The mechanism for proton–coupled electron transfer from tyrosine in a model complex and comparisons with Y _Z oxidation in photosystem II

Abstract

In the water-oxidizing reactions of photosystem II (PSII), a tyrosine residue plays a key part as an intermediate electron-transfer reactant between the primary donor chlorophylls (the pigment P 680 ) and the water-oxidizing Mn cluster. The tyrosine is deprotonated upon oxidation, and the coupling between the proton reaction and electron transfer is of great mechanistic importance for the understanding of the water-oxidation mechanism. Within a programme on artificial photosynthesis, we have made and studied the proton-coupled tyrosine oxidation in a model system and been able to draw mechanistic conclusions that we use to interpret the analogous reactions in PSII.

Theoretical studies of proton-coupled electron transfer: Models and concepts relevant to bioenergetics

“…Sjödin et al explained the pH dependence of the experimentally measured rate constant in terms of a PCET mechanism in which the proton transfers from tyrosine to bulk water. [15][16][17][18] Our theoretical calculations on this model system were consistent with this interpretation. [32] Alternative interpretations of this type of pH-dependence have also been proposed.…”

“…[11][12][13][14][15][16][17][18] For example, Sjodin et al studied tyrosine-bound ruthenium-tris-bipyridine model systems with the flash-quench method. [15][16][17][18] In these experiments, the excited state of Ru is quenched by an external quencher, followed by electron transfer from the tyrosine to the photo-oxidized Ru. Sjödin et al explained the pH dependence of the experimentally measured rate constant in terms of a PCET mechanism in which the proton transfers from tyrosine to bulk water.…”

“…[11][12][13][14][15][16][17][18] In some cases, photoexcitation of the metal in tyrosine-bound metal polypyridyl systems induces electron transfer from the tyrosine to the metal with concurrent proton transfer from the tyrosine to the solvent or the buffer. [14][15][16][17][18] Other experiments involve the chemical oxidation of phenols in solution. [11,12] Often the data are interpreted in the context of Marcus theory for electron transfer.…”

Photo-induced cytotoxicity and anti-metastatic activity of ruthenium(ii)–polypyridyl complexes functionalized with tyrosine or tryptophan

“…18,19 Both complexes were characterized using a range of spectroscopic and analytical techniques. All these spectroscopic data, as well as the results of elemental analysis, confirm the desired purity for both complexes utilized in our studies ( fig.…”

Coupled electron transfers in artificial photosynthesis

“…In contrast, the structure of the cluster, and the mechanism by which it catalyses water oxidation, are controversial and not yet fully understood. The reactions and mechanistic principles of PSII provide important inspiration and information for biomimetic chemistry, which attempts to mimic the fundamental photosynthetic processes in synthetic molecular systems ( Wasielewski 1992;Ruttinger & Dismukes 1997;Gust et al 2001;Sun et al 2001; Hammarströ m 2003; Mukhopadhyay et al 2004;Alstrum-Acevedo et al 2005). The goals are to improve and generalize our understanding and to ultimately pave the way for a molecular solar-energy-conversion technology by artificial photosynthesis.…”