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.
NiFe hydrogenases are widespread among microorganisms and catalyze the reversible cleavage of molecular hydrogen. However, only a few bacteria, such as Ralstonia eutropha H16 (Re), synthesize NiFe ...hydrogenases that perform H(2) cycling in the presence of O(2). These enzymes are of special interest for biotechnological applications. To gain further insight into the mechanism(s) responsible for the remarkable O(2) tolerance, we employ FTIR and EPR spectroscopy to study mutant variants of the membrane-bound hydrogenase (MBH) of Re-carrying substitutions of a particular cysteine residue in the vicinity of the NiFe active site that is characteristic of O(2)-tolerant membrane-bound NiFe hydrogenases. We demonstrate that these MBH variants, despite minor changes in the electronic structure and in the interaction behavior with the embedding protein matrix, display all relevant catalytic and noncatalytic states of the wild-type enzyme, as long as they are still located in the cytoplasmic membrane. Notably, in the oxidized Ni(r)-B state and the fully reduced forms, the CO stretching frequency increases with increasing polarity of the respective amino acid residue at the specific position of the cysteine residue. We purified the MBH mutant protein with a cysteine-to-alanine exchange to apparent homogeneity as dimeric enzyme after detergent solubilization from the membrane. This purified version displays increased oxygen sensitivity, which is reflected by detection of the oxygen-inhibited Ni(u)-A state, an irreversible inactive redox state, and the light-induced Ni(a)-L state even at room temperature.
Quinoprotein alcohol dehydrogenases use the pyrroloquinoline quinone (PQQ) cofactor to catalyze the oxidation of alcohols. The catalytic cycle is thought to involve a hydride transfer from the ...alcohol to the oxidized PQQ, resulting in the generation of aldehyde and reduced PQQ. Reoxidation of the cofactor by cytochrome proceeds in two sequential steps via the PQQ radical. We have used a combination of electron nuclear double resonance and density functional theory to show that the PQQ radical is not protonated at either O-4 or O-5, a result that is at variance with the general presumption of a singly protonated radical. The quantum mechanical calculations also show that reduced PQQ is unlikely to be protonated at O-5; rather, it is either singly protonated at O-4 or not protonated at either O-4 or O-5, a result that also challenges the common assumption of a reduced PQQ protonated at both O-4 and O-5. The reaction cycle of PQQ-dependent alcohol dehydrogenases is revised in light of these findings.
Covalent dimers, particularly pentacenes, are the dominant platform for developing a mechanistic understanding of intramolecular singlet fission (iSF). Numerous studies have demonstrated that a ...photoexcited singlet state in these structures can rapidly and efficiently undergo exciton multiplication to form a correlated pair of triplets within a single molecule, with potential applications from photovoltaics to quantum information science. One of the most significant barriers limiting such dimers is the fast recombination of the triplet pair, which prevents spatial separation and the formation of long-lived triplet states. There is an ever-growing need to develop general synthetic strategies to control the evolution of triplets following iSF and enhance their lifetime. Here, we rationally tune the dihedral angle and interchromophore separation between pairs of pentacenes in a systematic series of bridging units to facilitate triplet separation. Through a combination of transient optical and spin-resonance techniques, we demonstrate that torsion within the linker provides a simple synthetic handle to tune the fine balance between through-bond and through-space interchromophore couplings that steer iSF. We show that the full iSF pathway from femtosecond to microsecond timescales is tuned through the static coupling set by molecular design and structural fluctuations that can be biased through steric control. Our approach highlights a straightforward design principle to generate paramagnetic spin pair states with higher yields.
The first consistent series of mononuclear 17‐electron complexes of three Group 7 elements has been isolated in crystalline form and studied by X‐ray diffraction and spectroscopic methods. The ...paramagnetic compounds have a composition of M0(CO)(CNp‐F‐ArDArF2)4 (M=Mn, Tc, Re; ArDArF2=2,6‐(3,5‐(CF3)2C6H3)2C6H2F) and are stabilized by four sterically encumbering isocyanides, which prevent the metalloradicals from dimerization. They have a square pyramidal structure with the carbonyl ligands as apexes. The frozen‐solution EPR spectra of the rhenium and technetium compounds are clearly anisotropic with large 99Tc and 185,187Re hyperfine interactions for one component. High‐field EPR (Q band and W band) has been applied for the elucidation of the EPR parameters of the manganese(0) complex.
A series of structurally analogous monomeric manganese(0), technetium(0) and rhenium(0) monoradicals has been stabilized by means of sterically encumbered m‐terphenyl isocyanides. They possess square‐pyramidal structures. Their electron spin of S=
1/2
${{ 1/2 }}$
allows the detection of resolved EPR spectra. High field EPR (Q band and W band) is shown to be a valuable tool for a complete analysis of the tensor components of the manganese compound.
Tuning Spin Dynamics in Crystalline Tetracene Bayliss, Sam L; Kraffert, Felix; Wang, Rui ...
The journal of physical chemistry letters,
04/2019, Letnik:
10, Številka:
8
Journal Article
Recenzirano
Odprti dostop
Tetracene is an archetypal material undergoing singlet fissionthe generation of a pair of triplet excitons from one singlet exciton. Here, using time-resolved electron spin resonance, we show how ...the spin dynamics in tetracene crystals are influenced by temperature and morphology. Upon cooling from 300 to 200 K, we observe a switch between singlet fission and intersystem crossing generated triplets, manifesting as an inversion in transient spin polarization. We extract a spin dephasing time of approximately 40 ns for fission-generated triplets at room temperature, nearly 100 times shorter than the dephasing time that we measure for triplets localized on isolated tetracene molecules. These results highlight the importance of morphology and thermal activation in singlet fission systems.
Influencing proteins: A protein multichromophore system (photosystem I) is exposed to gold nanoparticles (NPs) and silver island films. In the presence of these nanostructures an altered fluorescence ...response of the chromophores is observed (see scheme), indicating a change in the protein function. A model to understand these plasmonic effects is generally applicable to other multichromophore systems.
We report transient electron paramagnetic resonance (trEPR) measurements with submicrosecond time resolution performed on a polymer:fullerene blend consisting of poly(3-hexylthiophene) (P3HT) and ...6,6-phenyl C sub(61)-butyric acid methyl ester (PCBM) at low temperatures. The trEPR spectrum immediately following photoexcitation reveals signatures of spin-correlated polaron pairs. The pair partners (positive polarons in P3HT and negative polarons in PCBM) can be identified by their characteristic g values. The fact that the polaron pair states exhibit strong non-Boltzmann population unambiguously shows that the constituents of each pair are geminate, i.e., originate from one exciton. We demonstrate that coupled polaron pairs are present even several microseconds after charge transfer and suggest that they embody the intermediate charge transfer complexes that form at the donor/acceptor interface and mediate the conversion from excitons into free charge carriers.
Sensory photoreceptor proteins underpin light-dependent adaptations in nature and enable the optogenetic control of organismal behavior and physiology. We identified the bacterial ...light-oxygen-voltage (LOV) photoreceptor PAL that sequence-specifically binds short RNA stem loops with around 20 nM affinity in blue light and weaker than 1 µM in darkness. A crystal structure rationalizes the unusual receptor architecture of PAL with C-terminal LOV photosensor and N-terminal effector units. The light-activated PAL-RNA interaction can be harnessed to regulate gene expression at the RNA level as a function of light in both bacteria and mammalian cells. The present results elucidate a new signal-transduction paradigm in LOV receptors and conjoin RNA biology with optogenetic regulation, thereby paving the way toward hitherto inaccessible optoribogenetic modalities.
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