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Abstract

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 preservation while irradiating the specimen. To provide statistically significant results, diffraction spots corresponding to spacings of 3Å(1200 spots) and 7Å(600 spots) were measured. A substantial increase of the lifetime of high resolution spots was found using liquid nitrogen as a coolant, whereas further structural preservation at liquid helium temperature was significant but smaller. It appears likely therefore that high resolution images are accessible even at liquid nitrogen temperature. Mechanical stability and the absence of thermal specimen drift are certainly of equivalent importance for successful high resolution imaging.