This paper is a report of work in progress. Completely satisfactory structures have not been obtained for either the a or /3 forms of poly-L-alanine but considerable progress has been made towards this end and an outline of the present position is given. Results obtained with other polypeptides are briefly referred to and a comparison is made with the various silk structures. I. THE X FORM SUMMARY OF RESULTS ON POLY-y-METHYL-L-GLUTAMATEThe first test of Pauling's helical structure 1% 2 was made on poly-y-methyl-Lglutamate (fig. 1 and 2). The unit cell and intensity distribution agreed well with those expected for a helical configuration with 3.63 residues/turn 3 s 4 but a detailed examination revealed a number of difficulties. Some of these have been discussed by Bamford et aZ.,5 others arose when attempts were made to account for the observed intensities. The main points of interest were as follows :

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Reduced human haemoglobin exists in three polymorphous modifications which all have the same space group (P21), but different molecular arrangements. Patterson projections of the simplest of the three forms indicate that the two molecules in the unit cell are nearly parallel to each other and centred approximately at :t:(x = 0-225, y = 0.25, z = 0.20). The peaks near the origin in two of the Patterson projections of this form resemble those in corresponding projections of horse methaemoglobin, and suggest similarities in the molecular structure of the two proteins.