Theoretical study of temperature induced phase transitions in poly(β-benzyl-l-aspartate) and itʹs copolymer

In polypeptides and proteins, the phenomenon of conformational transitions has been successfully explained on the basis of Zimm and Bragg model, as a function of temperature, pressure, solvent concentration and pH etc. The Zimm and Bragg theory of helix↔coil transition has been modified to explain the temperature induced helix→helix transitions in solid films of poly(β-benzyl-l-aspartate) and in its copolymer copoly(β-stearyl-l-aspartate-β-benzyl-l-aspartate) with varying degrees of stearylation. An expression for the degree of order is obtained from the grand partition function for the entire chain in terms of nucleation parameter, which controls the transition width. At low degree of stearylation, an α→ω transition occurs with the increase in temperature, whereas, at degree higher than 40%, the reversal of α helix sense from right-handed to left-handed takes place. The theoretical transition curves are found to be in good agreement with the experimental data.