Heat capacity studies of solid poly(amino acids)

Heat capacities of sixteen poly(amino acids), based on the twenty naturally occurring amino acids, and four copolymers of poly(amino acids) were analyzed using approximate group vibrations and fitting of the skeletal vibrations to a two parameter (θ₁, θ₃) Tarasov function. New experimental heat capacities were measured by differential scanning calorimetry in the temperature range 220 to 390 K. Good agreement between our experimental data and the calculated data was observed for all but two of the biopolymers, poly(L-methionine) and poly(L-serine). Previous investigations on the three simplest poly(amino acids) - polyglycine, poly(L-alanine), and poly(L-valine) showed agreement between calculation and reported experimental data for only limited, low temperature ranges. At higher temperatures, discrepancies of up to 55% existed between experiment and calculation. It is shown that systematic experimental errors plagued these earlier investigations. In fact, the most likely source of error in these earlier investigations was the presence of removable water. Recommended experimental data are revised on the basis of this investigation. Computed heat capacities are available for all the biopolymers studied in the solid state from 0 to 1000 K. For poly(L-serine) and poly(L-methionine), the lack of agreement between experiment and calculation is attributed to the presence of a broadened glass transition. For the remaining four poly(amino acids), attempts were made to calculate heat capacities based on the results of the sixteen that were measured.