GERALD OSTER, PH.D. Polytechnic Institute of Brooklyn, New York, N.Y.
The molecular morphology of materials of biological significance is determined by a long deductive process utilizing data obtained by a variety of physical methods. Perhaps the most direct method of the visualization of biomolecular structures is that of electron microscopy, which is treated elsewhere in this volume. The electron microscope suffers, however, in its inability in practice to resolve structural elements smaller than about 30 A (one angstrom, A, equals 10-8 cm). In proteins, for example, the interatomic distances in the polypeptide chain are of the order of 1.5 A and, hence, can be resolved only by rays having wavelengths of the same magnitude. All forms of matter scatter x-rays. If, as is usually the case, the material is periodic on a molecular level, then monochromatic x-rays of wavelength of, say, 1.54 A (the so-called copper Kα radiation) will be diffracted. From the diffraction pattern, the structure of the scattering material can, in principle, be deduced. Even if this material is not periodic, the resultant diffuse scattering can be used to deduce the disposition of the scattering elements. X-ray diffraction techniques have an additional feature not possessed by electron microscopy, in that the former method can be applied to determination of structures of gels and concentrated solutions of macromolecules.
The diffraction pattern may, in some cases, be extremely elaborate. For example, crystalline proteins may show an x-ray diagram containing more than a thousand spots. For this reason, other physical techniques, such as visible-light birefringence and infrared dichroism, are frequently employed in conjunction with the x-ray studies. This infor-