INCE insulin was first prepared crystalline1 in 1926, several efforts have been made to obtain X-ray photographs of the crystals. The first attempts of W. H. George2 by the powder method failed to show any pattern indicative of a crystal structure, and though later long spacings were reported by G. L. Clark and K. E. Korrigan3, it was impossible to base any unambiguous interpretation on their results. The fact that pepsin could be made to give a single crystal X-ray diffraction pattern4 suggested that the problem of insulin, which is in many respects a more stable crystalline species, could be attacked in the same way if large enough crystals could be grown. This was made possible by D. A. Scott's study of the crystallisation of insulin in the presence of salts of zinc and of other metals5.
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