Theoretical calculation of the conformation and crystal structure of poly-L-hydroxyproline

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Abstract
The intra-and intermolecular forces determining the conformation of poly-L-hydroxyproline have been examined. The vacuum conformation of the isolated molecule has the left-hand trans configuration of poly-L-proline II with Ψ = 295°. A second less favorable low-energy form is the right-hand superhelix, also previously described for polyproline. No cis conformation was found to be feasible. Introduction of intermolecular interaction functions shows that the conformation in the solid state is strongly modified to accommodate such interactions. The rotation angle corresponding to the minimum energy in the solid is found to be Ψ = 343°. The crystal structure predicted from these calculations is a hexagonal lattice with a0 = 12.5 Å and c0 = 9.1 Å with space group P32. Three helices are required in the fundamental structural unit. These results are in excellent agreement with X-ray evidence.