Specific heat measurements of poly-(L-alanine) at low temperatures (1.8-20 K) and aspects of one dimensionality.

Abstract

HOMOPOLYPEPTIDES have long been considered as model systems for the study of protein conformation1,2, and synthetic homopolypeptides are now available commercially. Solid state and polymer physics have advanced to where the simpler homopolypeptides can be treated theoretically by lattice dynamics and experimentally by the physical techniques of neutron, infrared and Raman spectroscopy, which are more suited to the examination of the optical vibrational modes of a molecule which are of energy higher than, say, 70 cm−1. Specific heat methods, on the other hand, have the advantage that by measuring at low temperatures (1–20 K) only the acoustic modes are appreciably excited thermally (a temperature of 1 K corresponds approximately to 0.7 cm−1). Moreover, the specimen need not be in single-crystal form, and at low temperature the vibrational spectrum is easier to interpret, and the useful picture of Debye3 holds well. The specific heat measurements reported here are believed to be, by a factor of 100, the lowest temperature specific heat measurements made on a biopolymer4. Such measurements can give information about the proportion of a protein which is in alpha-helical or beta-sheet structure, and also will be necessary in evaluating the basic thermodynamics of enzyme reactions5.