Bacteriochlorophyll triplet states

Carotenoids function in photosynthetic reaction centers (RC) as triplet quenchers of the primary donor chlorophyll or bacteriochlorophyll triplet states. The best studied RCs are those of purple photosynthetic bacteria where atomic models are available based on X-ray crystallography and optical as well as magnetic resonance spectroscopies have yielded a detailed picture of the flow of triplet energy transfer. Good reviews of these topics can be found in (Frank, 1992, 1993 Frank and Cogdell, 1996). 

Lutz M, Kleo J and Reiss-Husson F (1976) Resonance Raman scattering of bacteriochlorophyll, bacteriopheophytin and spheroidene in reaction centers of Rhodopseudomonas spheroides. Biochem Biophys Res Comm 69 711-717 Lutz M, Chinsky L and Turpin PY (1982) Triplet states of carotenoids bound to reaction centers ofphotosynthetic bacteria Time-resolved resonance Raman spectroscopy. Photochem Photobiol 36 503-515... 

Monger TG, Cogdell RJ and Parson WW (1976) Triplet states of bacteriochlorophyll and carotenoids in chromatophores of photosynthetic bacteria. Biochim Biophys Acta 449 136-153... 

A covalent Unkage between two bacteriochlorophyll a molecules results in a species which in the presence of the proper bifunctional ligand (e.g., ethanol) undergoes folding like the chlorophyll a dimer. It does not reproduce the in vivo absorption spectrum, absorbing at 803 nm instead of 870 nm. Yet, it does have a delocalized cation and a delocalized triplet state. 

Photosynthesis. In the photosynthetic process light energy is absorbed by carotenoids and bacteriochlorophylls and transferred to photosynthetic reaction centers. Time-resolved vibrational methods, including transient RR and TR techniques, are well-suited to elucidate the structure and dynamics of the excited (singlet and triplet) states of these chromophores. Nanosecond and picosecond transient RR studies of carotenoid excited states have been conducted by Atkinson and co-workers ... 

The transient optical spectroscopic data show that the borohydride-treated, spheroidene-reconstituted reaction center sample is less able to form carotenoid triplet states than the native Rb. sphaeroides R26 reaction centers that have been reconstituted with spheroidene to the same extent. However, before these results can be attributed to an involvement of the bacteriochlorophyll monomer in the triplet energy transfer process, it is necessary to provide compelling evidence that spheroidene is bound in a single site, in the same environment and with the same structure in both borohydride-treated and native Rb. sphaeroides R26 reaction center samples. This evidence is provided by the following arguments (1) The absorption spectral features shown in Figs. 1 and 2 for the carotenoid in borohydride-treated and untreated, spheroidene-reconstituted complexes are very similar to each other and very different from the carotenoid in either Triton X-100 detergent or pentane. (See Fig. 3.) (2)... 

The rate of another important process, the recombination of the primary product of the charge separation, i.e. the reduced primary acceptor (QA ) and oxidised primary donor, bacteriochlorophyl dimer (P+), falls from 103 to 102 s 1 when dynamic processes with uc = 103 s 1 monitored by the triplet labelling method occur. Very fast electron transfer from P+ to bacteriochlorophyl (Bchl) and from (Bchl) to QA does not depend on media dynamics and occurs via conformationally non-equilibrium states (Fig. 3.16). 

The T state was generated by excitation of the bacteriochlorophyll Qx absorption at 600 nm and subsequent triplet-energy transfer to the carotenoid, and the T Raman spectra were recorded by the use of 532 nm pulses [18]. 

In RCs where Bg is exchanged for [3-vinyl]-13 -hydroxy-bacteriochlorophyll a and then spheroidene-reconstituted, a MIA band shifts from 813 to 776 nm. This bleaching which has also been observed by MODS spectroscopy (Lous and Hoff, 1989) may be explained by a partial delocalization of the triplet excitation between spheroidene and Bg and allows the unequivocal determination of the ground state absorption of Bg (812 nm) which is obscured by the absorption of Ba (804 mn) and tbe upper exciton component of PV70 around 807 mn (Hartwich et al., 1995). The conclusion from these observations are that ... 

The luminescence originates from a Cp biq LLCT triplet. The complex shares some remarkable features with the photosynthetic reaction center which is characterized by a pair of two bacteriochlorophyll molecules in close proximity but in an oblique orientation ( special pair ). In the primary photochemical step charge separation takes place by excited state electron transfer from this special pair to an acceptor. In the case of [Cp2Zr biq] + the special pair is represented by both Cp ligands. However, in this complex charge separation takes place by a direct optical transition. 

Here we present experimental evidence that the monomeric bacteriochlorophyll is required for triplet energy transfer from the primary donor to the carotenoid in photosynthetic bacterial reaction centers. Our approach is to use sodium borohydride to extract the monomeric bacteriochlorophyll from the reaction centers of the carotenoidless mutant Rb. sphaeroides R26 [3, 4]. The borohydride treated reaction centers are then reconstituted with the carotenoid, spheroidene [5], and the ability of the reaction center complex to carry out the primary donor-to-carotenoid triplet transfer reaction was examined by transient optical spectroscopy. Steady state optical absorption and circular dichroism (CD) measurements demonstrate diat spheroidene reconstituted into borohydride-treated Rb, sphaeroides R26 reaction centers is bound in a single site, in the same environment and with the same structure as spheroidene reconstituted into native Rb. sphaeroides R26 reaction centers. It is shown herein that the primary donor-to-carotenoid triplet transfer reaction is inhibited in the absence of the accessory bacteriochlorophyll. 

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