In the dark the crystal suspensions were put on carbon-coated electron microscopy grids and frozen rapidly in liquid ethane (13). Images were taken at liquid nitrogen temperature using a high voltage Hitachi field emission microscope and a GATAN cold stage. Micrographs were recorded under low dose conditions using a spot scan procedure (14).
Scanned areas were processed with the MRC programs (12, 13, 15-17). Distortions of the crystalline areas were removed in an interactive two- or three-step procedure using a progressively improved reference of the crystal. The crystalline areas were boxed off before amplitudes and phases were extracted from the Fourier transform of the unbent images. All data were corrected for effects of the contrast transfer function.
Fig. 2: Fourier components of a crystalline area containing about 2000 unit cells. Signal to noise ratio 1: snr > 7; 2: snr >3.5; 3: snr > 2.3; 4: snr > 1.75. Tilt angle = 4.9°, defocus values: Fx= 7010 Å, Fy= 6060 Å, ø= 0°. Solid circle: first zero of contrast transfer function.
