An X‐ray diffraction study of poly(L‐lysine hydrobromide)

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Abstract
A structural study of the synthetic polypeptide poly(L‐lysine hydrobromide) has been made by X‐ray fiber techniques. The investigation was undertaken to determine whelther this polymer undergoes conformational transitions as a function of hydration in a manner similar to other chemically related basic polypeptides. Specifically, a comparison with the previously reported structures of the hydrochloride form of poly(L‐lysine) was sought. Homogeneous powder mixtures with various amounts of water and oriented fibers of poly(L‐lysine hydrobromide) at different relative humidities were X‐ray photographed. Reversible transitions amorphous state ⇌ β‐pleated sheet ⇌ α‐helix ⇌ isotropic solution as a function of increasing/decreasing degrees of hydration were found. The β‐pleated‐sheet conformation was observed between 33% and 76% relative humidities (containing about one and three molecules of water per residue, respectively). Each pleated sheet was formed by “antiparallel” chains, and the sheets were piled up along the b‐axis. The spacings of this conformation did not vary appreciably with hydration. The observed reflections at 52% relative humidity (1.4 molecules of water per residue) could be indexed satisfactorily in terms of an orthorhombic unit cell, of space group P21221, with a = 9.52 Å, b = 16.44 Å, and c = 6.80 Å. These dimensions were shown by models to be compatible with the proposed structure. The α‐helix conformation was present in specimens photographed at 76% relative humidity and up, and containing between three and fifteen molecules of water per residue. The helices were packed parallel to each other in a hexagonal array but randomly along or about their lengths. Increasing the hydration from five to fifteen molecules of water per residue causes the a‐axis to increase from 16.9 to 20.8 Å. Twenty molecules of water per residue produced an isotropic solution. Despite some structural differences between the hydrobromide and hydrochloride forms it is concluded that the role played by the anions is mainly related to determining the water content levels at which conformational changes occur. Therefore, the anions do not significantly influence the prevailing conformation in this particular system, but might affect the packing arrangement of the polypeptide chains.