Optimization of Beamline BL41XU for Measurement of Micro-Protein Crystal

A number of protein crystallography techniques have been improved in recent years. With this advancement, many kinds of not only soluble protein but also biological important molecule such as protein complexes and membrane proteins have been analyzed. When such proteins are crystallized, it is difficult to obtain a crystal with a suitable size for the diffraction experiment (~ 100 μm) though we commonly get a tiny crystal (~10 μm). Therefore, in order to extend the range of an applicable crystal size (< 20 μm), it is necessary to establish the data collection method for such micro-sized crystals. There are various difficulties in acquiring diffraction data with high accuracy from such crystals. However, it is very important to make the size of the beam irradiated to the crystal equal to or smaller than the size of the crystal to reduce the component of a cryoprotectant solution in background scattering. At beamline BL41XU, the highly brilliant beam made by undulator enables us to perform a structural analysis of a 30-μm-size crystal. However, it is not optimized to such crystals because the beam size usually used is about 100 × 100 μm2. Therefore, we have optimized the beamline setting from the second half of 2004, evaluating beam shape at the sample position. We tuned up the optical components, such as a monochromator, focusing mirrors and two sets of quadrant slits, by evaluating the direct beam. As a result, the beam size of 25 × 25 μm2 at the sample position was formed to at 1 Å. However, the stability of beam intensity deteriorated markedly. The beam intensity in the beam size of 25 × 25 μm2 decreased about 40% in 4 hours (Fig. 1) although the change in the intensity of the beam of 50 × 50 μm2 is less than 2% within the same period. On the basis of results of the study, it is thought that this instability is caused by the thermal distortion of the double-crystal monochromator. Compton scattering generated by the first crystal heats and distorts the cradle of the second crystal. Therefore, we re-designed a “Compton shield” that was installed into the second cradle to remove such an effect, and introduced a new one (Fig. 2). Consequently, the change in beam intensity is suppressed to within 0.2% in 4 hours at the beam size of 25 × 25 μm2 (Fig. 1). We also changed the aperture size of the front-end slit installed upstream of the monochromator. From the first half of 2005, the aperture size was reduced from 0.5 × 0.5 mm2 to 0.3 × 0.3 mm2 to improve the parallelity of the beam at a small beam size. Optimization of Beamline BL41XU for Measurement of Micro-Protein Crystal