Abstract. We report the measurement of the room-temperature optical properties of antenna chlorophyll proteins (prepared with sodium dodecyl sulfate membrane solubilization) from six plant species of five different orders: pea (Pisum sativum), soy (Glycine max), barley (Hordeum vulgare), spinach (Spinacea oleracea), yew (Taxus canadensis), and fern (Onoclea sensibilis). All six species' antenna protein preparations exhibitopticalspectra which are similarin great detail, implying similar internal chromophore arrangements from species to species. Features of the spectra thought due to chromophore interactions disappear upon heat denaturation. A model proposedby Van Metter for spinachchlorophyll protein has been revised and isconsistent with all the data presented here.
INTRODUcnONMost of the energy of the light captured and processed by green plants flows through a large number of chlorophyll molecules on its way to the primary reaction centers. These chlorophylls are held in place by bonding to proteins and act as an "antenna," insofar as they effectively increase the absorption cross section of the reaction centers with which they communicate. This chlorophyll-protein nature of the antenna structure has been known for perhaps fifteen years. I The need for a nonrandom, fixed, and fairly dense arrangement for efficient antenna function is twofold: the chromophores must be close enough together to provide reliable excitation transfer pathways by resonance transfer; and they must not combine as dimers which can act as quenchers.A relatively small chlorophyll-protein of molecular weight -35,000 was isolated by Thornber and colleagues in 1966: It has been found in many species' and generally contains three chlorophyll b, three chlorophyll a, and one carotenoid.' We will use the designation CP-II for this protein so prepared, since it is largely associated with photosystem II.' The ordinary absorption spectra of all CP-II appear identical. Scott and Gregory" and Van Metter" measured the circular dichroism spectrum in spinach CP-II. Van Metter also measured fluorescence emission and polarization excitation spectra, for which a consistent model chromophore arrangement was hypothesized. The water insolubility of CP-II prevents an X-ray structure determination such as that done on the bacteriochlorophyll protein of green bacteria."The purpose of this study was to investigate the possible variation of the chromophore arrangment within the CP-II extracted from different higher plants. All of Van Metter's measurements were repeated on six species and special attention was given to the analysis of the circular dichroism (CD) spectra. In the following sections we describe materials and methods, our experimental results, and a CD analysis using a hypothetical chromophore arrangement. We then summarize our findings, one of which is that CP-II (as prepared) appears to have virtually identical chromophore arrangements in the six species studied.
MATERIALS AND METHODSThe antenna protein was isolated from six plant species: pea (Pisu...
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