Two-dimensional infrared spectroscopy of the alanine dipeptide in aqueous solution

The linear-infrared and two-dimensional infrared (2D IR) spectra in the amide-I' region of the alanine dipeptide and its C-13 isotopomers in aqueous solution (D2O) are reported. The two amide-I' IR transitions have been assigned unambiguously by using C-13 isotopic substitution of the carbonyl group; the amide unit at the acetyl end shows a lower transition frequency in the unlabeled species. The ratio of their transition dipole strengths remains almost unchanged upon C-13 substitution, indicating the absence of intensity transfer between two vibrators. The 2D IR cross peaks directly associated with intramode coupling in this case show a small off-diagonal anharmonicity (0.2 +/- 0.2 cm(-1)), leading to a small coupling constant (1.5 +/- 0.5 cm(-1)). The coupling and the 2D IR spectra in two different polarizations (< zzzz > and < zxxz >) are as expected for a polyproline-II (PPII)-like conformation for dialanine, with the backbone dihedral angles (phi, psi) determined to be in the range of (-70 degrees +/- 25 degrees, + 120 degrees +/- 25 degrees). Ab initio DFT calculations and normal mode decoupling analysis in the Ramachandran subspace in the neighborhood of PPII conformation confirm the presence of a region where the coupling is vanishingly small and support these experimental findings. The relationship between the coupling and off-diagonal anharmonicity is consolidated by examining the distribution of the latter from an ensemble averaged Hamiltonian incorporating uncorrelated diagonal frequency distributions and a small coupling (< 2 cm(-1)); it is found that the most probable value for the off-diagonal anharmonicity falls into the range of experimental observations. Further, incorporating DFT results, the simulated linear-IR and 2D IR can reproduce the essential features of the measurements, including the transition frequency positions and apparent peak intensities. All the experimental results and simulations are consistent with a PPII-Iike conformation for the alanine dipeptide in aqueous solution, in which two amide-I' modes are highly localized and whose frequency distributions are uncorrelated.