Direct observation of vibrationally self-trapped states

J. Edler, R. Pfister, V. Pouthier, C. Falvo and P. Hamm

Physikalisch-Chemisches Institut, Universitat Zurich, Wintherthurerstr. 190, 8057 Zurich, Switzerland and
Laboratoire de Physique Moleculaire, UMR CNRS 6624, Faculte des Sciences, La Bouloie, Universite de Franche-Comte, 25030 Besan\c{c}on Cedex, France

We performed femtosecond infrared pump-probe experiments on the NH and the amide~I mode of acetanilide and N-methylacetamide. Both molecules form hydrogen bonded crystals, which are regarded to be model systems for polypeptides and proteins. The experiments prove that vibrational self-trapping occurs in the NH and amide~I band. The phonons that mediate self trapping are identified. In a second step we investigated the NH mode of a very stable $\alpha$-helix. The pump probe spectra reveal two excited state absorption bands, which disappear upon unfolding of the helix. Numerous test experiments prove that the bands originate only from the NH band and that they are caused by a coupling mechanism between peptide units and not within individual peptide units. A quantitative comparison with polaron theory shows that the two bands correspond to two types of two-vibron bound states. To our knowledge this is the first direct observation of vibrationally self-trapped states in an alpha-helix.