Time-resolved two-dimensional vibrational spectroscopy of a short #alpha#-helix in water

摘要

Nonlinear two-dimensional (2D) vibrational spectroscopy has been used to investigate the amide I band of an alanine-based 21-residue #alpha#-helical peptide in aqueous solution, Whereas the linear absorption spectrum consists of a single, broad amide I band, the 2D vibrational spectrum clearly reveals that this band is composed of two amide I transitions, which are assigned to the A and E_1 modes. The A-E_1 frequency splitting is found to be approximately 10 cm~(-1). We find that the amide I band is inhomogeneously broadened due to conformational disorder of the helix. The 2D line shapes can be well described using distributions of the dihedral angles (#PHI##PSI#) around their average values with a width of 20 deg, confirming previous molecular-dynamics studies. Time-resolved 2D measurements show that the conformation fluctuates on a time scale of picoseconds.