Effect of 13C-, 18O- and 2H-labeling on the infrared modes of UV-induced phenoxyl radicals

PCET-Based Ligand Limits Charge Recombination with an Ir(III) Photoredox Catalyst

Upon photoinitiated electron transfer, charge recombination limits the quantum yield of photoredox reactions for which the rates for the forward reaction and back electron transfer are competitive. Taking inspiration from a proton-coupled electron transfer (PCET) process in Photosystem II, a benzimidazole-phenol (BIP) has been covalently attached to the 2,2'-bipyridyl ligand of [Ir(dF(CF₃)ppy)₂(bpy)][PF₆] (dF(CF₃)ppy = 2-(2,4-difluorophenyl)-5-(trifluoromethyl)pyridine; bpy = 2,2'-bipyridyl). Excitation of the [Ir(dF(CF₃)ppy)₂(BIP-bpy)][PF₆] photocatalyst results in intramolecular PCET to form a charge-separated state with oxidized BIP. Subsequent reduction of methyl viologen dication (MV²⁺), a substrate surrogate, by the reducing moiety of the charge separated species demonstrates that the inclusion of BIP significantly slows the charge recombination rate. The effect of ∼24-fold slower charge recombination in a photocatalytic phthalimide ester reduction resulted in a greater than 2-fold increase in reaction quantum efficiency.

Pivotal role of the redox-active tyrosine in driving the water splitting catalyzed by photosystem II

TyrZ oxidation state triggers hydrogen bond modification in the water oxidation catalysis.

Three redox states of Trypanosoma brucei alternative oxidase identified by infrared spectroscopy and electrochemistry

Electrochemistry coupled with Fourier transform infrared (IR) spectroscopy was used to investigate the redox properties of recombinant alternative ubiquinol oxidase from Trypanosoma brucei, the organism responsible for African sleeping sickness. Stepwise reduction of the fully oxidized resting state of recombinant alternative ubiquinol oxidase revealed two distinct IR redox difference spectra. The first of these, signal 1, titrates in the reductive direction as an n = 2 Nernstian component with an apparent midpoint potential of 80 mV at pH 7.0. However, reoxidation of signal 1 in the same potential range under anaerobic conditions did not occur and only began with potentials in excess of 500 mV. Reoxidation by introduction of oxygen was also unsuccessful. Signal 1 contained clear features that can be assigned to protonation of at least one carboxylate group, further perturbations of carboxylic and histidine residues, bound ubiquinone, and a negative band at 1554 cm(-1) that might arise from a radical in the fully oxidized protein. A second distinct IR redox difference spectrum, signal 2, appeared more slowly once signal 1 had been reduced. This component could be reoxidized with potentials above 100 mV. In addition, when both signals 1 and 2 were reduced, introduction of oxygen caused rapid oxidation of both components. These data are interpreted in terms of the possible active site structure and mechanism of oxygen reduction to water.

Fourier transform infrared (FTIR) spectroscopy

Fourier transform infrared (FTIR) spectroscopy probes the vibrational properties of amino acids and cofactors, which are sensitive to minute structural changes. The lack of specificity of this technique, on the one hand, permits us to probe directly the vibrational properties of almost all the cofactors, amino acid side chains, and of water molecules. On the other hand, we can use reaction-induced FTIR difference spectroscopy to select vibrations corresponding to single chemical groups involved in a specific reaction. Various strategies are used to identify the IR signatures of each residue of interest in the resulting reaction-induced FTIR difference spectra. (Specific) Isotope labeling, site-directed mutagenesis, hydrogen/deuterium exchange are often used to identify the chemical groups. Studies on model compounds and the increasing use of theoretical chemistry for normal modes calculations allow us to interpret the IR frequencies in terms of specific structural characteristics of the chemical group or molecule of interest. This review presents basics of FTIR spectroscopy technique and provides specific important structural and functional information obtained from the analysis of the data from the photosystems, using this method.

Isotopic labelling of photosystem II in Thermosynechococcus elongatus

This report describes a protocol to incorporate isotopically labelled aromatic amino acids into the proteins of the thermophilic cyanobacterium Thermosynechoccus elongatus. By using the EPR signal of the two redox active tyrosines of Photosystem II, Tyr(D)(*) and Tyr(Z)(*), as spectroscopic probes it is shown that labelled tyrosines can be incorporated with a high yield in this cyanobacterium. The production of a fully (13)C- or (2)H-labelled enzyme is also described.

High sensitivity of human centrin 2 toward radiolytical oxidation: C-terminal tyrosinyl residue as the main target

Centrins are calcium-binding proteins that play a significant role in the maintenance of the centrosomal organization, mainly in the continuity between centrosome and microtubular network. Recent data showed that centrosome duplication abnormalities, like overduplication for example, could be due to hydrogen peroxide, suggesting an important impact of oxidative stress. To challenge this hypothesis, we performed one-electron oxidation experiments with human centrin 2, starting from azide radicals. Our results first revealed several intermolecular cross-links generating dimers, tetramers, hexamers, and higher molecular mass species. Dimers result from covalent bond linking the C-terminal tyrosines of each monomer. Second, the methionyl residue at position 19 was oxidized on the monomeric centrin. Further, electron microscopy experiments on centrin 2 showed a preexisting hexameric organization that was stabilized by covalent bonds as a result of irradiation. Overall, these results show that centrin 2 is highly sensitive to ionizing radiation, which could have important consequences on its biological functions.

Vibrational spectroscopy to study the properties of redox-active tyrosines in photosystem II and other proteins

Tyrosine radicals play catalytic roles in essential metalloenzymes. Their properties--midpoint potential, stability...--or environment varies considerably from one enzyme to the other. To understand the origin of these properties, the redox tyrosines are studied by a number of spectroscopic techniques, including Fourier transform infrared (FTIR) and resonance Raman (RR) spectroscopy. An increasing number of vibrational data are reported for the (modified-) redox active tyrosines in ribonucleotide reductases, photosystem II, heme catalase and peroxidases, galactose and glyoxal oxidases, and cytochrome oxidase. The spectral markers for the tyrosinyl radicals have been recorded on models of (substituted) phenoxyl radicals, free or coordinated to metals. We review these vibrational data and present the correlations existing between the vibrational modes of the radicals and their properties and interactions formed with their environment: we present that the nu7a(C-O) mode of the radical, observed both by RR and FTIR spectroscopy at 1480-1515 cm(-1), is a sensitive marker of the hydrogen bonding status of (substituted)-phenoxyl and Tyr*, while the nu8a(C-C) mode may probe coordination of the Tyr* to a metal. For photosystem II, the information obtained by light-induced FTIR difference spectroscopy for the two redox tyrosines TyrD and TyrZ and their hydrogen bonding partners is discussed in comparison with those obtained by other spectroscopic methods.

Tyrosyl radicals in Photosystem II

In PSII, there are two redox-active tyrosines, D and Z, with different midpoint potentials and different reduction kinetics. The factors responsible for these functional differences have not yet been elucidated. Recent model compound studies of tyrosinate and of tyrosine-containing dipeptides have demonstrated that perturbations of the amino and amide/imide group occur when the tyrosyl aromatic ring is oxidized [J. Am. Chem. Soc. 124 (2002) 5496]. Accompanying density functional calculations suggested that this perturbation is due to spin density delocalization from the aromatic ring onto the amino nitrogen. The implication of this finding is that spin density delocalization may occur in redox-active, tyrosine-containing enzymes, like Photosystem II. In this paper, we review the supporting evidence for the hypothesis that tyrosyl radical spin density delocalizes into the peptide bond in a conformationally sensitive, sequence-dependent manner. Our experimental measurements on tyrosyl radicals in dipeptides have suggested that the magnitude of the putative spin migration may be sequence-dependent. Vibrational spectroscopic studies on the tyrosyl radicals in Photosystem II, which are consistent with spin migration, are reviewed. Migration of the unpaired spin may provide a mechanism for control of the direction and possibly the rate of electron transfer.

Vibrations of the phenoxyl radical

Phenoxyl radical (C(6)H(5)O) was prepared photochemically in low-temperature argon matrices. The infrared absorption spectra were obtained for C(6)H(5)O and for the isotopically labeled species C(6)D(5)O and 1-(13)C(12)C(5)H(5)O. All but one IR-active fundamental vibrations were detected, most of them not previously observed. Combination of results from IR linear dichroism measurements on photooriented samples, determination of absolute IR intensities with the help of internal standards, analysis of isotopic shifts, and quantum chemical predictions (B3LYP/cc-pVTZ) led to a detailed assignment of phenoxyl radical vibrations. Significant frequency shifts are observed with respect to previously reported data based on resonance Raman studies in polar solutions. For some vibrations, these shifts reflect environment-induced structural changes, such as increase of the quinoid character of the phenoxyl radical in polar media. In particular, the frequency of the CO stretching vibration, readily observable in both IR and Raman experiments, is extremely sensitive to the environment and can thus be used to probe its polarity.

Hydrogen bonding of redox-active tyrosine Z of photosystem II probed by FTIR difference spectroscopy

The TyrZ./TyrZ FTIR difference spectrum is reported for the first time in Mn-depleted photosystem II (PS II)-enriched membranes of spinach, in PS II core complexes of Synechocystis sp. PCC 6803 WT, and in the mutant lacking TyrD (D2-Tyr160Phe). In Synechocystis, the v7'a(CO) and delta(COH) infrared modes of TyrZ are proposed to account at 1279 and 1255 cm-1. The frequency of these modes indicate that TyrZ is protonated at pH 6 and involved in a strong hydrogen bond to the side chain of a histidine, probably D1-His190. A positive signal at 1512 cm-1 is assigned to the v(CO) mode of TyrZ. on the basis of the 27 cm-1 downshift observed upon 13C-Tyr labeling at the Tyr ring C4 carbon. A second IR signal, at 1532 cm-1, is tentatively assigned to the v8a(CC) mode of TyrZ.. The frequency of the v(CO) mode of TyrZ. at 1512 cm-1 is comparable to that observed at 1513 cm-1 for the Tyr. obtained by UV photochemistry of tyrosinate in solution, while it is higher than that of TyrD. in WT PS II at 1503 cm-1 and that of non-hydrogen-bonded TyrD. in the D2-His189Gln mutant at 1497 cm-1 [Hienerwadel, R., Boussac, A., Breton, J., Diner, B. A., and Berthomieu, C. (1997) Biochemistry 36, 14712-14723]. This latter work and the present FTIR study suggest that hydrogen bonding induces an upshift of the v(CO) IR mode of tyrosyl radicals and that TyrZ. forms (a) stronger hydrogen bond(s) than TyrD. in WT PS II. Alternatively, the frequency difference between TyrZ. and TyrD. v(CO) modes could be explained by a more localized positive charge near the tyrosyl radical oxygen of TyrD. than TyrZ.. The TyrZ./TyrZ spectrum obtained in Mn-depleted PS II membranes of spinach shows large similarities with the S3'/S2' spectrum characteristic of radical formation in Mn-containing but Ca(2+)-depleted PS II, in support of the assignment using ESEEM of TyrZ. as being responsible for the split EPR signal observed upon illumination in these conditions [Tang, X.-S., Randall, D. W., Force, D. A., Diner, B. A., and Britt, R. D. (1996) J. Am. Chem. Soc. 118, 7638-7639]. The peak at 1514 cm-1 is assigned to the v(CO) mode of TyrZ. in these preparations, which indicates that Mn depletion only very slightly perturbs the immediate environment of TyrZ. phenoxyl.