Adiabatic vibrational excitons: Amide I states in α-helices as an example

Based on a correct quantum dynamical description, exciton formation and self-trapping in α-helical polypeptides is reinvestigated. The studies are focused on the amide I high-frequency vibration of the peptide group and account for their modulation by longitudinal helix vibrations. First, the adiabatic single- and two-exciton states are analyzed for a chain of 10 peptide units, whereby the presence of two types of self-trapped amide I two-exciton states is confirmed. In a second step, by using the multi-configuration time-dependent Hartree method an exact quantum description is achieved for the complete exciton helix-vibrational states. The computational approach incorporating up to nine helix coordinates is used to calculate the infrared linear absorbance. Moreover, excitation energy motion through the chain is characterized.