Variations thermiques, entre 1 et 300°K, de la chaleur specifique de la L ‐alanine, tri( L ‐alanine), et de la poly( L ‐alanine) sous formes α et β

The heat capacities of L ‐Alanine, tri( L ‐alanine), and poly ( L ‐alanine) (α helicoidal form and β pleated sheet structure) have been measured between 1.5 and 300°K with a standard adiabatic calorimeter. In the solid state, the heat capacity is in general dut to three parts which are additive in first‐order approximation. (1) The lattice vibrations or “acoustical modes” which are the largest at low temperatures. The low‐temperature lattice specific heat is proportional to T , T 2 , or T 3 for an ideal one‐, two‐ or three‐dimensional solid, respectively. (2) The so‐called group vibrations or “optical modes” which, due to their high frequencies, usually take effect only at higher temperatures. (3) The defects and unharmonic effects. The α‐amino acid and its trimer present a specific heat thermal variation characteristic of molecular solids which is correctly fitted with an empirical law proposed by Kitaigorodskii. This author assumes that, for such solids, the molecular lattice point has six degrees of freedom (three of translation and three of rotation). Thus the lattice contribution of the specific heat satisfies the Debye approximation in agreement with the doubling of the number of degrees of freedom per molecule (compared with atomic crystals). The polypeptide behavior is, however, different. The specific heat for each form exhibits a thermal dependence connected with a strong vibrational anisotropy. The model proposed earlier by Tarasov accounts well for these results. In the case of the β form, we have observed the predicted three‐ and two‐dimensional behavior due to the intermolecular H bondings responsible for the sheet structure. For the α form we observed a one‐dimensional pattern at higher temperature, since each peptidic chain vibrates separately. The comparison with other spectroscopic and theoretical investigations shows a large discrepancy. However, we have attemped to account for the “optical contribution” to the specific heat of poly( L ‐alanine) by using a continuum of mean frequencies as suggested by Wunderlich. Vibrational frequency spectra are proposed to explain our results, but the overlapping of acoustical and optical branches in the case of the α form outlines the limits of macroscopic models. It is quite likely that the acoustical spectrum is greatly affected by the intramolecular H bonding. At low temperature the specific heat is a physical property sensitive to the long‐range order of the macromolecule, and therefore further spectroscopic and theoretical investigations are necessary to explain correctly these experimental results.