Contrast variation and the versatility of deuterium in structural studies of biological macromolecules

The method of contrast variation has its origins in microscopy and the numerous stains and staining techniques used to improve the visibility of particular structures. The microscopic method has remained largely qualitative, but effective, especially in cell biology. With scattering techniques where images cannot be formed, one of the first uses of contrast variation was by Bragg & Perutz [Acta Cryst. (1952), 5, 277-283] in their early X-ray diffraction studies of haemoglobin crystals, but extensive applications began twenty years later in studies of macromolecules in solution. At this time a theoretical basis and quantitative method were developed, and neutron scattering as well as X-ray scattering from macromolecules became possible. The value of hydrogen/ deuterium (H/D) changes for contrast variation was quickly recognized, and neutron scattering with H/D manipulation is now the dominant form of contrast variation used in scattering experiments.