Model systems for flavoenzyme activity: a tuneable intramolecularly hydrogen bonded flavin-diamidopyridine complex

The redox potential of flavin derivatives as a mediator in biosensors

The two-electron reduction potential for a set of 393 flavin derivatives is presented in this article. These derivatives are substituted flavin on carbon 6, 7, 8, and 9 by coinage transition metals (Cu, Ag, and Au) and conjugated double bond hydrocarbons; and both groups are examined with and without functional groups such as OH, Cl, CH₃, COOH, and NO₂. In order to show the validity of the results, the reduction potential of human life molecules, which have experimental values, such as flavin adenine dinucleotide (FAD) and riboflavin (vitamin B₂) is calculated. The experimental value for FAD is - 0.22 V, while the obtained theoretical value is - 0.21 V, and the corresponding values for riboflavin are - 0.18 and - 0.19 V, respectively. Theoretical calculations have been carried out by DFT procedure with a 6-31+G** basis set and BLYP xc-functional for coinage transition metals substitution, and MPW1PW9 xc-functionals for conjugated double bond hydrocarbon substitution. Both xc-functionals are chosen by the DFT calibration procedure.

Recognition of a Flavin Analogue by Novel Bile Acid-Based Receptors: Effects of Hydrogen Bonding and Aromatic π-Stacking Interactions

Molecular recognition properties are reported for novel bile acid-based receptors that incorporate 2,6-diaminopyridine as a recognition unit. Apart from hydrogen-bonding interactions, the bile acid receptors exhibit significant aromatic π-stacking interactions with the aromatic fused ring of the flavin derivative. These studies provide rationalisation for the differences in binding behaviour of bile acid receptors having differing aromatic arm lengths towards a flavin analogue.

Modelling flavoenzymatic charge transfer events: development of catalytic indole deuteration strategies

The formation and chemistry of flavin-indole charge transfer (CT) complexes has been studied using a model cationic flavin. The ability to form a CT complex is sensitive to indole structure as gauged by spectroscopic, kinetics and crystallographic studies. Single crystals of sufficient quality of a flavin-indole CT complex, suitable for X-ray diffraction, have been grown, allowing solid-state structural analysis. When CT complex formation is conducted in d4-methanol, an efficient and synthetically useful C-3 indole deuteration is observed.

Overlapping Electronic States with Nearly Parallel Transition Dipole Moments in Reduced Anionic Flavin Can Distort Photobiological Dynamics

Chromophoric biomolecules are exploited as reporters of a diverse set of phenomena, acting as internal distance monitors, environment and redox sensors, and endogenous imaging probes. The extent to which they can be exploited is dependent on an accurate knowledge of their fundamental electronic properties. Arguably of greatest importance is a precise knowledge of the direction(s) of the absorption transition dipole moment(s) (TDMs) in the molecular frame of reference. Such is the case for flavins, fluorescent redox cofactors utilized for ground- and excited-state redox and photochemical processes. The directions of the TDMs in oxidized and semiquinone flavins were characterized decades ago, and the details of charge redistribution in these forms have also been studied by Stark spectroscopy. The electronic structure of the fully reduced hydroquinone anionic state, FlH^-, however, has been the subject of unfounded assumptions and estimates about the number and direction of TDMs in FlH^-, as well the electronic structure changes that occur upon light absorption. Here we have used Stark spectroscopy to measure the magnitude and direction of charge redistribution in FlH^- upon optical excitation. These data were analyzed using TD-DFT calculations. The results show unequivocally that not one but two nearly orientation-degenerate electronic transitions are required to explain the 340-500 nm absorption spectral range, demolishing the commonly held assumption of a single transition. The difference dipole moments for these states show that electron density shifts toward the xylene ring for both transitions. These measurements force a reappraisal of previous studies that have used erroneous assumptions and unsubstantiated estimates of these quantities. The results put future optical studies of reduced flavins/flavoproteins on a firm photophysical footing.

Aromatic stacking interactions in flavin model systems

Flavins feature multiple attributes that explain their widespread occurrence in nature, including photostability, reversible electrochemistry, and especially the tunability of their optical, electronic, and redox properties by supramolecular interactions and modification of their chemical structure. Flavins are important redox cofactors for enzymatic catalysis and are central to a wide variety of processes, including biosynthesis, electron transport, photosynthesis, and DNA repair. The wide range of processes catalyzed by flavins makes them promising leads for synthetic catalysts. Their properties are also relevant to organic electronic and optoelectronic devices, where they have the potential to serve as photoactive electron carriers, a very uncommon property in current photovoltaic systems. In flavoenzymes, the flavin cofactor binds to the active site of the apoenzyme through noncovalent interactions. These interactions regulate cofactor recognition and tune the redox behavior of the flavin cofactor. In this Account, we describe the creation of host-guest systems based on small molecule, polymer, and nanoparticle scaffolds that explore the role of aromatic stacking on the redox properties of the flavin and provide insight into flavoenzyme function. We also describe the creation of synthetic flavin-based interlocked structures featuring aromatic stacking interactions, along with the use of aromatic stacking to direct self-assembly of flavin-based materials. The interplay between redox events and aromatic stacking interactions seen in these synthetic models is important for fundamental understanding of biological systems including the flavoenzymes. The precise control of aromatic interactions and binding of flavins not only underpins their biological activity but gives them the potential to be developed into novel organic optoelectronic materials based on tuned synthetic flavin-receptor assemblies. In a broader context, the redox properties of the flavin provide a very concise tool for looking at the role of electronics in aromatic stacking, an issue of general importance in biological and supramolecular chemistry.

'Lock and key' control of optical properties in a push-pull system

We report the modulation of the absorbance of a flavin push-pull derivative through specific recognition by a complementary diamidopyridine (DAP), shifting the flavin intramolecular charge transfer band by approximately 30 nm.