Chemically modified reaction centers of photosystem II: Exchange of pheophytin a with 7-deformyl-7-hydroxymethyl-pheophytin b.

Abstract The native pheophytin a (Pheo a ) in isolated reaction centers of photosystem II (PSII RCs) has been chemically exchanged with extraneous 7-deformyl-7-hydroxymethyl-Pheo b (7 1 -OH-Pheo b ) which differs from Pheo a by the C-7 substituent (hydroxymethyl instead of methyl). The two pigments have similar reduction potentials in vitro [M. Meyer, Dissertation, Universitat Munchen, 1997], while their absorption spectra show small but distinct differences in the visible region. The resulting 7 1 -OH-Pheo b -modified reaction center preparations were characterized by high-performance liquid chromatography, electronic absorption and light-induced Fourier transform infra red absorption difference spectroscopies, together with photoaccumulation of the reduced pheophytin electron acceptor and NaBH 4 -treatment. About 70% of the total Pheo a molecules are found to be replaced by 7 1 -OH-Pheo b molecules in modified preparations, indicating that both the photochemically active (Pheo D1 ) and inactive (Pheo D2 ) binding sites were subjected to pigment exchange. The 7 1 -OH-Pheo b molecule located at the Pheo D1 site is able to functionally replace the native Pheo a , participating in primary charge separation as an electron acceptor. The Q x absorption band of this modified pheophytin molecule is localized at ~ 546 nm; its Q y band is blue-shifted with respect to the absorption of other reaction center core pigments, being located at ~ 665 nm. The Q y and Q x optical transitions of the 7 1 -OH-Pheo b molecule exchanged into the Pheo D2 site are identified at 677 and 543.5 nm, respectively. The photochemically active double-modified PSII RCs additionally containing 7-deformyl-7-hydroxymethyl-13 1 -deoxo-13 1 -hydroxy-Pheo b at the Pheo D2 site were obtained by treatment of the 7 1 -OH-Pheo b -modified RCs with NaBH 4 .