If the spectrum shows a shift in the calibration, the monochromator is set to the proper position. This process is typically carried out at the beginning and the end of each fill, consuming less than 1 minute of beamtime for each check. During a recent MAD experiment, a series of reference scans done at the beginning of 4 fills over an 8 hour period showed an energy drift of less than one eV. While this degree of stability was often observed, variations of up to 4 eV have also been seen. Clearly, close monitoring of the calibration of the monochromator is still essential for a successful MAD experiment. Using this facility, we have recently completed a MAD experiment in collaboration with Menachem Shoham of Case Western University studying the copper-containing protein rusticyanin.
Data processing is currently underway.
The results of some earlier MAD experiments demonstrate the effectiveness of the F2 beamline. In August 1992, Aneel Aggarwal's group from Columbia collected MAD data from the restriction endonuclease bamHI in which 5 methionine residues were replaced by seleno-methionines. A total of 5 crystals were shot at 3 wavelengths relative to the selenium absorption edge, corresponding to the edge, peak, and a remote wavelength above the edge. From this structure, the researchers have noted that the protein resembles EcoRI despite the lack of sequence similarity. Finally, Tom Steitz' group from Yale carried out MAD experiments on F2 resulting in the solution of two crystal structures.