"X-ray diffraction of protein crystals"

Date:
1954-1956
Reference:
PP/CRI/H/1/33
Catalogue details

Licence: In copyright

Credit: "X-ray diffraction of protein crystals". Source: Wellcome Collection.

    19/45
    17 unit cell; namely the volatile material, such as water or alcohol, and the non-volatile non -protein material (such as salt) remaining after drying. The former is found by tskt measuring the percentage loss of weight in taking the crystal fro» the original state (in which the density can he measuréà) to the vacuum-dried condition. The latter must then he found by chemical methods. What remains after subtracting these _ corrections is considered to be the molecular weight of the protein. f ere are three obvious choices for the *'well~d®fln®d w state of the crystal: the wet crystal, the air~drigj| crystal ; H y and the vacuum-dried crystal. Experience has shown * that the last of these is not suitable. The effect of vacuum drying is to remove the solvent from 4 he spaces between the protein molecules, and to leave holes. If the density of such a crystal is now determined (by immersion in a fluid) the fluid enter! f '■ these holes, and the observed density does not correspond to Ì4 the density of the unit cell, but ìm more nearly that of the protein molecule itself. One must therefore start from either the wet crystal or the air-dried crystal* At the moment of writing it is not clear which of these methods Is the better for the enzymologist. In theory the -et crystal should yield a more accurate figure » but in practice the technique is more difficult, and this may perhaps lead to errors. On the other hand the air-dried method can be carried out usinr only rat'• • er small crystals, whereas the wet crystal method cannot be. Both methods will therefore be described. They are folio -.ved by detailed accounts of the techniques of density determination and composition measurement.