Photoinduced electron transfer in proteins

Leben ist die Daseinsweise der Eiweifikorper, und diese Daseinsweise besteht wesentlich in der bestdndigen Selbsterneuerung der chemischen Bestandteile dieser Korper. [Life is the existence of protein structures, and this existence consists essentially in the constant selfrenewal of the chemical components of these structures.] 

In this chapter we review the application of metal complexes and light in the study of biological electron transfer events. 

Bioinorganic Photochemistry Grazyna Stochel, Malgorzata Brindell, Wojciech Macyk, Zofia Stasicka, Konrad Szacilowski 2009 Grazyna Stochel, Malgorzata Brindell, Wojciech Macyk, Zofia Stasicka, Konrad Szacilowski. ISBN 978-1 -405-16172-5 

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Callis PR, Liu T (2006) Short range photoinduced electron transfer in proteins QM-MM simulations of tryptophan and flavin fluorescence quenching in proteins. Chem Phys 326 (l) 230-239... 

Most of the interest in mimicing aspects of photosynthesis has centered on a wide variety of model systems for electron transfer. Among the early studies were experiments involving photoinduced electron transfer in solution from chlorophyll a to p-benzoquinone (21, 22) which has been shown to occur via the excited triplet state of chlorophyll a. However, these solution studies are not very good models of the in vivo reaction center because the in vivo reaction occurs from the excited singlet state and the donor and acceptor are held at a fixed relationship to each other in the reaction-center protein. 

Thermal and Photoinduced Long Distance Electron Transfer in Proteins and in Model Systems... 

Willner I, Willner B. Vectorial photoinduced electron-transfer in tailored redox-active proteins and supramolecular nanoparticle arrays. Coord Chem Rev 2003 245 139-51. 

Fig. 17 Photoinduced electron transfer in Re-derivatized Pseudomonas aeruginosa azurin (Az) by hopping through a proximal tryptophan Re,(CO)3(Me2-phen)(H124)l(W122)IAzCu. Only the part of the protein structure that bears the Re chromophore (at the left end of the chain) is shown. The trp indole group is nearly parallel with the phen ligand, sticking up from the chain. The Cu center is on the right. Reprinted with permission from [43]...

A. Chakraborty, D. Seth, P. Setua, and N. Sarkar, J. Phys. Chem. B, 110, 16607 (2006). Photoinduced Electron Transfer in a Protein-Surfactant Complex Probing the Interaction of SDS with BSA. 

The amide functionality plays an important role in the physical and chemical properties of proteins and peptides, especially in their ability to be involved in the photoinduced electron transfer process. Polyamides and proteins are known to take part in the biological electron transport mechanism for oxidation-reduction and photosynthesis processes. Therefore studies of the photochemistry of proteins or peptides are very important. Irradiation (at 254 nm) of the simplest dipeptide, glycylglycine, in aqueous solution affords carbon dioxide, ammonia and acetamide in relatively high yields and quantum yield (0.44)202 (equation 147). The reaction mechanism is thought to involve an electron transfer process. The isolation of intermediates such as IV-hydroxymethylacetamide and 7V-glycylglycyl-methyl acetamide confirmed the electron-transfer initiated free radical processes203 (equation 148). 

Experimental evidence for long-range electron transfer in polypeptides and proteins had been early accrued.The value of using a metal center as a marker is apparent from the above. The approach can be extended to electron transfer between two proteins which are physiological partners. Metal substitution (e. g. Zn for Fe) can be used to alter the value of AG° and permit photoinduced initiation. The parabolic behavior predicted by (5.86) has been verified for the electron transfer rate constant vs AG° within the adduct between cyt c and cyt bj." ... 

In recent years, RMs were reported to be applicable in diverse areas other than proteins and enzymes, which deserve a note here. These include extraction and determination of metals and metal ions, photoinduced electron transfer, RMs in supercritical liquids, extraction of antibiotics/drugs, synthesis of nanoparticles, etc. 

A chromophore such as the quinone, ruthenium complex, C(,o. or viologen is covalently introduced at the terminal of the heme-propionate side chain(s) (94-97). For example, Hamachi et al. (98) appended Ru2+(bpy)3 (bpy = 2,2 -bipyridine) at one of the terminals of the heme-propionate (Fig. 26) and monitored the photoinduced electron transfer from the photoexcited ruthenium complex to the heme-iron in the protein. The reduction of the heme-iron was monitored by the formation of oxyferrous species under aerobic conditions, while the Ru(III) complex was reductively quenched by EDTA as a sacrificial reagent. In addition, when [Co(NH3)5Cl]2+ was added to the system instead of EDTA, the photoexcited ruthenium complex was oxidatively quenched by the cobalt complex, and then one electron is abstracted from the heme-iron(III) to reduce the ruthenium complex (99). As a result, the oxoferryl species was detected due to the deprotonation of the hydroxyiron(III)-porphyrin cation radical species. An extension of this work was the assembly of the Ru2+(bpy)3 complex with a catenane moiety including the cyclic bis(viologen)(100). In the supramolecular system, vectorial electron transfer was achieved with a long-lived charge separation species (f > 2 ms). 

A simple approach to understanding the factors which control the conductivity of proteins towards photoinduced electron tunneling is to develop small molecule model systems to mimic intramolecular electron transfer in the protein systems. Appropriate models obviously require that the donor and acceptor be held at fixed distances and orientations which correspond to those in the protein-protein complexes. Models of this type have recently been obtained and investigated [296, 297], In these models the protein matrix is replaced by a simple synthetic spacer which separates two porphyrin molecules. By changing the chemical structure of the spacer, a series of molecules with different reaction distances and geometries has been synthesized. Typical examples of such molecules are presented in Fig. 34. 

In the natural photosynthetic reaction center, ubiquinones (QA and QB), which are organized in the protein matrix, are used as electron acceptors. Thus, covalently and non-covalently linked porphyrin-quinone dyads constitute one of the most extensively investigated photosynthetic models, in which the fast photoinduced electron transfer from the porphyrin singlet excited state to the quinone occurs to produce the CS state, mimicking well the photo synthetic electron transfer [45-47]. However, the CR rates of the CS state of porphyrin-quinone dyads are also fast and the CS lifetimes are mostly of the order of picoseconds or subnanoseconds in solution [45-47]. A three-dimensional it-compound, C60, is super-... 

The extremely rapid rate of formation of bathorhodopsin as compared to isomerization rates observed in model protonated Schiff bases (a factor of 103) suggested the idea that electron transfer between an amino acid residue (e.g., tyrosine or tryptophan) in the protein and the chromophore may catalyse isomerization. Thus, a photoinduced electron transfer leading to a radical anion chromophore, instead of complete cis-trans isomerization, was considered as a plausible alternate mechanism for the primary event [200], This mechanism, however, is difficult to reconcile with the known photoreversibility but thermal irreversibility of the bleaching process. Thermal irreversibility of the light-induced electron transfer would require geometrical separation of donor and acceptor moieties which would then not allow photoreversibility [201]. 

The combined systems of the photosensitizer with micelle, LB film, protein, and so on are interesting, because we can expect that new functions will be found in such systems. In some pioneering works, such ideas were applied to stereoselective photoinduced electron transfer reactions. We believe that one can construct the new photoreaction with such combined systems and also expect the enhancement of stereoselectivity in such systems. 

The 7i-stacked bases of ds DNA might be expected to provide a better medium for bridge-mediated electron transfer than the sigma bonds of proteins or hydrocarbons. It has in fact been proposed by Turro and Barton [18d] that ultrafast photoinduced electron transfer processes involving intercalated donors and acceptors can occur with little or no distance dependence. According to this paradigm, duplex DNA can function as a molecular wire or r-way . 

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