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

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 degrees, confirming previous molecular-dynamics studies. Time-resolved 2D measurements show that the conformation fluctuates on a time scale of picoseconds. (C) 2001 American Institute of Physics.