Biological Sciences: One and Two-dimensional Structure of Alpha-Helix and Beta-Sheet Forms of Poly(L-Alanine) shown by Specific Heat Measurements at Low Temperatures (1.5–20 K)

Abstract

HOMOPOLYPEPTIDES provide good model systems for various aspects of proteins^1,2. Recent advances in high polymer and solid state physics have enabled the vibrational aspects of the simpler homopolypeptides to be treated as normal—but complicated—polymers by the theoretical techniques of lattice dynamics based on the experimental methods of neutron, infrared and Raman spectroscopy. Basically, however, these latter methods examine the optical vibrational modes of a system, that is, those modes which are of energy higher than, for example, 70 cm⁻¹. The important lower-energy modes are well examined by specific heat measurements, which do not require single crystal samples, at low temperatures (for example, 1–20 K, where only the acoustic modes are excited thermally to any appreciable extent—1 K∼0.7 cm⁻¹). The low temperature measurements presented here are the first made on a polypeptide in alpha and beta-conformation, and clearly show the one-dimensional nature of the α-helix and the two-dimensional nature of the β-sheet structure. Quantitatively, these measurements also provide tests of theories of published vibrational models of homopolypeptides, and will give hydrogen bond strengths.