Light-driven proton transfer

In the purple bacterium Halobactium halobium, light driven proton transfer through membranes is executed by bacteriorhodopsin [41a]. This protein consisting of seven helical fragments criss-crosses the membrane seven times,... 

The CP MAS NMR spectroscopy has been also extensively used for studies of proteins containing retinylidene chromophore like proteorhodopsin or bacteriorhodopsin. Bacteriorhodopsin is a protein component of purple membrane of Halobacterium salinarium.71 7 This protein contains 248 amino acids residues, forming a 7-helix bundle and a retinal chromophore covalently bound to Lys-216 via a Schiff base linkage. It is a light-driven proton pump that translocates protons from the inside to the outside of the cell. After photoisomerization of retinal, the reaction cycle is described by several intermediate states (J, K, L, M, N, O). Between L and M intermediate states, a proton transfer takes place from the protonated Schiff base to the anionic Asp85 at the central part of the protein. In the M and/or N intermediate states, the global conformational changes of the protein backbone take place. 

Gardner JM, Beyler M, Kamahl M, Tschierlei S, Ott S, Hammarstrom L (2012) Light-driven electron transfer between a photosensitizer and a proton-reducing catalyst Co-adsorbed to NiO. J Am Chem Soc 134(47) 19322-19325. doi 10.1021/ja3082268... 

The result is the net transfer of a proton from the cytoplasmic side of the membrane to the extracellular medium. Bacteriorhodopsin is therefore a light-driven proton pump. PRG was soon identified with a group located close to the extracellular side of the membrane, made of Glul94, Glu204 and a network of water molecules, but the precise protonation site has long been controversial. The results of a time-resolved IR study favoured water molecules, but more detailed experiments and free-energy calculations have shown that the proton is actually delocalised over the two Glu residues. ... 

In recent work, Ott efal. have prepared the biomimetic complex 174 in which the diiron unit is covalently linked to a [Ru(terpy)2] photosensitizer, in a first attempt toward making a light-driven proton reduction system. IR spectra of 174 are identical to the parent azadithiolate complex, suggesting that the metal centers are electronically isolated. The observed excited-state lifetime is 6.5 ns, being substantially shorter than related non-iron-containing compounds, and combined with electrochemical measurements this shows that electron transfer from the photogenerated [Ru(terpy)2] excited state to the diiron center is uphill by 0.59 eV. ... 

The light-driven, energy- and electron-transfer processes trigger reactions that finally lead to the oxidation of water, the reduction of NADP+, and the build-up of a proton gradient across the photosynthetic membrane to produce ATP These processes are catalyzed by two membrane-embedded pigment-protein complexes, called... 

Light-Driven Cyciic Eiectron Transfer Coupled to Proton Translocation... 

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