Heat capacity and phonons in poly(L-leucine)

Poly(L-leucine) is one of the polyamino acids having a bulky hydrophobic side-chain. For want of full phonon dispersion curves and density-of-states on this biopolymer Roleset al have interpreted their specific heat data in a limited way. In the present communication we report an analysis of the normal modes and their dispersion for poly(L-leucine) which leads to a very good agreement of the specific heat calculations with experimental measurements. It is observed that the main contribution to specific heat comes from the coupling of the back-bone skeletal and side-chain modes. Several other assignments have been revised. A special feature of some dispersion curves is their tendency to bunch in the neighbourhood of helix angle. It has been attributed to the presence of strong intramolecular interactions. Repulsion between the dispersion curves is also observed.