Computer-controlled spot-scan imaging of crotoxin complex crystals with 400 keV electrons at near-atomic resolution.

400 keV electrons yield a better relative image contrast than 100 keV electrons for a beam-sensitive organic crystal when spot-scan imaging is used [J. Brink and W. Chiu, J. Microscopy 161 (1991) 279]. A FORTRAN 77 program has been written to operate the spot-scan imaging system on a computer workstation under the VMS operating system which is interfaced serially to the JEOL4000 electron microscope. We demonstrate the application of this implementation by imaging crotoxin complex crystals embedded in either vitreous ice or glucose to 2.5 A resolution. The intensity strength of the structure factors of this protein crystal are different at low (> 10 A) resolution but similar at high resolution (< 10 A) for the two embedding media as expected from their scattering contrast difference. Based on our experience as judged from the electron diffraction patterns of highly tilted crystals, flat crystals embedded in glucose can be readily obtained. Furthermore, our spot-scan imaging system also has the option of correcting the focus gradient that is present in images of tilted specimens.