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Tyrosine deprotonation and associated hydrogen bond rearrangements in a photosynthetic reaction center.

ABSTRACT: Photosynthetic reaction centers from Blastochloris viridis possess Tyr-L162 located mid-way between the special pair chlorophyll (P) and the heme (heme3). While mutation of the tyrosine does not affect the kinetics of electron transfer from heme3 to P, recent time-resolved Laue diffraction studies reported displacement of Tyr-L162 in response to the formation of the photo-oxidized P(+•), implying a possible tyrosine deprotonation event. pK(a) values for Tyr-L162 were calculated using the corresponding crystal structures. Movement of deprotonated Tyr-L162 toward Thr-M185 was observed in P(+•) formation. It was associated with rearrangement of the H-bond network that proceeds to P via Thr-M185 and His-L168.

SUBMITTER: Ishikita H

PROVIDER: S-EPMC3200362 | biostudies-literature | 2011

REPOSITORIES: biostudies-literature

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Tyrosine deprotonation and associated hydrogen bond rearrangements in a photosynthetic reaction center.

Ishikita Hiroshi H

PloS one 20111024 10

Photosynthetic reaction centers from Blastochloris viridis possess Tyr-L162 located mid-way between the special pair chlorophyll (P) and the heme (heme3). While mutation of the tyrosine does not affect the kinetics of electron transfer from heme3 to P, recent time-resolved Laue diffraction studies reported displacement of Tyr-L162 in response to the formation of the photo-oxidized P(+•), implying a possible tyrosine deprotonation event. pK(a) values for Tyr-L162 were calculated using the corresp ...[more]

PMID: 22039551

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Project description:Photosynthetic reaction centers convert excitation energy from absorbed sunlight into chemical potential energy in the form of a charge-separated state. The rates of the electron transfer reactions necessary to achieve long-lived, high-energy charge-separated states with high quantum yields are determined in part by precise control of the electronic coupling among the chromophores, donors, and acceptors and of the reaction energetics. Successful artificial photosynthetic reaction centers for solar energy conversion have similar requirements. Control of electronic coupling in particular necessitates chemical linkages between active component moieties that both mediate coupling and restrict conformational mobility so that only spatial arrangements that promote favorable coupling are populated. Toward this end, we report the synthesis, structure, and photochemical properties of an artificial reaction center containing two porphyrin electron donor moieties and a fullerene electron acceptor in a macrocyclic arrangement involving a ring of 42 atoms. The two porphyrins are closely spaced, in an arrangement reminiscent of that of the special pair in bacterial reaction centers. The molecule is produced by an unusual cyclization reaction that yields mainly a product with C(2) symmetry and trans-2 disubstitution at the fullerene. The macrocycle maintains a rigid, highly constrained structure that was determined by UV-vis spectroscopy, NMR, mass spectrometry, and molecular modeling at the semiempirical PM6 and DFT (B3LYP/6-31G**) levels. Transient absorption results for the macrocycle in 2-methyltetrahydrofuran reveal photoinduced electron transfer from the porphyrin first excited singlet state to the fullerene to form a P(•+)-C(60)(•-)-P charge separated state with a time constant of 1.1 ps. Photoinduced electron transfer to the fullerene excited singlet state to form the same charge-separated state has a time constant of 15 ps. The charge-separated state is formed with a quantum yield of essentially unity and has a lifetime of 2.7 ns. The ultrafast charge separation coupled with charge recombination that is over 2000 times slower is consistent with a very rigid molecular structure having a small reorganization energy for electron transfer, relative to related porphyrin-fullerene molecules.

Dataset Information

Tyrosine deprotonation and associated hydrogen bond rearrangements in a photosynthetic reaction center.

Publications

Tyrosine deprotonation and associated hydrogen bond rearrangements in a photosynthetic reaction center.

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