X-Ray Studies on Compounds of Biochemical Interest

Abstract

Every three years, a review paper on this subject has appeared in the Annual Review of Biochemistry. These papers have thus ade­ quately reviewed the field until the end of 1941. Since then, while enough papers have appeared to warrant a review, the war has, never­ theless, sharply reduced the published research. The reviewer be­ lieves, therefore, that he may justifiably devote part of his allotted space to an analysis of the general approach of x-ray methods to the study of materials of biochemical interest. All x-ray scattering experiments are similar in principle. A well 'defined beam of monochromatic or nearly monochromatic x-rays is allowed to fall on· the specimen. This radiation is scattered in all di­ rections and is recorded by means of photographic film. The pattern of scattered radiation is determined by the geometry of the experi­ mental set-up and by the scattering specimen. Because it is deter­ mined by the structure of the specimen, one attempts to set up a one-to-one correspondence between pattern and specimen structure. However, while one can determine the pattern of scattered x-rays if the structure is known, the reverse proposition is not true, and only sometimes (rarely for the complex structures that interest us here) can the structure of the specimen be determined from the x-ray scat­ tering. Nevertheless, much useful information is obtainable from the x-ray diagrams. The widest use is for purposes of identification. Here identification is used in its broader sense-referring not only to identification of materials but also to tracing of chemical and physi­ cal changes. Heretofore, the powder (Debye-Scherrer) method has been used almost exclusively for such purposes. This method is satis­ factory for comparatively simple materials. However, when applied to complex biological materials it is far from satisfactory, for often the deceptively simple x-ray diagrams, e.g., as obtained with proteins, are far from adequate to characterize the complex structures respon­ sible for them. The general problem will be considered from three different view-