Thin three-dimensional, hexagonal crystals of the light—harvesting chlorophyll a/b—protein complex (LHC II) from pea chloroplast membranes diffract electrons to 3.7 Å resolution when preserved in glucose or tannin. The symmetry of the diffraction pattern (6mm), the dimensions of the unit cell in projection (a = 127 Å) and micrographs of negatively stained specimens suggested that the hexagonal crystals were stacks of two-dimensional crystals of p321 symmetry. Low—dose (1 —2 electrons/Å2) electron micrographs of thin three-dimensional crystals preserved in tannin were recorded in an attempt to determine the structure of this integral membrane protein complex at high resolution, initially in projection. The best images showed sharp diffraction spots at 12 — 14 Å resolution when examined by optical diffraction. Image areas measuring up to 40 x 40 mm2 were densitometered at a step size corresponding to 3 Å or less at the specimen and computer processed to correct for lattice distortions, using programmes by R.Henderson and J.M.Baldwin. Fourier transforms of the distortion—corrected images showed reflections above background level to 6 Å resolution.
Determination of the aggregate size in detergent solution of the light-harvesting chlorophyll a/b-protein complex from chloroplast membranes