The structure of some proteins as revealed by an x-ray scattering method

Abstract

The intensity of scattering of monochromatic X-rays by isotropic specimens of thirty proteins (or protein fractions) and two synthetic polypeptides has been measured by means of a proportionalcounter diffractometer. The curves fall into only three essentially different types, calleda, /? and y. The a-proteins are the most numerous and some twenty have been examined; all the intensity curves are very similar and suggest that a single polypeptide-chain configuration is common to all as the predominant constituent. This predominant configuration is considered to be best represented by one of the variants of the a-helix of Pauling & Corey (1951 a ) and Pauling, Corey & Branson (1951) to which the term aj-helix is given. The method of interpretation used was to calculate the intensity of scattering that would be produced by independent polypeptide chains in random orientation, various regular configurations being assigned to them in accordance with existing hypotheses. In this way, the scattering characteristics of the following helical chain configurations were derived: a, y, 7r, 4 13 , 3 10 , 3 8 and 2 7 ; in the first five mentioned, the two alternative positions of the ^-carbon atom were considered. Comparison with the observed curves for a -proteins showed that the curve for theaj-helix was in best agreement with experiment, and with the synthetic polypeptides the correlation was particularly convincing. It was further shown that the several helices, or parts of helices, that constitute a whole protein molecule are usually packed together compactly in a nearly parallel fashion; in certain cases, a more pronounced skewness is indicated.