Multidimensional wave packet dynamics in polypeptides: Coupled amide-exciton chain-vibrational motion in an α-helix

Wave packet dynamics in eight dimensions and for eight coupled “diabatic” states are discussed in using the multi configuration time-dependent Hartree method. For this purpose a reduced dimensionality model for an α-helical polypeptide is used which combines intra amide unit high frequency vibrations with the low frequency longitudinal vibrations of the units along a single chain of hydrogen bonds. The model comprises the possibility to form delocalized vibrational Frenkel exciton states as well as to localize them by an appropriate chain compression. As it has been already shown [D.V. Tsivlin, H.-D. Meyer, V. May, J. Chem. Phys. 124 (2006) 134907; D.V. Tsivlin, V. May, J. Chem. Phys. 125 (2006) 224902] the used model is also ready to give a satisfactory reproduction of infrared transient absorption spectra taken either in the absorption range of CO amide vibrations or NH amide vibrations. Wave packet dynamics following special initial states are discussed based on the reduced single chain coordinate probability distribution as well as on the expectation values of the chain coordinates themselves. The high frequency excitation energy transfer through the chain is found to be anisotropic due to the intrinsic asymmetry of the exciton chain vibrational coupling. It is accompanied by a displacement impulse of the amide units.