The photogenerated triplet states of riboflavin and flavin mononucleotide (FMN) have been examined by time-resolved electron paramagnetic resonance (EPR) spectroscopy at low temperature (T = 80 K). ...Because of the high time resolution of the utilized EPR instrumentation, the triplets are for the first time observed in the nonequilibrated electron-spin polarized state and not in their equilibrated forms with the population of the triplet sublevels governed by Boltzmann distribution. The electron-spin polarization pattern directly reflects the anisotropy of the intersystem crossing from the excited singlet-state precursor. Spectral analysis of the resulting enhanced absorptive and emissive EPR signals yields the zero-field splitting parameters, |D| and |E|, and the zero-field populations of the triplet at high accuracy. These parameters are sensitive probes for the protonation state of the flavin's isoalloxazine ring, as becomes evident by a comparison of the spectra recorded at different pH values of the solvent. The three protonation states of the flavins can furthermore be distinguished by the kinetics of the transient EPR signals, which are dominated by spin−lattice relaxation. The fastest decays are observed for the protonated FMN and riboflavin triplets, followed by the deprotonated flavin triplets. Slow decays are measured for the triplet states of neutral FMN and riboflavin. Because proton transfer is found to be slow on the time scale of spin-polarized triplet detection by transient EPR, the pH-dependent spin-relaxation and zero-field splitting parameters offer a novel approach to probe the protonation state of flavins in their singlet ground state through the characterization of their triplet-state properties.
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•Synchrotron based frequency-domain Fourier-transform THz-EPR (FD-FT THz-EPR).•Assign spin-coupling parameters of high-spin systems with large zero-field splittings.•Instrumental ...foundations and technical peculiarities of FD-FT THz-EPR.•Selected applications of FD-FT THz-EPR.
We describe frequency-domain Fourier-transform THz-EPR as a method to assign spin-coupling parameters of high-spin (S>1/2) systems with very large zero-field splittings. The instrumental foundations of synchrotron-based FD-FT THz-EPR are presented, alongside with a discussion of frequency-domain EPR simulation routines. The capabilities of this approach is demonstrated for selected mono- and multinuclear HS systems. Finally, we discuss remaining challenges and give an outlook on the future prospects of the technique.
The geometric and electronic structures of the active sites in the oxidized Ni(r)-B state of the NiFe hydrogenases from Ralstonia eutropha H16 and Desulfovibrio vulgaris Miyazaki F were investigated ...in pulsed EPR and ENDOR experiments at two different microwave frequencies (X- and Q-band). Two hyperfine-couplings were clearly resolved in the frozen solution spectra arising from the beta-protons of the nickel-coordinating cysteine residues Cys549 and Cys586 from the Desulfovibrio vulgaris and Ralstonia eutropha hydrogenase, respectively. ESEEM spectroscopic experiments reveal the presence of a histidine in the second coordination sphere of the Ni. The spectroscopic data indicate that the electronic structures of the NiFe centers in both hydrogenases are identical in the Ni(r)-B state. However, additional spin couplings of the active site to further paramagnetic centers were identified for the Ralstonia eutropha hydrogenase. The respective couplings could be clearly resolved and simulated. The results from this study are discussed in view of the exceptional O(2)-tolerance of the Ralstonia hydrogenase.
Molecular spins are promising building blocks of future quantum technologies thanks to the unparalleled flexibility provided by chemistry, which allows the design of complex structures targeted for ...specific applications. However, their weak interaction with external stimuli makes it difficult to access their state at the single‐molecule level, a fundamental tool for their use, for example, in quantum computing and sensing. Here, an innovative solution exploiting the interplay between chirality and magnetism using the chirality‐induced spin selectivity effect on electron transfer processes is foreseen. It is envisioned to use a spin‐to‐charge conversion mechanism that can be realized by connecting a molecular spin qubit to a dyad where an electron donor and an electron acceptor are linked by a chiral bridge. By numerical simulations based on realistic parameters, it is shown that the chirality‐induced spin selectivity effect could enable initialization, manipulation, and single‐spin readout of molecular qubits and qudits even at relatively high temperatures.
Molecular spins are promising qudits but their weak interaction with external fields makes single‐molecule readout challenging. A solution that exploits chirality‐induced spin selectivity (CISS) in photoinduced electron transfer is envisioned in this work. Progress toward the detection of CISS at the molecular level by magnetic resonance is overviewed and it is shown how CISS could enable initialization, manipulation, and readout of molecular spins at high temperature.
Poly(heptazine imides) hosting cobalt ions as countercations are presented as promising electrocatalysts for the oxygen evolution reaction (OER). A facile mixed-salt melt-assisted condensation is ...developed to prepare such cobalt poly(heptazine imides) (PHI-Co). The Co ions can be introduced in well-controlled amounts using this method, and are shown to be atomically dispersed within the imide-linked heptazine matrix. When applied to electrocatalytic OER, PHI-Co shows a remarkable activity with an overpotential of 324 mV and Tafel slope of 44 mV dec
in 1 m KOH.
How carotenoids protect bacterial photosynthesis Cogdell, Richard J.; Howard, Tina D.; Bittl, Robert ...
Philosophical transactions of the Royal Society of London. Series B. Biological sciences,
10/2000, Letnik:
355, Številka:
1402
Journal Article
Recenzirano
Odprti dostop
The essential function of carotenoids in photosynthesis is to act as photoprotective agents, preventing chlorophylls and bacteriochlorophylls from sensitizing harmful photodestructive reactions in ...the presence of oxygen. Based upon recent structural studies on reaction centres and antenna complexes from purple photosynthetic bacteria, the detailed organization of the carotenoids is described. Then with specific reference to bacterial antenna complexes the details of the photoprotective role, triplet-triplet energy transfer, are presented.
Pulsed electron−electron double resonance (ELDOR) has been used to obtain structural information from a FAD-dependent sulfhydryl oxidase, Augmenter of Liver Regeneration (ALR). ALR is a homodimer ...with each subunit containing a noncovalently bound FAD cofactor. Both FADs may be converted into the blue neutral radical form by aerobic treatment with DTT. From three-pulse and four-pulse ELDOR experiments, a distance of 26.1 ± 0.8 Å could be determined between the FAD cofactors in human ALR. Taking into account the electron spin density distribution in a neutral flavin radical obtained from density functional theory calculations, a distance of 26.9 Å could be estimated for the separation of the spin centers in the X-ray structure of rat ALR. The good agreement confirms that rat ALR may be used as a model for mechanistic discussions of human ALR. The experiments also demonstrate that neutral flavin radicals have the appropriate properties to be used as intrinsic spin labels for distance determinations in proteins.
From organic electronics to biological systems, understanding the role of intermolecular interactions between spin pairs is a key challenge. Here we show how such pairs can be selectively addressed ...with combined spin and optical sensitivity. We demonstrate this for bound pairs of spin-triplet excitations formed by singlet fission, with direct applicability across a wide range of synthetic and biological systems. We show that the site sensitivity of exchange coupling allows distinct triplet pairs to be resonantly addressed at different magnetic fields, tuning them between optically bright singlet (S = 0) and dark triplet quintet (S = 1, 2) configurations: This induces narrow holes in a broad optical emission spectrum, uncovering exchange-specific luminescence. Using fields up to 60 T, we identify three distinct triplet-pair sites, with exchange couplings varying over an order of magnitude (0.3–5 meV), each with its own luminescence spectrum, coexisting in a single material. Our results reveal how site selectivity can be achieved for organic spin pairs in a broad range of systems.
Blue‐light sensitive photoreceptory BLUF domains are flavoproteins, which regulate various, mostly stress‐related processes in bacteria and eukaryotes. The photoreactivity of the flavin adenine ...dinucleotide (FAD) cofactor in three BLUF domains from Rhodobacter sphaeroides, Synechocystis sp. PCC 6803 and Escherichia coli have been studied at low temperature using time‐resolved electron paramagnetic resonance. Photoinduced flavin triplet states and radical‐pair species have been detected on a microsecond time scale. Differences in the electronic structures of the FAD cofactors as reflected by altered zero‐field splitting parameters of the triplet states could be correlated with changes in the amino‐acid composition of the various BLUF domains’ cofactor binding pockets. For the generation of the light‐induced, spin‐correlated radical‐pair species in the BLUF domain from Synechocystis sp. PCC 6803, a tyrosine residue near the flavin’s isoalloxazine moiety plays a critical role.
Light‐generated short‐lived radial pairs have been suggested to play pivotal roles in cryptochromes and photolyases. Cryptochromes are very probably involved in magnetic compass sensing in migratory ...birds and the magnetic‐field‐dependent behavior of insects. We examined photo‐generated transient states in the cryptochrome of Drosophila melanogaster and in the structurally related DNA‐repair enzyme Escherichia coli DNA photolyase. Using pulsed EPR spectroscopy, the exchange and dipolar contributions to the electron spin–spin interaction were determined in a straightforward and direct way. With these parameters, radical‐pair partners may be identified and the magnetoreceptor efficiency of cryptochromes can be evaluated. We present compelling evidence for an extended electron‐transfer cascade in the Drosophila cryptochrome, and identified W394 as a key residue for flavin photoreduction and formation of a spin‐correlated radical pair with a sufficient lifetime for high‐sensitivity magnetic‐field sensing.
Short‐lived spin‐correlated radical pairs are essential for the kind of magnetic compass that is believed to be used by migratory birds and insects to perceive direction. Such transient states were studied by pulsed EPR spectroscopy, and the distance‐dependent magnetic couplings between the unpaired spins were characterized.
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