Contrast transfer for frozen-hydrated specimens: determination from pairs of defocused images

Electron imaging of frozen-hydrated biological molecules allows density maps to be obtained directly, without the need for fixatives or stains. The appearance of such maps may, however, be strongly influenced by the contrast transfer properties, which have not previously been evaluated by quantitative experiments. Here we determine the contribution due to amplitude contrast in a typical (approximately 300 A thick) frozen specimen, consisting of arrays of acetylcholine receptor, by comparing pairs of images recorded with different defocuses. We find that this specimen is imaged as a "weak-phase-weak-amplitude" object and that the contribution due to amplitude contrast is 7%.