SPECTRAL PROPERTIES OF THE PHOTOSYNTHETIC REACTION UNITS RECONSTITUTED FROM BACTERIAL REACTION CENTERS AND ANTENNA PIGMENTS LOCATED IN LIPOSOMES SUSPENDED IN BUFFER OR ORDERED IN LANGMUIR-BLODGETT-FILMS

Artificial photosynthetic reaction units reconstituted from photosynth etic bacterial reaction centres and antenna pigments (bacteriochloroph yll a or chlorophyll a) and built in liposome vesicles were investigat ed. Samples were suspended in buffer or ordered in Langmuir-Blodgett f ilms. The absorption, photoacoustic and delayed luminescence spectra o f the liposomes in suspension and in Langmuir-Blodgett films deposited on quartz plates were measured. As a reference, suspensions or films containing reaction centres or antenna pigments only, both embedded in liposomes, were investigated, The absorption spectra of the samples c ontaining reaction centres and pigment mixtures were the sum of the ab sorptions of samples containing reaction centres and pigments separate ly. This result shows that, in the mixed sample, strong interactions d ue to the formation of ground state aggregates are absent. The photoac oustic and absorption spectra are different because of the different y ield of thermal deactivation of various chromophores. The yield of the rmal deactivation of the pigments in reaction centres is higher than t hat of additional artificial antenna molecules, This yield is at least partially due to the rather efficient excitation energy transfer from the antenna to reaction centres competing with the thermal deactivati on of excitation. All samples exhibit a delayed luminescence with the spectra located in the spectral regions of the prompt fluorescence of the investigated pigments. A comparison of the intensity of delayed lu minescence of the artificial photoreaction unit with the intensities o f reference samples demonstrates that excitation energy transfer from the antenna to the chromophores of the reaction centres occurs, In lip osome suspensions and in monolayers, the absorption spectra of the sam ples containing reaction centres and pigment mixtures are similar, but the photoacoustic spectra and delayed luminescence spectra are differ ent. This shows that the mutual interactions between the reaction cent re chromophores and antenna pigments in liposomes located in suspensio ns and in monolayers are different. (C) 1997 Elsevier Science S.A.