Selective 13C labelling reveals the electronic structure of flavocoenzyme radicals

Flavocoenzymes are nearly ubiquitous cofactors that are involved in the catalysis and regulation of a wide range of biological processes including some light-induced ones, such as the photolyase-mediated DNA repair, magnetoreception of migratory birds, and the blue-light driven phototropism in plants. One of the factors that enable versatile flavin-coenzyme biochemistry and biophysics is the fine-tuning of the cofactor’s frontier orbital by interactions with the protein environment. Probing the singly-occupied molecular orbital (SOMO) of the intermediate radical state of flavins is therefore a prerequisite for a thorough understanding of the diverse functions of the flavoprotein family. This may be ultimately achieved by unravelling the hyperfine structure of a flavin by electron paramagnetic resonance. In this contribution we present a rigorous approach to obtaining a hyperfine map of the flavin’s chromophoric 7,8-dimethyl isoalloxazine unit at an as yet unprecedented level of resolution and accuracy. We combine powerful high-microwave-frequency/high-magnetic-field electron–nuclear double resonance (ENDOR) with 13C isotopologue editing as well as spectral simulations and density functional theory calculations to measure and analyse 13C hyperfine couplings of the flavin cofactor in DNA photolyase. Our data will provide the basis for electronic structure considerations for a number of flavin radical intermediates occurring in blue-light photoreceptor proteins.

Photochemical synthesis of 4H-thieno[3,2-c]chromene and their optical properties

4 - [(2-Iodalyloxy) methyl] thiophene-2-carbaldehyde and 5-iodo-4- (aryloxymethyl)thiophene-2-carbaldehydes were obtained by interaction of phenols with 4-chloromethylthiophene-2-carbaldehyde and its 5-iodosubstituted derivative. These compounds underwent ring closure upon irradiation with UV light (254 nm) to form 4H-thieno[3,2-c]chromene-2-carbaldehyde in high yield. The formation of intermediate radical species was detected by EPR method. Comparative analysis of the ring closure methods showed that the photochemical cyclization of 5-iodo-4-(aryloxymethyl) thiophene-2-carbaldehyde is more rational than the Pd-catalyzed intramolecular arylation. A series of substituted 4H-thieno[3,2-c]chromene-2-carbaldehydes was synthesized by the photochemical cyclization of the corresponding precursors. The photophysical properties of the synthesized compounds were studied. The obtained 4H-thieno[3,2-c]chromene-2-carbaldehydes can be used as covert marking pigments.

Polyphenolic compounds as electron shuttles for sustainable energy utilization

For renewable and sustainable bioenergy utilization with cost-effectiveness, electron-shuttles (ESs) (or redox mediators (RMs)) act as electrochemical “catalysts” to enhance rates of redox reactions, catalytically accelerating electron transport efficiency for abiotic and biotic electrochemical reactions. ESs are popularly used in cellular respiratory systems, metabolisms in organisms, and widely applied to support global lives. Apparently, they are applicable to increase power-generating capabilities for energy utilization and/or fuel storage (i.e., dye-sensitized solar cell, batteries, and microbial fuel cells (MFCs)). This first-attempt review specifically deciphers the chemical structure association with characteristics of ESs, and discloses redox-mediating potentials of polyphenolics-abundant ESs via MFC modules. Moreover, to effectively convert electron-shuttling capabilities from non-sustainable antioxidant activities, environmental conditions to induce electrochemical mediation apparently play critical roles of great significance for bioenergy stimulation. For example, pH levels would significantly affect electrochemical potentials to be exhibited (e.g., alkaline pHs are electrochemically favorable for expression of such electron-shuttling characteristics). Regarding chemical structure effect, chemicals with ortho- and para-dihydroxyl substituents-bearing aromatics own convertible characteristics of non-renewable antioxidants and electrochemically catalytic ESs; however, ES capabilities of meta-dihydroxyl substituents can be evidently repressed due to lack of resonance effect in the structure for intermediate radical(s) during redox reaction. Moreover, this review provides conclusive remarks to elucidate the promising feasibility to identify whether such characteristics are non-renewable antioxidants or reversible ESs from natural polyphenols via cyclic voltammetry and MFC evaluation. Evidently, considering sustainable development, such electrochemically convertible polyphenolic species in plant extracts can be reversibly expressed for bioenergy-stimulating capabilities in MFCs under electrochemically favorable conditions.

Modeling of Miniemulsion Polymerization of Styrene with Macro-RAFT Agents to Theoretically Compare Slow Fragmentation, Ideal Exchange and Cross-Termination Cases

A 5-dimensional Smith-Ewart based model is developed to understand differences for reversible addition-fragmentation chain transfer (RAFT) miniemulsion polymerization with theoretical agents mimicking cases of slow fragmentation, cross-termination, and ideal exchange while accounting for chain length and monomer conversion dependencies due to diffusional limitations. The focus is on styrene as a monomer, a water soluble initiator, and a macro-RAFT agent to avoid exit/entry of the RAFT leaving group radical. It is shown that with a too low RAFT fragmentation rate coefficient it is generally not afforded to consider zero-one kinetics (for the related intermediate radical type) and that with significant RAFT cross-termination the dead polymer product is dominantly originating from the RAFT intermediate radical. To allow the identification of the nature of the RAFT retardation it is recommended to experimentally investigate in the future the impact of the average particle size (dp) on both the monomer conversion profile and the average polymer properties for a sufficiently broad dp range, ideally including the bulk limit. With decreasing particle size both a slow RAFT fragmentation and a fast RAFT cross-termination result in a stronger segregation and thus rate acceleration. The particle size dependency is different, allowing further differentiation based on the variation of the dispersity and end-group functionality. Significant RAFT cross-termination is specifically associated with a strong dispersity increase at higher average particle sizes. Only with an ideal exchange it is afforded in the modeling to avoid the explicit calculation of the RAFT intermediate concentration evolution.

Direct observation of the intermediate radical in the photodissociation of 1,3-cyclohexane dinitrite

The two-step photodissociation mechanism of 1,3-cyclohexane dinitrite is confirmed by observation of the laser-induced fluorescence spectrum of the intermediate 3-nitrosooxy cyclohexoxy radical.

Photochemical reaction dynamics of 2,2′-dithiobis(benzothiazole): direct observation of the addition product of an aromatic thiyl radical to an alkene with time-resolved vibrational and electronic absorption spectroscopy

A carbon-centered intermediate radical of a thiol–ene reaction is directly probed on the picosecond time scale with time-resolved vibrational absorption spectroscopy.

Studies on Kinetics and Mechanism of the Oxidation of Cobalt(Iii) Bound And Unbound Alpha - Hydroxy Acids By Tetramethylammonium Chlorochromate

The kinetics of induced electron transfer reaction has been attempted presently with Tetra Methyl Ammonium Chloro Chromate (TMACC) and pentaammine cobalt (III) complexes of α-hydroxy acid in the presence of micelles. The Tetra Methyl Ammonium Chloro Chromate (TMACC) oxidizes cobalt (III) bound and unbound α-hydroxy acids. In Tetra Methyl Ammonium Chloro Chromate (TMACC) induced electron transfer in the complex, the intermediate radical formed dissociates in a nearly synchronous C-C bond cleavage and the rest of it proceeding with C-H fission yielding cobalt (III) complex. With increase in surfactant Sodium Lauryl Ethersulphate (SLES) concentration, an increase in the rate was observed.