Femtosecond IR Pump-Probe Spectroscopy of Nonlinear Energy Localization in Protein Models and Model Proteins

This paper reviews our experimental and theoretical efforts toward understanding vibrational self-trapping of the amide I and N-H mode of crystalline acetanilide (ACN), other similar hydrogen-bonded crystals, as well as of model peptides. In contrast to previous works, we used nonlinear IR spectroscopy as the experimental tool, which is specifically sensitive to the anharmonic contributions of the intramolecular interactions (as the nonlinear IR response of set of harmonic oscillators vanishes exactly). Our work reconfirms the previous assignment of the two bands of the amide I mode of ACN as being a self-trapped and a free exciton state, but in addition also establishes the lifetimes of these states and identifies the relevant phonons. Furthermore, we provide evidence for vibrationally self-trapped states also in model alpha-helices. However, given the short lifetime, any biological relevance in the sense of Davydov's initial proposal can probably be ruled out.