Dynamics of amide-I modes of the alanine dipeptide in D2O

The equilibrium dynamics of the acetyl and amino amide-I groups of the alanine dipeptide were examined separately using C-13 isotopic selection and 2D IR. The population relaxation times of the amide transitions were measured to be in the range 500 fs by means of heterodyne transient grating methods. The vibrational frequency correlation functions consisted in all cases of a motionally narrowed part, a component near 800 fs, and a constant part representing a distribution of structures that is static on the few ps time scale. The intermediate time scale is attributed to fluctuations in the stretching and bending of hydrogen bonds between the carbonyl and water.