Energy Transfer Dynamics in Light-Harvesting Complex 2 Variants Containing Oxidized B800 Bacteriochlorophyll a

Abstract

Excitation energy transfer (EET) in light-harvesting proteins is vital for photosynthetic activities. The pigment compositions and their organizations in these proteins are responsible for the EET functions. Thus, changing the pigment compositions in light-harvesting proteins contributes to a better understanding of EET mechanisms. In this study, we investigated the EET dynamics of two light-harvesting complex 2 (LH2) variants, in which nine B800 bacteriochlorophyll (BChl) a pigments were entirely or half converted to 3-acetyl chlorophyll (AcChl) a. The AcChl a pigments showed a Qy band, which was blue-shifted by 107 nm from B800 BChl a in the two variants. EET from AcChl a to B850 BChl a was observed in both fully oxidized and half-oxidized LH2 variants, but the EET rates were lower than that from B800 to B850 BChl a. EET from AcChl a to the co-present B800 was barely detected in the half-oxidized LH2. The preferential EET from AcChl a to B850 instead of B800 was rationalized by little spectral overlap of AcChl a with B800 BChl a and the pigment geometry in the protein. The EET rate from B800 to B850 BChl a in the half-oxidized LH2 was analogous to that in native LH2, indicating that partial oxidation of B800 did not disturb the EET channel from the residual B800 to B850.

Conversion of B800 Bacteriochlorophyll a to 3-Acetyl Chlorophyll a in the Light-Harvesting Complex 3 by In Situ Oxidation

“…Notably, the rate of the intracomplex EET observed in oxidized LH3 was slightly higher than that in oxidized LH2 (3.6 ps), under the same conditions . Given that the geometries of the chromophores are almost identical between LH3 and LH2, this difference can be rationalized by assuming an increase in the spectral overlap between the luminescence band of AcChl a and the long tail of the B820 BChl a absorption band in oxidized LH3 caused by a blue shift of the B820 absorption band compared with the B850 absorption band in LH2.…”

“…The time constants of the EET processes in native LH3 and oxidized LH3 by the current TA measurements are summarized in Table . The difference in the time constant for native LH3 based on the temporal change of the energy donor from that based on the temporal change of the energy acceptor is mostly ascribable to simultaneous relaxation of the B820 excited state with EET. , …”

“…Chlorophyllous pigments, which were extracted from LH3 samples using methanol, were analyzed by reverse-phase high-performance liquid chromatography (HPLC). Transient absorption (TA) measurements were conducted on native LH3 and oxidized LH3 samples by excitation at 800 and 690 nm, respectively (Figure S1), according to previously published reports . Differential absorbance (Δabsorbance) of the samples was obtained by subtracting the absorbance without the photoexcitation from that with the excitation.…”

“…In this context, the replacement of native (B)­Chl chromophores with other chromophores in photosynthetic antenna proteins is an effective approach to unraveling the photofunctional mechanisms of these proteins. In the case of LH2, the replacement of the energy-donating B800 BChl a has been realized via two strategies: the substitution of BChl a with other (B)­Chl pigments via the acid-induced removal of B800 BChl a followed by pigment reconstitution, − and the selective oxidation of B800 BChl a with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) to produce Chl-type pigments. − Given the ease of implementation, the latter approach is particularly advantageous, and it results in a large blue shift of the Q y band of the energy donor. In this study, we applied this strategy to the LH3 protein from the purple photosynthetic bacterium R.…”