Direct observation of self-trapped vibrational states in alpha-helices.

Femtosecond infrared pump-probe spectroscopy of the N-H mode of a stable alpha-helix reveals two excited-state absorption bands, which disappear upon unfolding of the helix. A quantitative comparison with polaron theory shows that these two bands reflect two types of two-vibron bound states connected to the trapping of two vibrons at the same site and at nearest neighbor sites, respectively. The latter states originate from an acoustic phonon in the helix, which correlates adjacent sites.