In an ongoing effort to understand the thermodynamic properties of proteins, solid-state heat capacities of poly(amino acid)s of all 20 naturally occurring amino acids and 4 copoly(amino acid)s have been previously 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 measurements from 10 to 340 K. In addition, analyses were performed on literature data of a first protein, zinc bovine insulin dimer C508H752O150N130S12Zn, using both the ATHAS empirical addition scheme and computation with an approximate vibrational spectrum for the protein. For the solid state, agreement with the measurement could be accomplished to +/-1.6% for PLMET, +/-3.5% for PLPHEA, and +/-3.2% for insulin, linking the macroscopic heat capacity to its microscopic cause, the group and skeletal vibrational motion. For each polymer, one set of parameters, Theta(1) and Theta(3), of the Tarasov function representing the skeletal vibrational contribution to the heat capacity are obtained from a new optimization procedure [PLMET : 542 K and 83 K (number of skeletal 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.