Spectral Properties of Chlorophyll f in the B800 Cavity of Light‐harvesting Complex 2 from the Purple Photosynthetic Bacterium Rhodoblastus acidophilus

The interactions of chlorophyll (Chl) and bacteriochlorophyll (BChl) pigments with the polypeptides in photosynthetic light‐harvesting proteins are responsible for controlling the absorption energy of (B)Chls in protein matrixes. The binding pocket of B800 BChl a in LH2 proteins, which are peripheral light‐harvesting proteins in purple photosynthetic bacteria, is useful for studying such structure–property relationships. We report the reconstitution of Chl f, which has the formyl group at the 2‐position, in the B800 cavity of LH2 from the purple bacterium Rhodoblastus acidophilus. The Qy absorption band of Chl f in the B800 cavity was shifted by 14 nm to longer wavelength compared to that of the corresponding five‐coordinated monomer in acetone. This redshift was larger than that of Chl a and Chl b. Resonance Raman spectroscopy indicated hydrogen bonding between the 2‐formyl group of Chl f and the LH2 polypeptide. These results suggest that this hydrogen bonding contributes to the Qy redshift of Chl f. Furthermore, the Qy redshift of Chl f in the B800 cavity was smaller than that of Chl d. This may have arisen from the different patterns of hydrogen bonding between Chl f and Chl d and/or from the steric hindrance of the 3‐vinyl group in Chl f. Chlorophyll (Chl) f, which has the formyl group at the 2‐position of the cyclic tetrapyrrole, was reconstituted into the binding site of B800 bacteriochlorophyll a in LH2 protein from the purple photosynthetic bacterium Rhodoblastus acidophilus. The 2‐formyl group of Chl f was hydrogen‐bonded with the LH2 polypeptide. This hydrogen bonding is suggested to contribute to the redshift of the Qy band of Chl f by insertion into the B800 cavity.