Appendix. Crystallographic measurements on the anti-pernicious anaemia factor

Since May 1948, five different samples of crystalline anti-pernicious anaemia factor have been examined in Oxford by various crystallographic techniques. These all, from their general characteristics, particularly the intensities of a number of X-ray reflexions, contain the same molecular structure. But the different samples have shown small variations in unit-cell dimensions and in crystal habit, which are probably due mainly to differences in solvent content, combined with traces of different impurities. Small changes in, for example, some side chain in the molecule, might also conceivably contribute to these effects. The crystals, as grown both from water and from aqueous acetone, are dark red and show marked pleochroism . They vary in habit from long thin needles to short thick prisms on which different crystal faces appear (figures A 1 and A 2). They all contain solvent, probably water, of crystallization, part at least of which they lose slowly on exposure to the air. Crystals kept in their mother-liquor are transparent and show beautifully clear reflecting faces; they give sharp X-ray reflexions extending to spacings of 1·1Å. On removal from the liquid they tend to crack and to become opaque; the faces are distorted and the X-ray reflexions become first multiple, and then blurred, corresponding to the presence of disorder within the dried crystal structure. But the rate of loss of solvent appears to vary both with the size of crystals and with the different samples studied. Individual air-dried crystals for example, have been observed which have given good and sharp X-ray reflexions some weeks after exposure to the atmosphere.