Macromolecular Crystallography at Chess

CHESS activity report F many years, most synchrotron stations for crystallography, such as CHESS A1, used X-ray sensitive film as the primary detector and xray radiation at about 8 keV (1.5 Å). Two years ago CHESS constructed station F1 on a wiggler line that is tuneable from about 7-14 keV. In practice, this station operates with a standard energy of 0.91 Å. The benefits of using higher energies soon became apparent in terms of improved X-ray diffraction data that resulted from decreased absorption, decreased sample decay and lower background scatter. The crystallographic results from this station have been spectacular, with experimenters reporting much better data using fewer crystals. Because of the high energy operation of CESR, the critical energy of the F line wiggler is about 24 keV or 0.5 Å, and a useful flux to the sample can be obtained at 0.3 Å or less. Therefore, it is currently possible only at CHESS to determine whether or not the benefits derived from lowering the X-ray wavelength from 1.5 Å to 0.9 Å can be extended to the high energy range (0.3-0.5 Å). In order to test these ideas we have performed preliminary experiments on station F2 using lysozyme and nucleoside deoxyribosyl transferase, an enzyme with high sensitivity to radiation damage at room temperature. Test lysozyme data was measured using the oscillation method and image plate detectors at the critical wavelength of 0.5 Å and just above and below the Ba absorption edge at 0.34 Å. The experiment near the Ba absorption edge was to determine whether or not the sensitivity of the image plate, which contains Ba, could be enhanced by measuring data above the absorption edge. The figures shows a typical lysozyme oscillation frame measured at wavelengths of 0.5 Å (top) and 0.34 Å (bottom). Both patterns were recorded using unfocussed radiation at F2, over an oscillation angle of 1.0 degree. The data is somewhat noisy because of insufficient shielding and/ or collimation, but the data were easily processed at all three energies and gave good statistics to 1.5 Å resolution. These experiments showed that data collection from macro-molecular specimens at these energies is perfectly feasible. The Ba edge experiment was inconclusive although the higher energy data showed significantly more background and therefore poorer overall signal to noise. Protein crystallography at high energies at F2