Core antenna complexes, CP43 and CP47, of higher plant photosystem II. Spectral properties, pigment stoichiometry, and amino acid composition

The core antenna complexes of photosystem II CP43 and CP47, were purified from two higher plants by anion-exchange chromatography, using a combination of the chaotropic agent LiClO4 and the nonionic detergent beta-dodecyl maltoside. The Qy transition was resolved at 48 K into two main bands near 682.3 and 671.5 nm for CP43, while the CP47 spectrum showed a more complex structure with main bands at 688, 681.2, 676, 670. 667, and 661 nm. Emission bands (77 K) were detected at 683 and 695 nm for CP43 and CP47, respectively. Fluorescence excitation spectra showed high efficiency of energy transfer between the different transitions of the chlorophylls and a somewhat lower efficiency from beta-carotene. The circular dichroism spectrum of CP47 indicated the presence of excitonic interactions between some chlorophylls. In contrast, CP43 showed a single negative circular dichroism band at 670 nm. The pigment content of the complexes was determined by both spectroscopic measurements and HPLC. Contents of 18 chlorophylls alpha and 5 beta-carotenes per CP43 polypeptide and 19 chlorophylls alpha and 3 beta-carotenes per CP47 polypeptide were found, using the methods of Lowry or Bradford for protein quantitation. When the protein concentration was determined from the amino acid analysis, 20 chlorophylls alpha and 5 beta-carotenes per CP43 and 21-22 chlorophylls alpha and 4 beta-carotenes per CP47 were obtained. Thus, a content of 46-48 chlorophylls alpha was obtained for the core complex, assuming 4-6 chlorophylls per reaction center, in agreement with the composition obtained experimentally using a highly purified oxygen-evolving core complex. This suggested that no pigments were lost during the purification procedure. Moreover, the amino acid analysis of the purified complexes revealed a high homology with the amino acid composition derived from the gene sequences reported for other higher plants.