Synthesis and electronic properties of π-extended flavins

Tuning the Photophysical Properties of Flavins by Attaching an Aryl Moiety via Direct C-C Bond Coupling

Palladium-catalyzed Suzuki reactions of brominated flavin derivatives (5-deazaflavins, alloxazines, and isoalloxazines) with boronic acids or boronic acid esters that occur readily under mild conditions were shown to be an effective tool for the synthesis of a broad range of 7/8-arylflavins. In general, the introduction of an aryl/heteroaryl group by means of a direct C-C bond has been shown to be a promising approach to tuning the photophysical properties of flavin derivatives. The aryl substituents caused a bathochromic shift in the absorption spectra of up to 52 nm and prolonged the fluorescence lifetime by up to 1 order of magnitude. Moreover, arylation of flavin derivatives decreased their ability to generate singlet oxygen.

Molecular flavin catalysts for C-H functionalisation and derivatisation of dehydroamino acids

In nature, the isoalloxazine heterocycle of flavin cofactors undergoes reversible covalent bond formation with a variety of different reaction partners. These intermediates play a crucial role inter alia as the signalling states and in selective catalysis reactions. In the organic laboratory, covalent adducts with a new carbon-carbon bond have been observed with photochemically excited flavins but have, so far, only been regarded as dead-end side products. We have identified a series of molecular flavins that form adducts resulting in a new C-C bond at the C4a-position through allylic C-H activation and dehydroamino acid oxidation. Typically, these reactions are of radical nature and a stepwise pathway is assumed. We could demonstrate that these adducts are no dead-end and that the labile C-C bond can be cleaved by adding the persistent radical TEMPO leading to flavin regeneration and alkoxyamine-functionalised substrates. Our method allows for the catalytic oxidation of dehydroamino acids (16 examples) and we show that the acylimine products serve as versatile starting points for diversification. The present results are envisioned to stimulate the design of further catalytic reactions involving intermediates at the flavin C4a-position and their reactivity towards metal complexes or other persistent organic radicals. Our method for dehydrobutyrine derivatisation is orthogonal to the currently used methods (i.e., nucleophilic attack or radical addition) and offers new perspectives for peptide natural product diversification.

Tunable Properties of Nature-Inspired N,N'-Alkylated Riboflavin Semiconductors

A series of novel soluble nature-inspired flavin derivatives substituted with short butyl and bulky ethyl-adamantyl alkyl groups was prepared via simple and straightforward synthetic approach with moderate to good yields. The comprehensive characterization of the materials, to assess their application potential, has demonstrated that the modification of the conjugated flavin core enables delicate tuning of the absorption and emission properties, optical bandgap, frontier molecular orbital energies, melting points, and thermal stability. Moreover, the thin films prepared thereof exhibit smooth and homogeneous morphology with generally high stability over time.

Flavin Conjugated Polydopamine Nanoparticles Displaying Light-Driven Monooxygenase Activity

A hybrid of flavin and polydopamine (PDA) has been explored as a photocatalyst, drawing inspiration from natural flavoenzymes. Light-driven monoxygenase activity has been demonstrated through the oxidation of indole under blue light irradiation in ambient conditions, to afford indigo and indirubin dyes. Compared to riboflavin, a flavin-polydopamine hybrid is shown to be more resistant to photobleaching and more selective toward dye production. In addition, it has been demonstrated that it can be recycled from the solution and used for up to four cycles without a marked loss of activity, which is a significant improvement compared to other heterogenous flavin catalysts. The mechanism of action has been explored, indicating that the PDA shell plays an important role in the stabilization of the intermediate flavin-peroxy species, an active component of the catalytic system rather than acting only as a passive nanocarrier of active centers.

Novel Riboflavin-Inspired Conjugated Bio-Organic Semiconductors

Flavins are known to be extremely versatile, thus enabling routes to innumerable modifications in order to obtain desired properties. Thus, in the present paper, the group of bio-inspired conjugated materials based on the alloxazine core is synthetized using two efficient novel synthetic approaches providing relatively high reaction yields. The comprehensive characterization of the materials, in order to evaluate the properties and application potential, has shown that the modification of the initial alloxazine core with aromatic substituents allows fine tuning of the optical bandgap, position of electronic orbitals, absorption and emission properties. Interestingly, the compounds possess multichromophoric behavior, which is assumed to be the results of an intramolecular proton transfer.