Circular Dichroism Spectra and the Molecular Arrangement of Bacteriochlorophylls in the Reaction Centers of Photosynthetic Bacteria

Chromatophores, or membrane fragments, prepared from photosynthetic bacteria exhibit a variety of properties which demonstrate that they are able to carry out the quantum conversion steps characteristic of the intact bacterial cells.", 2 The pigment molecules consist of two distinct functional types: an antenna, which includes over 90 per cent of the bacteriochlorophyll molecules, and a smaller fraction incorporated into reaction centers.The primary function of the antenna is to absorb incident photons and to transfer the resulting elec- tronic excitation energy to the reaction center bacteriochlorophyll molecules.It is in these reaction centers that the actual transformation to chemical energy is initiated.Recently, Clayton and others have shown that it is possible to treat chromato- phores of Rhodopseudomonas spheroides so as to remove the antenna bacterio- chlorophylls without harming the reaction centers.3IThese preparations retain the ability to exhibit photoinduced, reversible absorption changes characteristic of bacteriochlorophyll in untreated chromatophores or in intact bacterial cells.In the reaction center preparations, an absorption band at 865 nm (P870) undergoes complete bleaching and a second, larger band at 803 nm (P800) exhibits a concomitant blue shift to 797 nm upon illumination.Added oxidizing agents, such as ferricyanide or chloroiridate, will induce the same absorption changes in the dark, which indicates that the bleaching of the P870 by illumination is a con- sequence of oxidation of bacteriochlorophyll.We have examined the absorption and circular dichroism (CD) spectra of the reaction center preparations both in the reduced (dark) and in the oxidized (illuminated or chemically oxidized) state.The CD spectrum exhibits dramatic changes that accompany the absorption changes.Analysis of the spectral properties suggests that the reduced reaction center contains a trimer of bacteriochlorophyll molecules that are closely coupled and exhibit pronounced exciton interaction.Upon oxidation, one of these three molecules is converted to a form with an entirely different absorption spectrum, and the remaining two molecules no longer exhibit evidence of exciton interaction.Experimental.-Preparations of reaction centers from R. spheroides were obtained from R. K. Clayton and D. W. Reed.Two types of preparations were studied: preliminary