Poly(L-leucine) is one of the poly(amino acids) having a bulky hydroph
obic side chain. For want of full phonon dispersion curves and density
-of-states on this biopolymer Roles ct al. have interpreted their heat
capacity data in a limited way. In the present paper, we report the F
ourier transform infra-red (FTi.r.) spectra and an analysis of the nor
mal modes and their dispersion based on the calculations for an infini
te chain and Urey Bradley force field with intrachain interactions onl
y. The results thus obtained agree well with our FTi.r. spectra and th
e Raman frequencies reported by Koenig er nl. Several earlier assignme
nts have been revised. A special feature of some dispersion curves is
their tendency to bunch in the neighbourhood of the helix angle. This
has been attributed to the presence of strong intramolecular coupling
between different types of motions. Repulsion between the dispersion c
urves is also observed. The heat capacity obtained from the dispersion
curves via density-of-states is in very good agreement with the exper
imental measurements beyond 220 K. It is observed that the main contri
bution to heat capacity comes from the modes involving the coupling of
the backbone skeletal and side-chain motions. Copyright (C) 1996 Else
vier Science Ltd.