Chain arrangement and sense of the α-helix in poly-L-alanine fibres

The X-ray diffraction pattern of poly-L-alanine fibres has been compared with optical diffraction patterns of a-helices. With bond lengths and angles not significantly different from those found in simple compounds, good agreement is found with right-handed (but not with left-handed) helices. It is necessary to suppose that the direction of the peptide sequence of chains in the crystallites is random. Helices with a long repeat distance are found to pack in a way which produces a limited sequence of residues, spaced at 4.5 A, in which displacements from steric effects can be expected. It is shown how this may produce a meridian reflexion at 4.4 A, as observed. A complete account of all features of the packing is not given, however, and hardly seems to be practicable.

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