Partial characterization of six chlorophyll a-protein complexes isolated from a blue-green alga by a nondetergent...

The chlorophyll a-protein complexes of a blue-green alga, Phormidium luridum, are resolved in the absence of detergent, by the combination of a Sepharose 4B column and sucrose density gradient centrifugation, into six chlorophyll a-containing zones. These six complexes are termed here as F15, F25, F35, F40, F60, and F65, according to their appearance in the sucrose density gradient after centrifugation. The absorption spectra of these six isolated complexes are reported, as well as the fluorescence emission spectra at room temperature and liquid-nitrogen temperature. The F60 complex was enriched in Photosystem I, while the F35 and F40 complexes contained both Photosystem I and II. The F60 complex is the predominant band and accounts for about 49% of the total chlorophyll a of the cells. The extinction coefficient of this complex is determined to be 68.7 mM-1 cm-1 at 680 nm in 50 mM tris(hydroxymethyl)amino methane buffer at pH 8.0. In addition, the effect of the detergent, sodium dodecyl sulfate, on these spectra are also reported for comparison. The chemically induced difference spectra of F35, F40, and F60 complexes also indicate the presence of the reaction center, P700, of Photosystem I. These three complexes have been shown to contain P700 in a ratio of approximately one reaction center molecule per 100 light-harvesting chlorophyll molecules. The simple exposure of the F60 fraction to sodium dodecyl sulfate results in an "apparent" enhancement of the P700 to chlorophyll ratio to one P700 per 51 light harvesting chlorophyll. Room temperature electron spin resonance measurements of photooxidized F60 are consistent with the presence of P700 and with the chlorophyll/P700 ratio observed by chemical assay. The amino acid compositions of F60 and F65 complexes are studied. Gel electrophoresis patterns of these six isolated complexes are presented and are significantly different from those reported for detergent-treated chlorophyll-protein complexes.