Orientation of pigments in phycobilisomes of Porphyridium sp. Lewin. A linear dichroism study utilizing electric and gel orientation methods

The linear dichroism spectra of oriented membranes prepared from the red algae Porphyridium sp. Lewin, of isolated phycobilisomes, and of fractions containing either phycoerythrin alone or phycoerythrin and phycocyanin, oriented by pulsed electric fields and by squeezed polyacrylamide gel orientation techniques, were studied. The intact membranes and the phycobilisomes were stabilized by glutaraldehyde fixation. The phycoerythrin- and phycocyanin-containing fractions were derived from dissociated phycobilisomes by osmotic shock and sucrose density gradient centrifugation. In intact membranes the 545 and 570 nm transitions of phycoerythrin, the 620 nm phycocyanin and 655 nm allophycocyanin transitions, as well as the Q_y transition moment of chlorophyll a, appear to have an overall parallel inclination with respect to the membrane plane. Utilizing both the electric field and gel-squeezing methods, similar linear dichroism spectra are obtained for isolated phycobilisomes, except in the region of 570 nm. This difference is ascribed to a partial dissociation of phycoerythrin rods from the phycobilisomes in polyacrylamide gels. A further degradation of the rods into their component discs appears to occur in the presence of glycerol. The linear dichroism spectra of the phycoerythrin, and phycoerythrin + phycocyanin fractions, suggest that the 545 nm phycoerythrin transition is perpendicular to the plane of the discs, while the approx. 570 nm phycoerythrin and the 620 nm phycocyanin transition moments tend to be oriented more parallel to the plane of the discs. In polyacrylamide gels the linear dichroism spectra are superpositions of contributions from intact and from smaller degradation fragments of phycobilisomes. Glutaraldehyde-fixed phycobilisomes in aqueous suspensions (without glycerol or polyacrylamide) appear to be intact as verified by sensitized fluorescence spectroscopy; the electric linear dichroism spectra of these suspensions thus accurately represent the linear dichroism characteristics of integral phycobilisome structures.