Carotenoid triplet yields in normal and deuterated Rhodospirillum rubrum

Abstract

Quantum yields of carotenoid triplet formation in Rhodospirillum rubrum wild type and fully deuterated cells and chromatophores were determined in weak laser flashes for excitation wavelength λ i = 530 nm (mainly absorbed by the carotenoid spirilloxanthin) and for λ i = 608 nm (mainly absorbed by bacteriochlorophyll) in the presence and absence of magnetic fields. All experiments were performed at room temperature and in the absence of oxygen. The quantum yield of reaction center bacteriochlorophyll oxidation in wild type preparations, in which all reaction centers are in state P I X, at λ i = 608 nm is close to unity, whereas the quantum yield of antenna carotenoid triplet formation is low (about 5%); P is the primary electron donor, a bacteriochlorophyll dimer, I the primary acceptor, a bacteriopheophytin, and X the secondary acceptor, an iron-ubiquinone complex. In cells in which the reaction centers are in the state P +I X (−), the antenna carotenoid triplet yield is about 0.2. In contrast, at λ i = 530 nm, the quantum yield of P + formation is relatively low (0.3) and the yield of the antenna carotenoid triplet state in state P I X unusually high (0.3). At increasing light intensities of 530 nm only about 3 carotenoids per reaction center of the 15 carotenoids present are efficiently photoconverted into the triplet state, which indicates that there are two different pools of carotenoids, one with a low efficiency for transfer of electronic excitation to bacteriochlorophyll and a high yield for triplet formation, the other with a high transfer efficiency and a low triplet yield. The absorption difference spectrum of the antenna carotenoid triplet, excited by 608 or 530 nm light in the state P +I X (−) does not show the peak at 430 nm, that is present in the difference spectrum of the reaction center carotenoid triplet, mainly observed at λ i = 608 nm with weak flashes. The yield of the reaction center carotenoid triplet, generated in chromatophores in the state P I X −, is decreased by about 10% by a magnetic field of 0.6 T. In a magnetic field of 0.6 T the yield of the carotenoid triplet, formed by 530 nm excitation in chromatophores at ambient redox potential, is decreased by about 45%. The high quantum yield of formation and the pronounced magnetic field effect for the carotenoid triplet generated by direct excitation at 530 nm can be explained by assuming that this triplet is not formed by intersystem crossing, but by fission of the singlet excitation into two triplet excitations and subsequent annihilation (triplet pair mechanism), or by charge separation and subsequent recombination (radical pair mechanism). Fully deuterated bacteria give essentially the same triplet yields, both in the reaction center and in the antenna carotenoids and show the same magnetic field effects as non-deuterated samples. This indicates that hyperfine interactions do not play a major role in the dephasing of the spins in the radical pair P +I − nor in the formation of the antenna carotenoid triplet.