OPTICAL MONITORING OF PROTEIN CRYSTALS IN TIME-RESOLVED X-RAY EXPERIMENTS - MICROSPECTROPHOTOMETER DESIGN AND PERFORMANCE

We describe a single crystal microspectrophotometer for optical monito ring of protein crystals during time-resolved x-ray diffraction experi ments. It is a single beam device with a diode array detector for simu ltaneous multiwavelength measurement of optical spectra. The time reso lution of the microspectrophotometer is 2.5 ms, which matches the ms t ime resolution of the x-ray experiments obtained at existing synchrotr on sources. Optical densities of single crystals can be reliably measu red up to 2.5 OD, with the linear range extending to 2 OD. The tight f ocusing of the light beam allows monitoring of crystals as small as 30 -50 mum, an important feature since successful reaction initiation and optical monitoring of crystals in time-resolved x-ray diffraction exp eriments often require very small crystals. The compact design of the microspectrophotometer and the long objective-to-crystal distance make it compatible with the crowded environment of time-resolved x-ray dif fraction experiments.