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

Abstract
The X-ray diffraction pattern of poly-L-alanine fibres has been compared with optical diffraction patterns of $\alpha $-helices. With bond lengths and angles not significantly different from those found in simple compounds, good agreement is found with right-handed (but not with left-handed) helices. It is necessary to suppose that the direction of the peptide sequence of chains in the crystallites is random. Helices with a long repeat distance are found to pack in a way which produces a limited sequence of residues, spaced at 4$\cdot $5 angstrom, in which displacements from steric effects can be expected. It is shown how this may produce a meridian reflexion at 4$\cdot $4 angstrom, as observed. A complete account of all features of the packing is not given, however, and hardly seems to be practicable.