Riboflavin interactions with oxygen-a survey from the photochemical perspective

Lightening flavin by amination for fluorescent sensing

Monitoring of reactive oxygen species (ROS), such as O2˙-, etc., in organisms is of great significance, not only for their essential role in biological processes, but their excessive production may also result in many diseases. Flavin (FL) is a fluorophore that naturally exists in flavoenzymes, and its fluorescent emission (FE) becomes negligible when reduced. This enables the application of FL derivatives as fluorescent sensors for ROS. We presented a theoretical investigation to address the impact of amino substitution on the photophysical properties of aminoflavins (AmFLs). Resulting from the interplay of electronic and positional effects, amination at C8 enhances the electronic coupling between the ground state and the first singlet excited state by enlarging the adiabatic energy change of the electronic transitions and the emission transition dipole moments, weakens the vibronic coupling by decreasing the contribution of isoalloxazine to the frontier molecular orbitals, redshifts the absorption band, and enhances the fluorescent emission drastically in 8AmFL. The theoretically estimated fluorescent emission intensity of 8AmFL is ∼40 times that of FL, suggesting its potential application as a fluorescent sensor.

Exploring the Reactivity of Flavins with Nucleophiles Using a Theoretical and Experimental Approach

Covalent adducts of flavin cofactors with nucleophiles play an important role in non-canonical function of flavoenzymes as well as in flavin-based catalysis. Herein, the interaction of flavin derivatives including substituted flavins (isoalloxazines), 1,10-ethylene-bridged flavinium salts, and non-substituted alloxazine and deazaflavin with selected nucleophiles was investigated using an experimental and computational approach. Triphenylphosphine or trimethylphosphine, 1-nitroethan-1-ide, and methoxide were selected as representatives of neutral soft, anionic soft, and hard nucleophiles, respectively. The interactions were investigated using UV/Vis and 1H NMR spectroscopy as well as by DFT calculations. The position of nucleophilic attack estimated using the calculated Gibbs free energy values was found to correspond with the experimental data, favouring the addition of phosphine and 1-nitroethan-1-ide into position N(5) and methoxide into position C(10a) of 1,10-ethylene-bridged flavinium salts. The calculated Gibbs free energy values were found to correlate with the experimental redox potentials of the flavin derivatives tested. These findings can be utilized as valuable tools for the design of artificial flavin-based catalytic systems or investigating the mechanism of flavoenzymes.

A multifunctional role for riboflavin in the yellow nectar of Capsicum baccatum and Capsicum pubescens

A few Capsicum (pepper) species produce yellow-colored floral nectar, but the chemical identity and biological function of the yellow pigment are unknown. A combination of analytical biochemistry techniques was used to identify the pigment that gives Capsicum baccatum and Capsicum pubescens nectars their yellow color. Microbial growth assays, visual modeling, and honey bee preference tests for artificial nectars containing riboflavin were used to assess potential biological roles for the nectar pigment. High concentrations of riboflavin (vitamin B2) give the nectars their intense yellow color. Nectars containing riboflavin generate reactive oxygen species when exposed to light and reduce microbial growth. Visual modeling also indicates that the yellow color is highly conspicuous to bees within the context of the flower. Lastly, field experiments demonstrate that honey bees prefer artificial nectars containing riboflavin. Some Capsicum nectars contain a yellow-colored vitamin that appears to play roles in (1) limiting microbial growth, (2) the visual attraction of bees, and (3) as a reward to nectar-feeding flower visitors (potential pollinators), which is especially interesting since riboflavin is an essential nutrient for brood rearing in insects. These results cumulatively suggest that the riboflavin found in some Capsicum nectars has several functions.

Evaluation of visible light and natural photosensitizers against Staphylococcus epidermidis and Staphylococcus saprophyticus planktonic cells and biofilm

Antimicrobial photoinactivation (API) has shown some promise in potentially treating different nosocomial bacterial infections, however, its application on staphylococci, especially other than Staphylococcus aureus or methicillin-resistant S. aureus (MRSA) species is still limited. Although S. aureus is a well-known and important nosocomial pathogen, several other species of the genus, particularly coagulase-negative Staphylococcus (CNS) species such as Staphylococcus epidermidis and Staphylococcus saprophyticus, can also cause healthcare-associated infections and foodborne intoxications. CNS are often involved in resilient biofilm formation on medical devices and can cause infections in patients with compromised immune systems or those undergoing invasive procedures. In this study, the effects of chlorophyllin and riboflavin-mediated API on S. epidermidis and S. saprophyticus planktonic cells and biofilm are demonstrated for the first time. Based on the residual growth determination and metabolic reduction ability changes, higher inactivating efficiency of chlorophyllin-mediated API was determined against the planktonic cells of both tested species of bacteria and against S. saprophyticus biofilm. Some insights on whether aqueous solutions of riboflavin and chlorophyllin, when illuminated with optimal exciting wavelength (440 nm and 402 nm, respectively) generate O2-•, are also provided in this work.

UV-stimulated riboflavin exerts immunosuppressive effects in BALB/c mice and human PBMCs

Riboflavin (RF) as a photosensitizer has been used in corneal surgery and the inactivation of blood products. However, the effect of RF on immune cells after ultraviolet (UV) light stimulation has not been investigated. This study pioneered a novel application method of RF. Firstly, UV-stimulated RF was co-cultured with human peripheral blood mononuclear cells in vitro, and the apoptosis rate of lymphocyte subsets, cell proliferation inhibition rate and concentrations of IL-1β, IL-6, IL-10, TNF-α were assessed. UV-stimulated RF was then administered intravenously to mice via the tail vein for a consecutive period of 5 days. The levels of immunoglobulin (IgG, IgM, IgA), complement (C3, C4) and cytokines (IFN-γ, IL-4, IL17, TGF-β) were detected by ELISA. Flow cytometry was employed to analyze the populations of CD3+T, CD4+T, CD8+T and CD4+T/CD8+T cells in spleen lymphocytes of mice. The data showed that UV-stimulated RF can effectively induce apoptosis in lymphocytes, and different lymphocyte subtypes exhibited varying degrees of treatment tolerance. Additionally, the proliferative capacity of lymphocytes was suppressed, while their cytokine secretion capability was augmented. The animal experiments demonstrated that UV-stimulated RF led to a significant reduction observed in serum immunoglobulin and complement levels, accompanied by an elevation in IFN-γ, IL-17 and TGF-β levels, as well as a decline in IL-4 level. In summary, the results of both in vitro and in vivo experiments have demonstrated that UV-stimulated RF, exhibits the ability to partially inhibit immune function. This novel approach utilizing RF may offer innovative perspectives for diseases requiring immunosuppressive treatment.

Nature-Derived Gelatin-Based Antifungal Nanotherapeutics for combatting Candida albicans Biofilms

Infections caused by fungi are emerging global health challenges that are exacerbated by the formation of fungal biofilms. Further challenges arise from environmental contamination with antifungal agents, which promotes environmental acquisition of antifungal resistance. We report the generation of an efficient, sustainable, all-natural antifungal nanotherapeutic based on the integration of an antimicrobial natural essential oil into a gelatin-based nanoemulsion platform. Carvacrol-loaded gelatin nanoemulsions penetrated Candida albicans biofilms, resulting in death of C. albicans cells in biofilms, and displayed selective biofilm elimination without harmful effects on fibroblast cells in a fungal biofilm-mammalian fibroblast co-culture model. Furthermore, the nanoemulsions degraded in the presence of physiologically relevant biomolecules, reducing the potential for environmental pollution and ecotoxicity. Overall, the sustainability, and efficacy of the described gelatin nanoemulsion formulation provides an environmentally friendly strategy for treating biofilm-associated fungal infections, including those caused by drug-resistant fungi.

Photoinactivation of the bacteriophage PhiX174 by UVA radiation and visible light in SM buffer and DMEM-F12

OBJECTIVE

It has been observed that viruses can be inactivated by UVA radiation and visible light. The aim of this study is to investigate whether a medium that contains a photosensitizer might have an influence on viral reduction under irradiation by UVA, violet or blue light. Test virus is the bacteriophage PhiX174 in the photosensitizer-free SM buffer and DMEM-F12, which contains the known photosensitizer riboflavin.

RESULTS

The determined PhiX174 D90 doses in SM buffer and DMEM were 36.8 J/cm² and 13.6 J/cm² at 366 nm, 153.6 J/cm² and 129.1 J/cm² at 408 nm and 4988 J/cm² and 2477.1 J/cm² at 455 nm, respectively. It can be concluded that the medium has a large influence on the results. This might be caused by the photosensitizer riboflavin in DMEM-F12. As riboflavin is a key component in many cell culture media, irradiation experiments with viruses in cell culture media should be avoided if the investigation of intrinsical photoinactivation properties of viruses is aimed for.

Tetraacetyl riboflavin derivative mediates caspase 3/7 activation via MAPK in A431 cells upon blue light influence

An acetylated riboflavin derivative, 3-methyl-tetraacetyl riboflavin (3MeTARF), is a compound with high photostability and photophysical properties similar to riboflavin, including the ability to photogenerate singlet oxygen. In the present study, we compared the effects of irradiation on A431 cancer cells with blue LED light (438 nm) in the presence of 3MeTARF and riboflavin on MAPK phosphorylation, apoptosis, caspase 3/7 activation and PARP cleavage. We observed that photogenerated oxidative stress in this reaction activates MAPK by increasing phosphorylation of p38 and JNK proteins. Preincubation of cells with inhibitors specific for phosphorylation of p38 and JNK proteins (SB203580, SP600125), respectively, results in decreased caspase 3/7 activation and PARP cleavage. We showed that the tetraacetyl derivative more effectively activates MAPK and skin cancer cell death compared to riboflavin. These data, together with results of our previous study, support the hypothesis that 3MeTARF, of riboflavin, might be more useful and desirable as a compound for use in photodynamic oxidation processes, including its therapeutic potential.

Alkyne Oxidation by a Vitamin B2-Based Photocatalytic System with Both H2O and O2 as the Oxygen Source

The employment of readily available photocatalysts and green oxygen atom sources is recognized as a promising strategy to develop sustainable catalysis for oxidation reactions. We herein reported a sacrificial reagent-free system consisting of riboflavin tetraacetate (RFT), an ester of natural vitamin B2 as the photocatalyst, and Sc(OTf)3 and NaCl as the cocatalysts for alkyne oxidation under blue light or even sunlight irradiation to produce 1,2-diketone in which the oxygen atoms were from both water and molecular oxygen, respectively. A major Cl-/Cl• cycle was proposed to be involved and achieved by the excited [RFT-2Sc3+]* complex via single electron transfer for the first time, distinguished from the OCl- active species by a two-electron process in previous flavin-halide photo-oxidation systems.

Natural photosensitizers potentiate the targeted antimicrobial photodynamic therapy as the Monkeypox virus entry inhibitors: An in silico approach

BACKGROUND

Monkeypox is a viral zoonotic disease that has emerged as a threat to public health. Currently, there is no treatment approved specifically targeting Monkeypox disease. Hence, it is essential to identify and develop therapeutic approaches to the Monkeypox virus. In the current in silico paper, we comprehensively involve using computer simulations and modeling to insights and predict hypotheses on the potential of natural photosensitizers-mediated targeted antimicrobial photodynamic therapy (aPDT) against D8L as a Monkeypox virus protein involved in viral cell entry.

MATERIALS AND METHODS

In the current study, computational techniques such as molecular docking were combined with in silico ADMET predictions to examine how Curcumin (Cur), Quercetin (Qct), and Riboflavin (Rib) as the natural photosensitizers bind to the D8L protein in Monkeypox virus, as well as to determine pharmacokinetic properties of these photosensitizers.

RESULTS

The three-dimensional structure of the D8L protein in the Monkeypox virus was constructed using homology modeling (PDB ID: 4E9O). According to the physicochemical properties and functional characterization, 4E9O was a stable protein with the nature of a hydrophilic structure. The docking studies employing a three-dimensional model of 4E9O with natural photosensitizers exhibited good binding affinity. D8L protein illustrated the best docking score (-7.6 kcal/mol) in relation to the Rib and displayed good docking scores in relation to the Cur (-7.0 kcal/mol) and Qct (-7.5 kcal/mol).

CONCLUSIONS

The findings revealed that all three photosensitizers were found to obey the criteria of Lipinski's rule of five and displayed drug-likeness. Moreover, all the tested photosensitizers were found to be non-hepatotoxic and non-cytotoxic. In summary, our investigation identified Cur, Qct, and Rib could efficiently interact with D8L protein with a strong binding affinity. It can be concluded that aPDT using these natural photosensitizers may be considered an adjuvant treatment against Monkeypox disease.

Investigating the photosensitization activities of flavins irradiated by blue LEDs

Due to their ability to easily absorb light and to generate highly reactive species, photosensitizers emerged as promising tools in a wide variety of physico-chemical and biological processes. Natural photosensitizers have the benefit of a life-compatible toxicological profile. Porphyrins and flavins are such examples that already proved their efficiency as photo-dynamic therapeutics. The present article describes a reliable, easy-to-implement, readily available and reproducible method that can be used to characterize the photosensitizing activity of flavins. Several key factors were investigated during this study, the optimum parameters were: (i) a blue LED light source (λ _em = 455 nm) at 6.69 mW; (ii) a pH of 6 mimicking the tumoral environment; (iii) an air-saturated atmosphere reaction medium, (iv) a tetrazolium dye (MTT) was used to monitor the photosensitization efficacy via the generation of the colored MTT-formazan product. This method can be used to rank a series of flavins based on their photosensitizing activities. Such structure-photosensitization activity relationships are essential for the discovery of future potent photosensitizers for photodynamic therapy.

Effects of Dietary Riboflavin Supplementation on the Growth Performance, Body Composition and Anti-Oxidative Capacity of Coho Salmon (Oncorhynchus kisutch) Post-Smolts

The present study investigated the effects of dietary riboflavin on growth performance, body composition and anti-oxidative capacity of coho salmon (Oncorhynchus kisutch) post-smolts. Seven experimental diets were formulated with graded riboflavin levels of 0.00, 3.96, 8.07, 16.11, 31.81, 63.67 and 126.69 mg/kg, respectively. Each diet was fed to triplicate groups of 10 fish with an individually initial mean body weight of 186.22 ± 0.41 g in 21 cages (water volume, 1000-L/cage) and fed three times daily (7:30, 12:30 and 17:30) to apparent satiation for 12 weeks. Fish fed a diet with 31.81 mg/kg riboflavin had the highest specific growth rate (SGR), which was significantly higher than fish-fed diets with 0.00, 3.96, 8.07 and 126.69 mg/kg riboflavin (p < 0.05). Feed conversion ratio showed an inverse trend with SGR. No significant differences were observed in condition factor, hepatosomatic index, viscerosomatic index, muscle moisture, crude protein and ash contents among dietary groups. Muscle lipid had the highest content in the 31.81 mg/kg group and was significantly higher (p < 0.05) than those in the 0.00, 3.96 and 8.07 mg/kg groups. The alanine aminotransferase, aspartate aminotransferase and malondialdehyde contents in the liver and serum of fish were significantly decreased with the increase in dietary riboflavin level up to 31.81 mg/kg, and then increased as dietary riboflavin level further increased. An inverse trend was observed for total superoxide dismutase and catalase activities. Serum total cholesterol and triglyceride levels were significantly decreased with the dietary of riboflavin levels up to 31.81 and 63.67 mg/kg, respectively. The cubic curve regression analysis based on SGR indicated that the optimum dietary riboflavin level was estimated to be 35.26 mg/kg for coho salmon post-smolts.

Magnetically Boosted Generation of Intracellular Reactive Oxygen Species toward Magneto-Photodynamic Therapy

The generation of reactive oxygen species (ROS) in photodynamic therapy (PDT) involves excited-state intermediates with both singlet and triplet spin configurations, which provides possibilities to modulate the ROS production in PDT under an external magnetic field. Here, we present that magnetically modulated ROS production can promote PDT efficacy and develop a magnetic-field-assisted PDT (magneto-PDT) method for effectively and selectively killing cancer cells. The photosensitization reaction between excited-state riboflavin and oxygen molecules is influenced by the applied field, and the overall magnetic field effect (MFE) shows a moderate increase at a low field (<1000 G) and then a boost up to the saturation ∼100% at a high field (>1000 G). It is found that the spin precession occurring in radical ion pairs (electron transfer from riboflavin to oxygen) facilitates the O₂^•- generation at the low field. In comparison, the spin splitting in an encounter complex (energy transfer from riboflavin to oxygen) benefits the production of ¹O₂ species at the high field. The field modulation on the two types of ROS in PDT, i.e., O₂^•- and ¹O₂, is also demonstrated in living cells. The magneto-PDT strategy shows the capability to inhibit the proliferation of cancer cells (e.g., HeLa, RBL-2H3, and MCF-7) effectively and selectively, which reveals the potential of using the MFE on chemical reactions in biological applications.

Organic dyes supported on silicon-based materials: synthesis and applications as photocatalysts

The most important advance in photocatalysis in the last decade has been the synthesis and application of organic compounds to promote this process.

Amide Bond Formation via Aerobic Photooxidative Coupling of Aldehydes with Amines Catalyzed by a Riboflavin Derivative

We report an effective, operationally simple, and environmentally friendly system for the synthesis of tertiary amides by the oxidative coupling of aromatic or aliphatic aldehydes with amines mediated by riboflavin tetraacetate (RFTA), an inexpensive organic photocatalyst, and visible light using oxygen as the sole oxidant. The method is based on the oxidative power of an excited flavin catalyst and the relatively low oxidation potential of the hemiaminal formed by amine to aldehyde addition.

Riboflavin-based carbon dots with high singlet oxygen generation for photodynamic therapy

Photodynamic therapy, as an effective treatment for superficial tumors, has attracted more and more attention. The development of safe, biocompatible and in vivo photosensitive materials is helpful to promote photodynamic therapy. Here we report green fluorescent carbon quantum dots prepared from a natural vitamin, riboflavin (VB2), as a photosensitizer. The VB2-based carbon dots have excellent water solubility and biocompatibility, and their singlet oxygen generation ability is much stronger than that of riboflavin itself. Through endocytosis, the carbon dots can easily enter the cells and show bright green fluorescence. In vivo experiments show that after photodynamic therapy the carbon dots can significantly inhibit the growth of tumors, and will not have toxic and side effects on other organs.

Photosensitization Reactions of Biomolecules: Definition, Targets and Mechanisms

Photosensitization reactions have been demonstrated to be largely responsible for the deleterious biological effects of UV and visible radiation, as well as for the curative actions of photomedicine. A large number of endogenous and exogenous photosensitizers, biological targets and mechanisms have been reported in the past few decades. Evolving from the original definitions of the type I and type II photosensitized oxidations, we now provide physicochemical frameworks, classifications and key examples of these mechanisms in order to organize, interpret and understand the vast information available in the literature and the new reports, which are in vigorous growth. This review surveys in an extended manner all identified photosensitization mechanisms of the major biomolecule groups such as nucleic acids, proteins, lipids bridging the gap with the subsequent biological processes. Also described are the effects of photosensitization in cells in which UVA and UVB irradiation triggers enzyme activation with the subsequent delayed generation of superoxide anion radical and nitric oxide. Definitions of photosensitized reactions are identified in biomolecules with key insights into cells and tissues.

Flavin-Conjugated Nanobombs: Key Structural Requirements Governing Their Self-Assemblies' Morphologies

In contrast to artificial molecules, natural photosensitizers have the benefit of excellent toxicity profiles and of life-compatible activating energy ranges. Flavins are such photosensitizers that were selected by nature in a plethora of light-triggered biochemical reactions. Flavin-rich nanoparticles could thus emerge as promising tools in photodynamic therapies and in active-targeting drug delivery. Self-assembled flavin-conjugated phospholipids improve the pharmacokinetics of natural flavins and, in the case of controlled morphologies, reduce photobleaching phenomena. The current article presents a proof of concept for the design of riboflavin-rich nanoparticles of tunable morphology from multilamellar patches to vesicular self-assemblies. Coarse-grained simulations of the self-assembling process revealed the key interactions governing the obtained nanomaterials and successfully guided the synthesis of new flavin-conjugates of predictable self-assembly. The obtained flavin-based liposomes had a 65 nm hydrodynamic diameter, were stable, and showed potential photosensitizer activity.

Effectiveness of Riboflavin and Rose Bengal Photosensitizer Modified Adhesive Resin for Orthodontic Bonding

This study aimed to evaluate the effect of riboflavin (RF) and Rose Bengal (RB) photosensitizer modified adhesive resin on the degree of conversion (DC), and antimicrobial capacity after bonded to tooth surface. Different concentrations of RB and RF were prepared by homogenization method. An ultraviolet light source A (UVA) (375 nm wavelength, 3 mW/cm² power) was used for 30 min irradiation. FTIR was performed for control and test adhesives to analyze the DC. Antibacterial testing was performed using the MTT assay. Metal brackets were bonded using the modified adhesives and subjected for SEM examination. The surfaces of teeth and metal brackets were examined at ×10 magnification for assessing adhesive remnant index (ARI) after PDT, 24 h and thermocycling. For DC, control group, 0.1% RB and RF after PDT showed the highest value. SEM imaging indicated lowest growth of Streptococcus mutans over 0.5% of RB-PDT and RF-PDT as compared to the control group. The MTT assay outcomes reported that the activity of S. mutans substantially decreased with the addition of a high amount of either RB or RF (p < 0.01). Mean ARI scores showed a significant difference between all groups. This study concluded that 0.1% of either RB or RF after PDT can be used for bonding orthodontic brackets to the tooth surface with substantial antibacterial properties.

Challenging microalgal vitamins for human health

BACKGROUND

Vitamins' deficiency in humans is an important threat worldwide and requires solutions. In the concept of natural biofactory for bioactive compounds production, microalgae represent one of the most promising targets filling many biotechnological applications, and allowing the development of an eco-sustainable production of natural bioactive metabolites. Vitamins are probably one of the cutting edges of microalgal diversity compounds.

MAIN TEXT

Microalgae can usefully provide many of the required vitamins in humans, more than terrestrial plants, for instance. Indeed, vitamins D and K, little present in many plants or fruits, are instead available from microalgae. The same occurs for some vitamins B (B12, B9, B6), while the other vitamins (A, C, D, E) are also provided by microalgae. This large panel of vitamins diversity in microalgal cells represents an exploitable platform in order to use them as natural vitamins' producers for human consumption. This study aims to provide an integrative overview on vitamins content in the microalgal realm, and discuss on the great potential of microalgae as sources of different forms of vitamins to be included as functional ingredients in food or nutraceuticals for the human health. We report on the biological roles of vitamins in microalgae, the current knowledge on their modulation by environmental or biological forcing and on the biological activity of the different vitamins in human metabolism and health protection.

CONCLUSION

Finally, we critically discuss the challenges for promoting microalgae as a relevant source of vitamins, further enhancing the interests of microalgal "biofactory" for biotechnological applications, such as in nutraceuticals or cosmeceuticals.

Photoinactivation results of Enterococcus moraviensis with blue and violet light suggest the involvement of an unconsidered photosensitizer

Microorganisms can be photoinactivated with 405 and 450 nm irradiation, due to endogenous photosensitizers, which absorb light of these wavelengths and generate reactive oxygen species that destroy the cells from within. The photosensitizers assumed to be responsible are porphyrins in the spectral region around 405 nm and flavins at about 450 nm. The aim of this study was to investigate this hypothesis on enterococci, considering that they do not contain porphyrins. In photoinactivation experiments with Enterococcus moraviensis, 405 nm and 450 nm irradiation both led to a reduction of the bacterial concentration by several orders of magnitude with 405 nm irradiation being much more efficient. The measurement and analysis of the fluorescence spectra revealed no signs of porphyrins whereas flavins seemed to be rapidly converted to lumichrome by 405 nm radiation. Therefore, probably none of the usual suspects, porphyrins and flavins, was responsible for the photoinactivation of Enterococcus moraviensis during 405 nm irradiation. Fluorescence experiments revealed the spectra of lumichrome and NADH, which are both known photosensitizers. Presumably, one of them or both were actually involved here. As NADH and flavins (and therefore their photodegradation product lumichrome) are abundant in all microorganisms, they are probably also involved in 405 nm photoinactivation processes of other species.

Flavins Act as a Critical Liaison Between Metabolic Homeostasis and Oxidative Stress in the Retina

Derivatives of the vitamin riboflavin, FAD and FMN, are essential cofactors in a multitude of bio-energetic reactions, indispensable for lipid metabolism and also are requisites in mitigating oxidative stress. Given that a balance between all these processes contributes to the maintenance of retinal homeostasis, effective regulation of riboflavin levels in the retina is paramount. However, various genetic and dietary factors have brought to fore pathological conditions that co-occur with a suboptimal level of flavins in the retina. Our focus in this review is to, comprehensively summarize all the possible metabolic and oxidative reactions which have been implicated in various retinal pathologies and to highlight the contribution flavins may have played in these. Recent research has found a sensitive method of measuring flavins in both diseased and healthy retina, presence of a novel flavin binding protein exclusively expressed in the retina, and the presence of flavin specific transporters in both the inner and outer blood-retina barriers. In light of these exciting findings, it is even more imperative to shift our focus on how the retina regulates its flavin homeostasis and what happens when this is disrupted.

Deazaflavin reductive photocatalysis involves excited semiquinone radicals

Flavin-mediated photocatalytic oxidations are established in synthetic chemistry. In contrast, their use in reductive chemistry is rare. Deazaflavins with a much lower reduction potential are even better suited for reductive chemistry rendering also deazaflavin semiquinones as strong reductants. However, no direct evidence exists for the involvement of these radical species in reductive processes. Here, we synthesise deazaflavins with different substituents at C5 and demonstrate their photocatalytic activity in the dehalogenation of p-halogenanisoles with best performance under basic conditions. Mechanistic investigations reveal a consecutive photo-induced electron transfer via the semiquinone form of the deazaflavin as part of a triplet-correlated radical pair after electron transfer from a sacrificial electron donor to the triplet state. A second electron transfer from the excited semiquinone to p-halogenanisoles triggers the final product formation. This study provides first evidence that the reductive power of excited deazaflavin semiquinones can be used in photocatalytic reductive chemistry.

Flavins and deazaflavins are well suited for photoredox processes but their application in photoreductions is challenging. Here, the authors provide direct evidence of the high reductive power of excited deazaflavin semiquinones and their application in catalytic photodehalogenations.

Flavinium Catalysed Photooxidation: Detection and Characterization of Elusive Peroxyflavinium Intermediates

Flavin-based catalysts are photoactive in the visible range which makes them useful in biology and chemistry. Herein, we present electrospray-ionization mass-spectrometry detection of short-lived intermediates in photooxidation of toluene catalysed by flavinium ions (Fl+ ). Previous studies have shown that photoexcited flavins react with aromates by proton-coupled electron transfer (PCET) on the microsecond time scale. For Fl+ , PCET leads to FlH.+ with the H-atom bound to the N5 position. We show that the reaction continues by coupling between FlH.+ and hydroperoxy or benzylperoxy radicals at the C4a position of FlH.+ . These results demonstrate that the N5-blocking effect reported for alkylated flavins is also active after PCET in these photocatalytic reactions. Structures of all intermediates were fully characterised by isotopic labelling and by photodissociation spectroscopy. These tools provide a new way to study reaction intermediates in the sub-second time range.

Vitamins in cell culture media: Stability and stabilization strategies

Nowadays, chemically defined cell culture media (CCM) have replaced serum- and hydrolysate-based media that rely on complex ingredients, such as yeast extracts or peptones. Benefits include a significantly lower lot-to-lot variability, more efficient manufacturing by reduction to essential components, and the ability to exclude components that may negatively influence growth, viability, or productivity. Even though current chemically defined CCMs provide an excellent basis for various mammalian biotechnological processes, vitamin instabilities are known to be a key factor contributing to the variabilities still present in liquid CCM as well as to short storage times. In this review, the chemical degradation pathways and products for the most relevant vitamins for CCM will be discussed, with a focus on the effects of light, oxygen, heat, and other CCM compounds. Different approaches to stabilize vitamins in solution, such as replacement with analogs, encapsulation, or the addition of stabilizing compounds will also be reviewed. While these vitamins and vitamin stabilization approaches are presented here as particular for CCM, the application of these concepts can also be considered relevant for pharmaceutical, medical, and food supplement purposes. More precise knowledge regarding vitamin instabilities will contribute to stabilize future formulations and thus decrease residual lot-to-lot variability.

Identification of Flavin Mononucleotide as a Cell-Active Artificial N6 -Methyladenosine RNA Demethylase

N⁶ -Methyladenosine (m⁶ A) represents a common and highly dynamic modification in eukaryotic RNA that affects various cellular pathways. Natural dioxygenases such as FTO and ALKBH5 are enzymes that demethylate m⁶ A residues in mRNA. Herein, the first identification of a small-molecule modulator that functions as an artificial m⁶ A demethylase is reported. Flavin mononucleotide (FMN), the metabolite produced by riboflavin kinase, mediates substantial photochemical demethylation of m⁶ A residues of RNA in live cells. This study provides a new perspective to the understanding of demethylation of m⁶ A residues in mRNA and sheds light on the development of powerful small molecules as RNA demethylases and new probes for use in RNA biology.

Morpholine-based buffers activate aerobic photobiocatalysis via spin correlated ion pair formation

The use of enzymes for synthetic applications is a powerful and environmentally-benign approach to increase molecular complexity. Oxidoreductases selectively introduce oxygen and hydrogen atoms into myriad substrates, catalyzing the synthesis of chemical and pharmaceutical building blocks for chemical production. However, broader application of this class of enzymes is limited by the requirements of expensive cofactors and low operational stability. Herein, we show that morpholine-based buffers, especially 3-(N-morpholino)propanesulfonic acid (MOPS), promote photoinduced flavoenzyme-catalyzed asymmetric redox transformations by regenerating the flavin cofactor via sacrificial electron donation and by increasing the operational stability of flavin-dependent oxidoreductases. The stabilization of the active forms of flavin by MOPS via formation of the spin correlated ion pair ³[flavin˙^--MOPS˙⁺] ensemble reduces the formation of hydrogen peroxide, circumventing the oxygen dilemma under aerobic conditions detrimental to fragile enzymes.

Iron(ii) tetrafluoroborate complexes of new tetradentate C-scorpionates as catalysts for the oxidative cleavage of trans-stilbene with H2O2

Iron(ii) complexes of two new tetradentate C-scorpionate ligands are characterized. Both catalyze stilbene cleavage using either H₂O₂ or a O₂/photocatalyst oxidant.

Boosting photobioredox catalysis by morpholine electron donors under aerobic conditions

Morpholine derivatives expand the applicability of photobiocatalysis towards stabilization of flavin-based bio- and photocatalysts.

Direct Electrochemical Vibrio DNA Sensing Adopting Highly Stable Graphene-Flavin Mononucleotide Aqueous Dispersion Modified Interface

A biofunctionalized graphene nanohybrid was prepared by simultaneously sonicating graphene and riboflavin 5'-monophosphate sodium salt (FMNS). FMNS, as a biodispersant, showed an efficient stabilization for obtaining highly dispersed graphene nanosheets in an aqueous solution. Due to the superior dispersion of graphene and the excellent electrochemical redox activity of FMNS, a direct electrochemical DNA sensor was fabricated by adopting the inherent electrochemical redox activity of graphene-FMNS (Gr-FMNS). The comparison between using traditional electrochemical indicator ([Fe(CN)₆]^3-/4-) and using the self-signal of Gr-FMNS was fully conducted to study the DNA-sensing performance. The results indicate that the proposed DNA-sensing platform displays fine selectivity, high sensitivity, good stability, and reproducibility using either [Fe(CN)₆]^3-/4- probe or the self-signal of Gr-FMNS. The two methods display the same level of detection limit: 7.4 × 10^-17 M (using [Fe(CN)₆]^3-/4-) and 8.3 × 10^-17 M (using self-signal), respectively, and the latter exhibits higher sensitivity. Furthermore, the sensing platform also can be applied for the DNA determination in real samples.

Photostability Issues in Pharmaceutical Dosage Forms and Photostabilization

Photodegradation is one of the major pathways of the degradation of drugs. Some therapeutic agents and excipients are highly sensitive to light and undergo significant degradation, challenging the quality and the stability of the final product. The adequate knowledge of photodegradation mechanisms and kinetics of photosensitive therapeutic entities or excipients is a pivotal aspect in the product development phase. Hence, various pharmaceutical regulatory agencies, across the world, mandated the industries to assess the photodegradation of pharmaceutical products from manufacturing stage till storage, as per the guidelines given in the International Conference on Harmonization (ICH). Recently, numerous formulation and/or manufacturing strategies has been investigated for preventing the photodegradation and enhancing the photostability of photolabile components in the pharmaceutical dosage forms. The primary focus of this review is to discuss various photodegradation mechanisms, rate kinetics, and the factors that influence the rate of photodegradation. We also discuss light-induced degradation of photosensitive lipids and polymers. We conclude with a brief note on different approaches to improve the photostability of photosensitive products.

Design of peptide-containing N5-unmodified neutral flavins that catalyze aerobic oxygenations

Simulation of the monooxygenation function of flavoenzyme (Fl-Enz) has been long-studied with N5-modified cationic flavins (FlEt+ ), but never with N5-unmodified neutral flavins (Fl) despite the fact that Fl is genuinely equal to the active center of Fl-Enz. This is because of the greater lability of 4a-hydroperoxy adduct of Fl, FlOOH , compared to those of FlEt+ , FlEtOOH , and Fl-Enz, FlOOH-Enz. In this study, Fl incorporated into a short peptide, flavopeptide (Fl-Pep), was designed by a rational top-down approach using a computational method, which could stabilize the corresponding 4a-hydroperoxy adduct (FlOOH-Pep) through intramolecular hydrogen bonds. We report catalytic chemoselective sulfoxidation as well as Baeyer-Villiger oxidation by means of Fl-Pep under light-shielding and aerobic conditions, which are the first Fl-Enz-mimetic aerobic oxygenation reactions catalyzed by Fl under non-enzymatic conditions.

Riboflavin as a bioorthogonal photocatalyst for the activation of a PtIV prodrug

Encouraging developments demonstrate that few transition metal and organometallic catalysts can operate in a bioorthogonal fashion and promote non-natural chemistry in living systems by minimizing undesired side reactions with cellular components. These catalytic processes have potential for applications in medicinal chemistry and chemical biology. However, the stringent conditions of the cell environment severely limit the number of accessible metal catalysts and exogenous reactions. Herein, we report an unorthodox approach and a new type of bioorthogonal catalytic reaction, in which a metal complex is an unconventional substrate and an exogenous biological molecule acts as a catalyst. In this reaction, riboflavin photocatalytically converts a PtIV prodrug into cisplatin within the biological environment. Due to the catalytic activity of riboflavin, cisplatin-like apoptosis is induced in cancer cells under extremely low doses of light, potentially preventing systemic off-target reactions. Photocatalytic and bioorthogonal turnover of PtIV into PtII species is an attractive strategy to amplify the antineoplastic action of metal-based chemotherapeutics with spatio-temporal control.

Visible-light-mediated oxidative demethylation of N6-methyl adenines

A novel solution for the chemical modulation of RNA epigenetics via biologically compatible photo-catalysis under visible light irradiation has been described.

Photophysical Characterization and in Vitro Phototoxicity Evaluation of 5,10,15,20-Tetra(quinolin-2-yl)porphyrin as a Potential Sensitizer for Photodynamic Therapy

Photodynamic therapy (PDT) is a selective and minimally invasive therapeutic approach, involving the combination of a light-sensitive compound, called a photosensitizer (PS), visible light and molecular oxygen. The interaction of these per se harmless agents results in the production of reactive species. This triggers a series of cellular events that culminate in the selective destruction of cancer cells, inside which the photosensitizer preferentially accumulates. The search for ideal PDT photosensitizers has been a very active field of research, with a special focus on porphyrins and porphyrin-related macrocycle molecules. The present study describes the photophysical characterization and in vitro phototoxicity evaluation of 5,10,15,20-tetra(quinolin-2-yl)porphyrin (2-TQP) as a potential PDT photosensitizer. Molar absorption coefficients were determined from the corresponding absorption spectrum, the fluorescence quantum yield was calculated using 5,10,15,20-tetraphenylporphyrin (TPP) as a standard and the quantum yield of singlet oxygen generation was determined by direct phosphorescence measurements. Toxicity evaluations (in the presence and absence of irradiation) were performed against HT29 colorectal adenocarcinoma cancer cells. The results from this preliminary study show that the hydrophobic 2-TQP fulfills several critical requirements for a good PDT photosensitizer, namely a high quantum yield of singlet oxygen generation (Φ∆ 0.62), absence of dark toxicity and significant in vitro phototoxicity for concentrations in the micromolar range.

Synthesis and electronic properties of π-extended flavins

Flavin derivatives with an extended π-conjugation were synthesized in moderate to good yields from aryl bromides via a Buchwald-Hartwig palladium catalyzed amination protocol, followed by condensation of the corresponding aromatic amines with violuric acid. The electronic properties of the new compounds were investigated by absorption and emission spectroscopy, cyclic voltammetry, density functional theory (DFT) and time dependent density functional theory (TDDFT). The compounds absorb up to 550 nm and show strong luminescence. The photoluminescence quantum yields ϕPL measured in dichloromethane reach 80% and in PMMA (poly(methyl methacrylate)) 77%, respectively, at ambient temperature. The electrochemical redox behaviour of π-extended flavins follows the mechanism previously described for the parent flavin.