| C-phycocyanin (CPC), an accessory photosynthetic pigment from the cyanobacterium Agmenellum quadruplicatum was examined using X-ray crystallographic techniques, electron microscopy, and absorbance and fluorescence spectroscopy. Crystals of CPC belong to space group P321 with a = b = 184.5 (ANGSTROM) and c = 60.5 (ANGSTROM). Three dimensional data were collected by the oscillation method. Three isomorphous derivatives were found and conditions refined.;Modeling studies show the crystallographic unit cell to be made up of three ((alpha)(beta))(,6) hexamers centered on the crystallographic tryads and being rotated 30(DEGREES) relative to one another. The hexamers themselves are made up to two ((alpha)(beta))(,3) trimers joined face to face and rotated 60(DEGREES) with respect to one another.;Rotation function studies demonstrated the presence of a molecular six-fold axis of symmetry parallel to the crystallographic tryads as well as molecular dyads intersecting the crystallographic two-fold rotation axes.;The visible absorption of solution CPC and a suspension of CPC crystals showed little difference while crystals of CPC and BPE (B-phycoerythrin, another phycobiliprotein, from the red alga Porphyridium cruentum) fluorescence was red shifted 50-60 nm relative to their solution forms. Biological implications are discussed.;Electron microscopy of crystals sectioned perpendicular to the crystallographic c-axis shows a hexagonal closest packing arrangement of donut-shaped particles about 100 (ANGSTROM) in diameter.;CPC and BPE crystals also demonstrated shortened fluorescence lifetimes relative to their solution counterparts. |
