title: X-ray diffraction studies on photosystem I fragments from a blue-green alga, Anabaena variabilis, and spinach.
abstract: Photosystem I fragments were prepared from thylakoid membranes of a blue-green alga (Anabaena variabilis) and spinach by treatment with a detergent, Triton X-100. Equatorial X-ray diffraction patterns were recorded on films for oriented specimens of thylakoid membranes and photosystem I fragments. The thylakoid membranes and photosystem I fragments gave essentially the same equatorial diffraction patterns in both Anabaena variabilis and spinach, indicating that the major X-ray scatterers in these thylakoid membranes are the molecular assembly of photosystem I. The equatorial X-ray diffraction from the photosystem I fragments of Anabaena variabilis and spinach extends to the reciprocal space of 1/7 A-1. The diffraction pattern exhibits six to nine distinct maxima though they are diffuse, indicating that the arrangement of the constituent molecules in photosystem I has a definite geometrical regularity. The radial autocorrelation functions indicate that the maximal sizes of photosystem I in these thylakoid membranes are about 100 A, and the geometrical regularity does not correspond to a crystalline order. The X-ray diffraction patterns from photosystem I fragments from Anabaena variabilis and spinach are quite similar to each other, suggesting the possibility that the molecular structures of photosystem I in Anabaena variabilis and spinach have a fundamental similarity. These diffraction patterns, however, are different from that of the chromatophore obtained from a photosynthetic bacterium, Rhodospirillum rubrum.
mesh_terms: D000458:Cyanobacteria / Q000032:analysis*; D009249:NADP / Q000032:analysis; D010446:Peptide Fragments / Q000032:analysis; D045322:Photosynthetic Reaction Center Complex Proteins; D045331:Photosystem I Protein Complex; D010940:Plant Proteins / Q000032:analysis*; D010944:Plants / Q000032:analysis*; D013056:Spectrophotometry, Ultraviolet; D014961:X-Ray Diffraction
chemical_list: D010446:Peptide Fragments; D045322:Photosynthetic Reaction Center Complex Proteins; D045331:Photosystem I Protein Complex; D010940:Plant Proteins; D009249:NADP