Anomalous X-ray scattering. Ch. 17

Anomalous X-ray diffraction originally conceived for the use in single-crystal studies has rapidly found very promising applications in other areas of X-ray structural studies. The investigation of membranes, of multilayers, of alloys and of macro-molecules in solution has been started. Anomalous X-ray diffraction is expected to become a powerful tool in fibre diffraction as well. This development is entirely due to the availability of synchrotron radiation (SR). The high spectral brilliance of SR can be tuned to any interval of X-rays of interest for anomalous X-ray diffraction studies. The bremsstrahlung of X-ray tubes would do the same at five or more orders lower spectral brilliance. The results from anomalous scattering of sulfur in purple membrane show that light atoms may be used as labels in structural research. The dispersion of phosphorus near the K absorption edge at λk=5.8x10-10m is now almost at hand. These two elements are ideal probes to investigate the interaction of proteins with membranes and nucleic acids. They are native labels. Contrary to phosphorus, sulfur in biomolecules may be present in different valence states giving rise to major shifts of the K absorption edge. Even the information of disulfide bridges (cysteine to cystine) leads to an additional peak 1 eV off the leading absorption peak. The regions of anomalous dispersion of sulfides and sulfates are separated by 10 eV allowing their independent processing in structure determination. (author). 43 refs.; 16 figs