ELECTRON-RADIATION DAMAGE TO PROTEIN CRYSTALS OF BACTERIORHODOPSIN ATDIFFERENT TEMPERATURES

A series of diffraction patterns from two-dimensional protein crystals of bacteriorhodopsin (purple membrane) at different temperatures (294 K, 98 K and 4 K) were recorded as the diffraction spots faded due to radiation damage. The patterns were then computationally evaluated in order to obtain a quantitative measurement of the structural preservat ion while irradiating the specimen. To provide statistically significa nt results, diffraction spots corresponding to spacings of 3 Angstrom (1200 spots) and 7 Angstrom (600 spots) were measured. A substantial i ncrease of the lifetime of high resolution spots was found using liqui d nitrogen as a coolant, whereas further structural preservation at li quid helium temperature was significant but smaller. It appears likely therefore that high resolution images are accessible even at liquid n itrogen temperature. Mechanical stability and the absence of thermal s pecimen drift are certainly of equivalent importance for successful hi gh resolution imaging.