CHLOROPHYLL-PROTEINS AND REACTION CENTER PREPARATIONS FROM PHOTOSYNTHETIC BACTERIA,ALGAE AND HIGHER PLANTS*,‡

Abstract— The use of sodium dodecyl sulfate to dissociate photosynthetic membranes followed by standard fractionation techniques yields chlorophyll-proteins and reaction center complexes with molecular weights of 500,000 or less. Much about the structure and function of photo-synthetic units in vivo can be deduced from the properties of the isolated complexes. The Bchl-protein from green bacteria is approximated by an incompletely filled sphere ? 80 Â in diameter consisting of four identical subunits. The five Bchl molecules in each subunit are 14 to 20Â apart. The related Chl a-proteins from a blue-green alga and various eukaryotic plants may have similar structural characteristics. The Chl a-protein from a blue-green alga contains one molecule of P700 per 60–90 Chl a molecules. The quantum requirement for P700 oxidation is 2.6 or less. The midpoint potential in various preparations ranges from 0.38 V to 0.42 V. Green algae and higher plants yield a Chl a-protein similar to that from the blue-green alga; in addition they yield another Chl-protein (mol. wt. = 2–3×10⁴) which contains an equal amount of Chl a and Chl b. These two Chl-proteins account for most of the chlorophyll in these organisms. Two photosynthetic bacteria (Rhodopseudomonas viridis and Chromatium) yield protein complexes containing Bchl, carotenoid, and bound cytochromes. The reaction center complex from R. viridis contains P960 (E_m, ₈= 0.39 to 0.42 V), cytochrome 558 (E_m,8= 0.33 V) and cytochrome 553 (E_m,7=— 0.02 V). Quantum requirements for P960 and C558 oxidation are ?2.2 and 3.0, respectively. Complex A from Chromatium contains Bchl 890, P883, cytochrome 556 (E_m,8= 0.34 V) and cytochrome 552 (E_m,7=?0.04 V). The quantum requirement for C556 oxidation is about 15. Both high- and low-potential cytochromes can donate electrons to the reaction center chlorophyll present in either complex. This fact supports the idea that only one kind of photochemical reaction center functions in photosynthetic bacteria. An hypothesis about the nature of the photosynthetic unit in purple bacteria is outlined.