Anomalous Dispersion with Edges in the Soft X-ray Region: First Results of Diffraction from Single Crystals of Trypsin Near the K-Absorption Edge of Sulfur

Anomalous dispersion of X-ray diffraction at wavelengths near the X-ray K-absorption edge of sulfur at wavelengths around 5 A has been applied to single crystals of trypsin obtained from an ammonium sulfate solution. The multiwavelength anomalous-dispersion method based on 775 unique reflections (+183 Bijvoet mates) measured at three wavelengths near the K-absorption edge of sulfur in trypsin (two methionines and disulfide bridges of six cystines) reproduces the known features of the trypsin structure of a resolution of 4 A. It appears that there is anisotropic anomalous scattering from the disulfide bridges of cystine. The multiwavelength anomalous solvent contrast shows up at wavelengths near the K-absorption edge of the sulfate ions, which is shifted by 10 eV to higher energies with respect to that of sulfur in trypsin. The influence of the complex contrast of trypsin in 2.5 M ammonium sulfate on the dispersion of a low-order reflection is analyzed. The measurement of anomalous dispersion of X-ray diffraction at long wavelengths beyond 5 A requires a special diffractometer, the features of which are presented. An outstanding one is a detector system consisting of four multiwire proportional counters. Its efficiency is compared with that of imaging plates. The influence of radiation damage with soft X-ray diffraction from single crystals of trypsin is presented and possible remedies are discussed.