Study on energy transfer between carotenoid and chlorophyll a in cytochrome b6f complex from Bryopsis corticulans

High-light induced superoxide radical formation in cytochrome b₆f complex from Bryopsis corticulans as detected by EPR spectroscopy

The generation of superoxide radical (O₂·⁻) in Cyt b₆f of Bryopsis corticulans under high light illumination was studied using electron paramagnetic resonance (EPR) spectroscopy. This could be evidenced by the addition of SOD which specifically reacted with O₂·⁻. The generation of O₂·⁻ was lost in the absence of oxygen and was found to be suppressed in the presence of NaN₃ and be scavenged by extraneous antioxidants such as ascorbate, β-carotene and glutathione which could also scavenged ¹O₂*. These results indicated that O₂·⁻ which produced under high light illumination in Cyt b₆f of B. corticulans might rise from a reaction which ¹O₂* could participated in. Also the photo-protection mechanism to Cyt b₆f complex by antioxidants which might contain in thylakoid was speculated.

High-light induced singlet oxygen formation in cytochrome b(6)f complex from Bryopsis corticulans as detected by EPR spectroscopy

Electron paramagnetic resonance (EPR) spectroscopy was used to detect the light-induced formation of singlet oxygen ((1)O(2)*) in the intact and the Rieske-depleted cytochrome b(6)f complexes (Cyt b(6)f) from Bryopsis corticulans, as well as in the isolated Rieske Fe-S protein. It is shown that, under white-light illumination and aerobic conditions, chlorophyll a (Chl a) bound in the intact Cyt b(6)f can be bleached by light-induced (1)O(2)*, and that the (1)O(2)* production can be promoted by D(2)O or scavenged by extraneous antioxidants such as l-histidine, ascorbate, beta-carotene and glutathione. Under similar experimental conditions, (1)O(2)* was also detected in the Rieske-depleted Cyt b(6)f complex, but not in the isolated Rieske Fe-S protein. The results prove that Chl a cofactor, rather than Rieske Fe-S protein, is the specific site of (1)O(2)* formation, a conclusion which draws further support from the generation of (1)O(2)* with selective excitation of Chl a using monocolor red light.

Singlet oxygen formation and chlorophyll a triplet excited state deactivation in the cytochrome b6f complex from Bryopsis corticulans

We have attempted to investigate the correlation between the detergent-perturbed structural integrity of the Cyt b (6) f complex from the marine green alga Bryopsis corticulans and its photo-protective properties, for which the nonionic detergents n-octyl-beta-D-glucopyranoside (beta-OG) and n-dodecyl-beta-D-maltoside (beta-DM), respectively, were used for the preparation of Cyt b (6) f, and the singlet oxygen ((1)O(2)*) production as well as the triplet excited-state chlorophyll a ((3)Chl a*) formation and deactivation were examined by spectroscopic means. Near-infrared luminescence of (1)O(2)* (approximately 1,270 nm) on photo-irradiation was detected for the beta-OG preparation where the complex is mainly in oligomeric state, but not for the beta-DM one in which the complex exists in dimeric form. Under anaerobic condition, photo-excitation of Chl a in the beta-DM preparation generated (3)Chl a* with a lower quantum yield of Phi(T) approximately 0.02 and a longer lifetime of approximately 600 micros with respect to those as in the case of beta-OG preparation, Phi(T) approximately 0.12 and 200-300 micros. These results prove that the enzymatically active and intact Cyt b (6) f complex on photo-excitation tends to produce little (3)Chl a* or (1)O(2)*, which implies that the pigment-protein assembly of Cyt b (6) f complex per se is crucial for photo-protection.

Hydrogen peroxide-induced chlorophyll a bleaching in the cytochrome b6f complex: a simple and effective assay for stability of the complex in detergent solutions

The instability of cytochrome b ( 6 ) f complex in detergent solutions is a well-known problem that has been studied extensively, but without finding a satisfactory solution. One of the important reasons can be short of the useful method to verify whether the complex suspended in different detergent is in an intact state or not. In this article, a simple and effective assay for stability of the complex was proposed based on the investigation on the different effects of the two detergents, n-octyl-beta-D: -glucopyranoside (OG) and dodecyl-beta-D: -maltoside (DDM), on the properties of the complex. DDM stabilizes the complex preparation more effectively whereas OG denatures the interactions of the heme groups and pigment molecules with the protein environment, leading to the bleaching of chlorophyll a induced by addition of hydrogen peroxide. The assay of the use of hydrogen peroxide to characterize the complex by studying the bleaching of chlorophyll induced by hydrogen peroxide and the peroxidase activity of the complex was discussed. This simple method will probably be useful to study the stability of the complex.

Ultrafast carotenoid-to-chlorophyll singlet energy transfer in the cytochrome b6f complex from Bryopsis corticulans

Ultrafast carotenoid-to-chlorophyll (Car-to-Chl) singlet excitation energy transfer in the cytochrome b(6)f (Cyt b(6)f) complex from Bryopsis corticulans is investigated by the use of femtosecond time-resolved absorption spectroscopy. For all-trans-alpha-carotene free in n-hexane, the lifetimes of the two low-lying singlet excited states, S(1)(2A(g)(-)) and S(2)(1B(u)(+)), are determined to be 14.3 +/- 0.4 ps and 230 +/- 10 fs, respectively. For the Cyt b(6)f complex, to which 9-cis-alpha-carotene is bound, the lifetime of the S(1)(2A(g)(-)) state remains unchanged, whereas that of the S(2)(1B(u)(+)) state is significantly reduced. In addition, a decay-to-rise correlation between the excited-state dynamics of alpha-carotene and Chl a is clearly observed. This spectroscopic evidence proves that the S(2)(1B(u)(+)) state is able to transfer electronic excitations to the Q(x) state of Chl a, whereas the S(1)(2A(g)(-)) state remains inactive. The time constant and the partial efficiency of the energy transfer are determined to be 240 +/- 40 fs and (49 +/- 4)%, respectively, which supports the overall efficiency of 24% determined with steady-state fluorescence spectroscopy. A scheme of the alpha-carotene-to-Chl a singlet energy transfer is proposed based on the excited-state dynamics of the pigments.