisotropy measurements. The structures of these conformers are assigned to C5 (carbonyl) and C5 (ester) conformations, where a five-membered ring is formed by the carbonyl or ester oxygen atom. A first principles simulation of the two color pump/probe signal based on the third order nonlinear polarization, assuming a homogeneous line width for the amide-I and amide-A modes, reproduces the data satisfactorily.