HEAT-CAPACITIES OF POLY(AMINO ACID)S AND PROTEINS

In an ongoing effort to understand the thermodynamic properties of pro teins, solid-state heat capacities of poly(amino acid)s of all 20 natu rally occurring amino acids and 4 copoly(amino acid)s have been previo usly reported on and were analyzed using our Advanced THermal Analysis System (ATHAS). We extend the heat capacities of poly(l-methionine) ( PLMET) and poly(L-phenylalanine) (PLPHEA) with new low temperature mea surements from 10 to 340 K. In addition, analyses were performed on li terature data of a first protein, zinc bovine insulin dimer C508H752O1 50N130S12Zn, using both the ATHAS empirical addition scheme and comput ation with an approximate vibrational spectrum for the protein. For th e solid state, agreement with the measurement could be accomplished to +/-1.6% for PLMET, +/-3.5% for PLPHEA, and +/-3.2% for insulin, linki ng the macroscopic heat capacity to its microscopic cause, the group a nd skeletal vibrational motion. For each polymer, one set of parameter s, Theta(1) and Theta(3), of the Tarasov function representing the ske letal vibrational contribution to the heat capacity are obtained from a new optimization procedure [PLMET: 542 K and 83 K (number of skeleta l vibrations N-s = 15); PLPHEA: 396 K and 67 K (N-s = 11); and insulin monomer: 599 K and 79 K (N-s = 628), respectively]. Enthalpy, entropy ,and Gibbs free energy have been derived for the solid state. (C) 1995 John Wiley & Sons, Inc.