CONTRAST TRANSFER FOR FROZEN-HYDRATED SPECIMENS .2. AMPLITUDE CONTRAST AT VERY LOW-FREQUENCIES

The contrast transfer properties of frozen-hydrated specimens were examined quantitatively for low spatial frequencies (1/350-1/160 angstrom-1). Specimens used were tubular crystals of two membrane proteins, the acetylcholine receptor and calcium-ATPase. These specimens were found to behave as weak-phase-weak-amplitude objects. The contribution of amplitude contrast was 5.8% at 120 kV and 4.8% at 200 kV accelerating voltage. Combined with previous results, the level of amplitude contrast showed little dependence on the spatial frequency over the range examined so far (1/350-1/35 angstrom-1).