Crystalline Morphology of Synthetic Polypeptides

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Abstract
In an attempt to growsingle crystals of synthetic polypeptides, polyglycine, poly‐l‐alanine, and poly‐l‐tyrosine have been precipitated from dilute solution and also cast as films by the slow evaporation of solvent. Polyglycine precipitated from solutions containing equal parts of trifluoroacetic acid (TFA) and trifluoroethanol (TFE) by the addition of water, crystallizes with the polyglycine II structure in the form of lamellar platelets with folded molecular chains. In films of polyglycine cast from these solutions, however, sheaves of ribbonlike crystals are produced, also chain folded and having the same structure. Selected‐area electron diffraction from these various crystals yields corroborative evidence for some of the structural features of polyglycine II. Poly‐l‐alanine cast from solution in TFA and TFE, on the other hand, crystallizes to give platelets and sheaves side by side in the same preparations. Both have the crystal structure of the α form of poly‐l‐alanine, and indirect evidence indicates that chain folding probably occurs in both instances. There is a suggestion that, as grown, the crystals contain solvent complexed chemically with the polymer. Poly‐l tyrosine can be precipitated from solution in mixtures of N,N‐dimethylformamide (DMF) and n‐heptanol in the form of hexagonal platelets and these too probably consist of folded molecules complexed with solvent. A tentative correlation between conformation in solution and crystalline morphology is suggested in the case of poly‐l‐tyrosine, but no definitive conclusion can be reached without further study of the conformation of the polymer held in solution under the conditions prevailing during crystallization.

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