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

The spectral features of energy donors and acceptors and the relationship between them in photosynthetic light-harvesting proteins are crucial for photofunctions of these proteins. Engineering energy donors and acceptors in light-harvesting proteins affords the means to increase our understanding of their photofunctional mechanisms. Herein, we demonstrate the conversion of energy-donating B800 bacteriochlorophyll (BChl) a to 3-acetyl chlorophyll (AcChl) a in light-harvesting complex 3 (LH3) from Rhodoblastus acidophilus by in situ oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone. AcChl a in the B800 site exhibited a Q y band that was 111 nm blue-shifted with respect to BChl a in oxidized LH3. The structure of LH3 was barely influenced by the oxidation process, based on circular dichroism spectroscopy and size-exclusion chromatography evidence. In oxidized LH3, AcChl a transferred excitation energy to B820 BChl a , but the rate of excitation energy transfer (EET) was lower than in native LH3. The intracomplex EET in oxidized LH3 was slightly faster than in oxidized light-harvesting complex 2 (LH2). This difference is rationalized by an increase in overlap of the luminescence band of AcChl a with the long tail of the B820 absorption band in oxidized LH3 compared with that of the B850 band in oxidized LH2.