The triple helix in equilibrium with coil conversion of collagen-like polytripeptides in aqueous and nonaqueous solvents. Comparison of the thermodynamic parameters and the binding of water to (L-Pro-L-Pro-Gly)n and (L-Pro-L-Hyp-Gly)n.
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The collagenlike peptides (LProLProGly)n and (LProLHypGly)n with n = 5 and 10, were examined in terms of their triple helix coil transitions in aqueous and nonaqueous solvents. The peptides were soluble in 1,2propanediol containing 3% acetic acid and they were found to form triplehelical structures in this solvent system. The water content of the solvent system and the amount of water bound to the peptides were assayed by equilibrating the solvent with molecular sieves and carrying out Karl Fischer titrations on the solvent phase. After the solvent was dehydrated, much less than one molecule of water per tripeptide unit was bound to the peptides. Since the peptides remained in a triplehelical conformation, the results indicated that water was not an essential component of the triplehelical structure. Comparison of peptides with the same chain length demonstrated that the presence of hydroxyproline increased the thermal stability of the triple helix even under anhydrous conditions. The results, therefore, did not support recent hypotheses that hydroxyproline stabilizes the triple helix of collagen and collagenlike peptides by a specific interaction with water molecules. Analysis of the thermal transition curves in several solvent systems showed that although the peptides containing hydroxyproline had tm values which were 18.6 to 32.7C higher, the effect of hydroxyproline on G was only 0.1 to 0.3 kcal per tripeptide unit at 25C. The results suggested, therefore, that the influence of hydroxyproline on helical stability may be explained by intrinsic effects such as dipoledipole interactions or by changes in the solvation of the peptides by alcohol, acetic acid, and water. A direct calorimetric measurement of the transition enthalpy for (LProLProGly)n in 3% or 10% acetic acid gave a value of 1.84 kcal per tripeptide unit for the coiltohelix transition. From the value for enthalpy and from data on the effects of different chain lengths on the thermal transition, it was calculated that the apparent free energy for nucleation was +5 kcal/mol at 25C (apparent nucleation parameter = 2 104 M2). The value was dependent on solvent and on chemical modification of end groups. Copyright 1977 John Wiley & Sons, Inc.
