Free energy of hydration of collagen models and the enthalpy of the transition between the triple‐helical coiled‐coil and single‐stranded conformations

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Abstract
Interactions with water make an important contribution to the free energy of stabilization of the collagen triple helix, but they do not alter the structure of the triple helix, i.e., the packing geometry of the three strands. Conformational energy computations have been carried out on poly(tripeptide) analogues of collagen, with the introduction of a newly developed form of a hydration shell model to compute the free energy of hydration. The most stable triple helix formed by poly(Gly‐Pro‐Pro), obtained earlier from conformational energy computations [M. H. Miller & H. A. Scheraga (1976) J. Polym. Sci. Polym. Symp. 54, 171], with a structure that is very closely similar to the observed structure, is strongly favoured over other three‐strand complexes, both in the absence and the presence of hydration. The hydration shell model does not provide an explanation for the increased stability of the poly(Gly‐Pro‐Hyp) triple helix as compared to poly(Gly‐Pro‐Pro). It appears that the difference should be attributed to specific binding of water, an effect that is not yet included in the present version of the hydration shell model. On the other hand, this model accounts for the observed enthalpy of unfolding of a poly(Gly‐Pro‐Pro) triple helix to isolated single chains in solution in terms of intramolecular noncovalent interactions and the free energy of hydration.