CHARACTERIZATION OF BONDING INTERACTIONS OF THE INTERMEDIARY ELECTRON-ACCEPTOR IN THE REACTION CENTER OF PHOTOSYSTEM-II BY FTIR SPECTROSCOPY

Molecular changes associated with the photoreduction of the pheophytin a intermediary electron acceptor in films of Photosystem II reaction center (D1D2 RC) were characterized by FTIR spectroscopy. Upon accumulation at 240 K of the photoreduced acceptor, three negative carbonyl bands are observed at 1739 cm-1, 1721 cm-1 and 1677 cm-1 in the light-minus-dark FTIR spectrum of D1D2 RC. The redox-induced FTIR spectrum of the pheophytin a anion generated electrochemically in tetrahydrofuran shows only two negative bands at 1743 cm-1 and 1706 cm-1 which are assigned to changes of absorption of the 10a-ester C=O and 9-keto C=O, respectively. These assignments are based upon the comparison between FTIR data obtained on radicals of pheophytin a and its pyroderivative lacking the 10a-ester C=O. Thus, the 1677 cm-1 band observed in vivo reflects an interacting 9-keto C=O in D1D2 RC. The close similarity observed between: (i) FTIR spectra obtained on Photosystem II and Rps. viridis reaction centers and (ii) amino-acid sequences of the L and D1 polypeptides leads to the assignment of the 1721 cm-1 band in D1D2 RC to a protein-bound 10a-ester C=O of the acceptor and the 1739 cm-1 band to a contribution from the protonated carboxylic group of Glu D1-130 which is proposed to be H-bonded to the 9-keto C=O of the pheophytin acceptor, in the same way as in the Rps. viridis reaction center, Glu L104 is interacting with the 9-keto C=O of HL. The FTIR data indicate that the interactions of the 9-keto C=O and of the 10a-ester of the intermediary acceptor with the protein are stronger in D1D2 RC than in Rps. viridis. These stronger interactions could account, at least in part, for the difference in accessibility to 1H-2H exchange of the H-bonded proton of the Glu D1-130 side-chain in D1D2 RC compared to Rps. viridis reaction center. © 1990.