Variability of light-induced circular dichroism spectra of photosystem I complexes of cyanobacteria

Abstract

Circular dichroism (CD) spectra of photosystem I (PSI) complexes of the cyanobacteria Thermosynechococcus elongatus, Arthrospira platensis and Synechocystis sp. PCC 6803 were studied. CD spectra of dark-adapted PSI trimers and monomers, measured at 77 K, show common bands at 669-670(+), 673(+), 680(-), 683-685(-), 696-697(-), 702(-) and 711(-) nm. The intensities of these bands are species specific. In addition, bands at 683-685(-) and 673(+) nm differ in intensity for trimeric and monomeric PSI complexes. CD difference spectra (P700(+)-P700) of PSI complexes at 283 K exhibit conservative bands at 701(-) and 691(+) nm due to changes in resonance interaction of chlorophylls in the reaction center upon oxidation of P700. Additional bands are observed at 671(-), 678(+), 685(-), 693(-) nm and in the region 720-725 nm those intensities correlate with intensities of analogous bands of antenna chlorophylls in dark-adapted CD spectra. It is suggested that the variability of CD difference spectra of PSI complexes is determined by changes in resonance interaction of reaction center chlorophylls with closely located antenna chlorophylls.