Phonon-Dispersion In Omega-Helical Poly-L-Phenylalanine

Poly(L-Phenylalanine) is the simplest polypeptide which goes into the rare omega form. Complete normal mode calculations and their dispersions are reported using Urey Bradley force field and Higgs method for infinite systems. The results indicate that the modes below 1350 cm(-1) show appreciable dispersion. Two main chacteristics of the dispersion curves are cross-over between torsional and bending modes at delta=psi/2 and repulsion with character exchange between junction and ring modes in the neighbourhood of delta=psi. The crossover within the Brillouin Zone (B-Z) is permissible between different species at the same frequency and it can be called as a ''nonfundamental resonance''. The regions of high density of states in the neighbourhood of delta=0.75 pi are critical points akin to Von Hove Singularities in lattice dynamics. Detailed assignments of various modes are also given.