COOPERATIVITY IN POLY-L-PROLINE I-2 TRANSITIONS

The poly-L-proline I–II transition is known to exhibit positive cooperativity. This effect has hitherto been only qualitatively explained. In this study, the contributions of intramolecular steric and electrostatic interactions to this phenomenon are computed. The calculations suggest that the total end effects are +11.5 kcal/ mol for form I but only +3.3 kcal/mol for form II. Similar calculations for a polyproline molecule with a junction between form I and form II, and between form II and form I, yield intramolecular junction free energies of –0.8 and +2.3 kcal/mol for these two structures. The minimum intramolecular junction free energy J for simultaneous formation of a I–11 and a II-I junction, +1.5 kcal/mol, arises from a +2.1-kcal/mol electrostatic term and a -0.6-kcal/mol van der Waals term. Since the observed value of J is +4 to +7 kcal/mol, these calculations suggest that, in contrast to the α-helix-random coil transition, the poly-L-proline I–II transition derives much of its cooperativity from polymer-solvent interactions. © 1969, American Chemical Society. All rights reserved.