The paths of excitation energy deactivation in LH1 reduced mutant and wild-type strains of Rhodobacter sphaeroides

The P3 mutant of Rhodobacter sphaeroides had an altered ratio of reaction c enter to core (LH1) and peripheral (LH2) antenna complexes compared to the wild-type strain. Intracytoplasmic membranes from these two strains were pu rified and then resuspended in buffer or immobilized in isotropic and stret ched polymer film. The absorption, photoacoustic, and delayed luminescence spectra were measured. The ratios of infrared absorption and photoacoustic bands (located at about 880 nm for LH1 and at 850 and about 800 nm for LH2) as well as the half-width of these bands are different for the LH2 and LH1 mutants and wild-type strain. The whole yields of thermal deactivation of the two strains were comparable, but in the absorption region of LH2 it was slightly lower in the case of the mutant than for the wild-type strain. Th e delayed luminescence main maxima were observed at about 860 and 700 nm. T he first one could be due to emission of bacteriochlorophyll a of LH2 compl exes. The emission at about 700 nm is probably due to dihydromesochlorophyl l, which is usually, to some extent, produced from bacteriochlorophyll a in bacterial complexes. The delayed luminescence emission is competing with e xcitation energy transfer to the reaction center. The intensity of the dela yed luminescence of the mutant strain was higher than that of the wild-type strain when both samples were excited in a region of carotenoid absorption . The mutant contains less carotenoids than the wild-type strain. Carotenoi ds work as efficient antenna. When they at a lower concentration the excita tion can be trapped more easily by some chlorophyll-like pigment isolated f rom the excitation energy chain. The dependences of delayed luminescence sp ectra on the light polarization and excitation wavelengths for the wild-typ e strain and for the mutant were different. The anisotropy of delayed lumin escence showed that bacteriochlorophyll a molecules of different orientatio ns were contributing to the mutant and the wild-type strain emission. All t he results suggest that the excitation energy transfer from the antenna to the reaction center in the mutant and the wild-type strain is similar.