Single-crystal diffractometry: strategy for rapidly decaying poorly diffracting crystals

A strategy allowing rapid data collection has been developed on a standard Philips PW1100 automatic four-circle diffractometer adapted for protein studies. Hardware modifications include: a long arm enabling a continuous adjustment of the crystal-to-counter distance, a helium pathway with variable apertures on the detector side, and a cooling system for temperatures down to 263 K using an axial flow of dry air blown onto the crystal along the axis. Modifications of the diffractometer software, providing a more efficient data collection strategy, are also described. An integrated set of Fortran programs has been implemented on a 16-bit minicomputer for the processing of the experimental measurements. During the treatment of the Bragg reflections, stored with their profiles, special care is taken for background estimation and decay correction. The set of programs and the strategy have been successfully tested with the structure determinations to high resolution of yeast tRN^Asp and of cardiotoxin IV from Naja mossambica mossambica. The modified hardware has also been successfully used for data collection of a large number of poorly diffracting small-molecule crystals.