A roadmap to the isotopolog space of flavocoenzymes

Flavocoenzymes with selective or universal stable isotope labeling are important tools for the investigation of flavoproteins using a variety of spectroscopic methods. Numerous selectively labeled flavin isotopologs can be generated by the combined application of chemical synthesis and in vitro biotransformation using commercially available enzymes and/or recombinant riboflavin biosynthesis enzymes. Notably, the complex reaction sequences can be rapidly carried out using enzyme-assisted one-pot reaction strategies.

Synthesis and characterization of naphthalenediimide-functionalized flavin derivatives

Two acceptor-acceptor dyads have been synthesized featuring a flavin moiety and a naphthalenediimide (NDI) unit. The NDI unit is linked to the flavin through a short spacer group via either the N(3) or N(10) positions of the flavin. We have investigated the UV-Vis and redox properties of these multi-electron accepting systems which indicate that these materials display the collective properties of their component systems. Fluorescence spectroscopy measurements have revealed that their emission properties are dominated by the flavin unit.

Antitumor studies. Part 3: Design, synthesis, antitumor activity, and molecular docking study of novel 2-methylthio-, 2-amino-, and 2-(N-substituted amino)-10-alkyl-2-deoxo-5-deazaflavins

Various novel 10-alkyl-2-deoxo-2-methylthio-5-deazaflavins have been synthesized by reaction of 6-(N-alkylanilino)-2-methylthiopyrimidin-4(3H)-ones with Vilsmeier reagent. The similar 2-(N-substituted amino) derivatives were prepared by nucleophilic replacement reaction of the 2-methylthio moiety by appropriate amines. The 2-oxo derivatives (i.e., 5-deazaflavins) were obtained by acidic hydrolysis of the 2-methylthio derivatives. The antitumor activities against CCRF-HSB-2 and KB cells and the antiviral activities against HSV-1 and HSV-2 have been investigated in vitro, and many compounds showed promising antitumor activities. Furthermore, AutoDock molecular docking into PTK has been done for lead optimization of these compounds as potential PTK inhibitors. Whereas, the designed 2-deoxo-5-deazaflavins connected with amino acids at the 2-position exhibited the good binding affinities into PTK with more hydrogen bonds.

Synthesis of 5-deazaflavin derivatives and their activation of p53 in cells

A family of 5-deazaflavin derivatives has been synthesised using a two-step convergent strategy. The biological activity of these compounds was evaluated in cells, by assessing their ability to stabilize and activate p53. These compounds may act as low molecular weight inhibitors of the E3 activity of HMD2 in tumours that retain wild-type p53. Importantly, we have demonstrated that the nitro group present in all three of the original lead compounds [1-3 (HL198C-E)] is not essential for observation of this biological activity.

Antitumor studies. Part 1: design, synthesis, antitumor activity, and AutoDock study of 2-deoxo-2-phenyl-5-deazaflavins and 2-deoxo-2-phenylflavin-5-oxides as a new class of antitumor agents

Novel 2-deoxo-2-phenyl-5-deazaflavins and 2-deoxo-2-phenylflavin-5-oxides were prepared as a new class of antitumor agents and showed significant antitumor activities against NCI-H 460, HCT 116, A 431, CCRF-HSB-2, andKB cell lines. In vivo investigation, 2-deoxo-10-methyl-2-phenyl-5-deazaflavin exhibited the effective antitumor activity against A 431 human adenocarcinoma cells transplanted subcutaneously into nude mouse. Furthermore, AutoDock study has been done by binding of the flavin analogs into PTK pp60(c-src), where a good correlation between their IC(50) and AutoDock binding free energy was exhibited. In particular, 2-deoxo-2-phenylflavin-5-oxides exhibited the highest potential binding affinity within the binding pocket of PTK.

Investigation of the Pathways of Excess Electron Transfer in DNA with Flavin-Donor and Oxetane-Acceptor Modified DNA Hairpins

Oxetane is a potential intermediate that is enzymatically formed during the repair of (6-4) DNA lesions by special repair enzymes (6-4 DNA photolyases). These enzymes use a reduced and deprotonated flavin to cleave the oxetane by single electron donation. Herein we report synthesis of DNA hairpin model compounds containing a flavin as the hairpin head and two different oxetanes in the stem structure of the hairpin. The data show that the electron moves through the duplex even over distances of 17 A. Attempts to trap the moving electron with N2O showed no reduction of the cleavage efficiency showing that the electron moves through the duplex and not through solution. The electron transfer is sequence dependent. The efficiency is reduced by a factor of 2 in GC rich DNA hairpins.

Model systems for flavoenzyme activity: relationships between cofactor structure, binding and redox properties

A series of flavins were synthesized bearing electron-withdrawing and -donating substituents. The electrochemical properties of these flavins in a nonpolar solvent were determined. The recognition of these flavins by a diamidopyridine (DAP) receptor and the effect this receptor has on flavin redox potential was also quantified. It was found that the DAP-flavin binding affinity and the reduction potentials (E(1/2)) for both the DAP-bound and unbound flavins correlated well with functions derived from linear free energy relationships (LFERs). These results provide insight and predictive capability for the interplay of electronics and redox state-specific interactions for both abiotic and enzymatic systems.

Synthesis and anti-HIV-1 activity of novel 10-thiaisoalloxazines, a structural analog of C-5 and/or C-6 substituted pyrimidine acyclonucleoside

A series of novel 10-thiaisoalloxazine derivatives bearing an alkoxymethyl or benzyloxymethyl moiety at the N-1 position has been synthesized through the bromination of 1-substituted-5-hydroxyuracils and subsequent condensation with aminobenzenethiol in a one-pot reaction. Contrary to the previous report, the formation of intermediary 5,6-diethoxy-5-hydroxy-5,6-dihydrouracil seems to be not the necessary factor for the formation of the thiaisoalloxazines, since the reaction proceeds in tetrahydrofuran (THF) or acetonitrile far more smoothly than in ethanol. The anti-human immunodeficiency virus (HIV)-1 activity of the resulted thiaisoalloxazine derivatives was evaluated in lymphocyte cells based on the inhibitory activity against the viral-induced cytopathic activity. Among the derivatives, compounds 6, 7, and 8 bearing an alkoxymethyl moiety at the N-1 position exhibited modest inhibitory activity towards the cytotopathic effect of HIV-1.

Synthesis, characterization and reactions of 2-deoxo-5-deazaalloxazines

5-Deazaflavins and their homologues have been known as potential riboflavin antagonists, bioreductives, and compounds with potent antitumor activity. 2-Amino-4-methylquinoline-3-carbonitrile (2) was prepared as unreported starting material for several interesting 2-deoxo-5-deazalloxazine derivatives. Cyclization of 2 using formamide afforded the 2,4-deoxo-5-deazaalloxazine derivative 7, which was subjected to deamination with nitrous acid to give the 2-deoxo-5-deazaalloxazine (8). The compound 8 was also obtained via 13 by treating the latter with refluxing formic acid or formamide and used as a precursor for synthesis of several 2-deoxo-5-deazaalloxazines 18, 19, 20, 21 and 22. The pharmacological and biological properties of these compounds are still under investigation.

Synthesis and antitumor activities of novel 5-deazaflavin-sialic acid conjugate molecules

6-Nitro-5-deazaflavin derivatives bearing O-(methyl 5-acetamido-4,7,8,9-tetra-O-acetyl-3,5-dideoxy-D-glycero-alpha- and beta-D-galacto-non-2-ulopyranosylonate)alkyl group (sialosylalkyl group) at N(3) or N(10) and 8-amino-5-deazaflavin substituted with the sialosylalkyl group at the amino group were synthesized and their physicochemical properties as well as antitumor effects on KB and L1210 cells have been investigated. The configurations of the glycosides were determined by 1H NMR and rate of hydrolysis of the glycosidic bond. It has been found that these conjugate molecules show significant antitumor activities. Combination of an 8-amino-5-deazaflavin with the sialosylalkyl group have been found to give rise to significant increase in antitumor activities of the compound. Antitumor effects of 6-nitro-5-deazaflavin-sialic acid conjugate molecules were similar or rather weak in comparison with those of the 6-nitro-5-deazaflavin derivatives without sialosylalkyl group.

Synthesis, DNA-binding properties and cytotoxic activity of flavin-oligopyrrolecarboxamide and flavin-oligoimidazolecarboxamide conjugates

The aim of this study was to develop novel series of photosensitizer-DNA minor groove binder hybrids composed of a flavin (isoalloxazine) chromophore linked to a moiety related to netropsin or distamycin. Three series (Fla-Pyr, Fla-Gly-Pyr and Fla-Gly-Im) were synthesized which differ by the number and the nature of the heterocyclic nuclei in the oligopeptide units, the nature of the linker and its anchoring position on the flavin. In terms of DNA binding and DNA specificity, satisfactory data are obtained in the Fla-Pyr and Fla-Gly-Pyr series; in terms of photo-induced cytotoxicity, the results are disappointing. The present study allows us to draw the following structure-activity relationships: (i) substitution of the flavin nucleus in either the N3 or the N10 position does not affect the activity; (ii) tris-pyrrolic hybrids are more efficient than bis- and tetra-pyrrolic analogs; (iii) the presence of a glycin in the linking chain does not suppress the DNA binding properties or the cytotoxic activities of the hybrids; and (iv) the replacement of the pyrrole nuclei by imidazoles has a drastic effect since it results in the loss of DNA affinity and cytotoxicity.

Synthesis and evaluation of nitro 5-deazaflavin-pyrrolecarboxamide(s) hybrid molecules as novel DNA targeted bioreductive antitumor agents

A series of 6-nitro-5-deazaflavins bearing at N(3) or N(10) position the pyrrolecarboxamide(s) group as DNA minor groove binder has been synthesized. These hybrid molecules show similar redox properties to those of 6-nitro-5-deazaflavins with no pyrrolecarboxamide(s) group, suggesting that they generate stable one- and two-electron reduction product(s). Electrolytic reductions of the hybrid molecules were carried out at a controlled potential under anaerobic conditions in the presence of plasmid pBR322 DNA. Significant conversions of the supercoiled circular pBR322 DNA (form I) to the open circular DNA (form II) have been found by treatment with the reductively activated 6-nitro-5-deazaflavin derivatives. Their DNA damaging effects have been found to be enhanced as the number of pyrrolecarboxamide group as the DNA binder increases. Antitumor activities of the hybrid molecules towards KB and L1210 cells were evaluated in vitro. It has been found that the antitumor effects of the compounds on KB cells slightly change and those on L1210 cells decrease as the number of the pyrrolecarboxamide group increases. These results reveal that the combination of 6-nitro-5-deazaflavin molecule with the pyrrolecarboxamide(s) group increase the DNA binding properties of the compounds, giving rise to promoted DNA damaging effects, and also suggest that the combination would affect the capacity of the compounds to act as the substrate for intracellular reductases and/or the cellular uptake of the compounds.

Poly(pyrrolecarboxamides) linked to photoactivable chromophore isoalloxazine. Synthesis, selective binding, and DNA cleaving properties

In an attempt to obtain DNA sequence-specific cleaving molecules, we have synthesized two types of hybrid groove binders composed of an isoalloxazine (flavin) chromophore linked through a polymethylenic chain to either a bis- or a tris(pyrrolecarboxamide) moiety related to netropsin and distamycin, respectively. In both types of molecules, the polymethylenic chain is linked to the alloxazine ring either in the N10 position or in the N3 position. As netropsin and distamycin, the hybrid derivatives preferentially bind to A + T-rich sequences and recognize sequences such as 5'-ATTT. Upon visible light irradiation the flavin moiety undergoes a redox cycling process generating superoxide anion and hydroxyl radical. Generation of oxy radicals appears to be more efficient with the hybrids in which the polymethylenic chain is linked at the N10 position. The generation of oxy radicals results in the occurrence of single strand break in supercoiled DNA. Breaks preferentially occur in the vicinity of A + T-rich sequences. The advantage of flavin relative to other oxy radicals generating compounds such as ferrous-EDTA is that it does not require chemical reduction but can be reduced either by visible light or by cellular enzymes, both conditions being compatible with pharmacological constraints.

Flavins as potential antimalarials. 2. 3-Methyl-10-(substituted-phenyl) flavins

A series of 3-methyl-10-(substituted-phenyl)flavins was prepared and tested for antimalarial activity against the lethal parasite Plasmodium vinckei in mice. Several of these analogues were found to be effective antimalarial agents. A quantitative structure-activity relationship study was undertaken with 44 analogues and no satisfactory relationship could be established.

Preparation and some properties of 6-substituted flavins as active site probes for flavin enzymes

6-Azidoflavins, 6-thiocyanatoflavins, and 6-mercaptoflavins at the lumiflavin, riboflavin, FMN, and FAD level were prepared from the corresponding 6-aminoflavins and some of their properties investigated. They are bound tightly by apoflavin enzymes which bind either riboflavin, FMN, or FAD. 6-Azidoflavins undergo facile photolysis. One major product was identified as 6-aminoflavin. A further product, which was formed also during acid decomposition of the azide, results from opening of the flavin benzene ring and is proposed to have a lumazine structure. 6-Thiocyanatoflavins are easily converted by dithiothreitol to 6-mercaptoflavins. The latter are stabilized against dimerization in the presence of reducing thiols. 6-Mercaptoflavins have a pK of 5.9, which corresponds to ionization of the 6-SH function. The neutral form is yellow, while the anion is green, due to a long-wavelength band (lambda max approximately 600 nm) extending beyond 700 nm. These properties suggest the use of these 6-substituted flavins for probing the active site of flavin enzymes. Because their reactive substituents are in close proximity to the flavin N(5)-position, these 6-substituted derivatives should also serve as useful probes of the environment around the flavin N(5), a position known to be involved in all flavin-mediated redox processes.

Hydroperoxyflavin-mediated oxidations of organosulfur compounds. Model studies for the flavin monooxygenase

Kinetic and product studies were carried out for the reaction of a synthetic hydroperoxyflavin with a series of organosulfur compounds as a model for the flavin-containing monooxygenase of mammalian liver (FMO). S-Oxidized products were identified, and the kinetics of the oxidation reactions were consistent with a mechanism involving attack of a sulfur nucleophile on the terminal oxygen atom of the hydroperoxyflavin. These results provide information about the substrate oxygenation step of FMO not available from steady-state enzyme kinetics.

Chemical synthesis and some properties of 6-substituted flavins

A number of derivatives of riboflavin and of 3-methyllumiflavin substituted in the 6 position have been synthesized starting with 6-nitro flavins, reduction to the 6-amino flavin, and diazotization, followed by reaction with the appropriate nucleophile. The absorption spectra, oxidation-reduction potentials, and the electron spin resonance spectra of the radical cationic forms of several of these synthetic compounds have been determined, including 6-S-cysteinyl-3-methyllumiflavin and 6-S-cysteinylriboflavin. The latter has been shown to be identical with the dephosphorylated form of the aminoacyl flavin isolated from trimethylamine dehydrogenase [Steenkamp, D. J., Kenney, W. C. & Singer, T. P. (1978) J. Biol. Chem. 253, 2812-2817; Steenkamp, D. J., McIntire, W., & Kenney, W. C. (1978) J. Biol. Chem. 253, 2818-2824] in regard to absorption specturm, photochemical properties, and mobility in high-voltage electrophoresis and in thin-layer chromatography. An unusually pronounced interaction between the amino group and the isoalloxazine ring system was deduced from the absorption spectra of 6-amino-3-methyllumiflavin and 6-aminoriboflavin.

(Photo)chemistry of 5-deazaflavin. A clue to the mechanism of flavin-dependent (de)hydrogenation

The catalytic action of 5-deazaflavin in the photochemical reduction of flavin and iron proteins [Massey, V. and Hemmerich, P. (1978) Biochemistry, 17, 9--17] is shown to be due to the highly reactive 5-deazaflavosemiquinone. This radical is generated in a complex sequence of reactions, which involves (a) covalent photoaddition of the substrate residue to the deazaflavin, (b) fast secondary photoreaction of this adduct with starting deazaflavin to yield a covalent radical dimer, accompanied by the liberation of the oxidized substrate, and (c) deazaflavin-sensitized cleavage of the radical dimer to the monomers. The structure and properties of this radical (redimerisation or dismutation) and the precursor intermediates as well as the mechanism of the photoreaction are described. Deazaflavins and their natural parent compounds are compared with respect to their different redox behavior and radical stability. The syntheses of 5-deuterated deazaflavins are described and their redox reactions are compared with those of normal deazaflavins.

Syntheses and biological activities of basically substituted isoalloxazines

A number of flavins possessing a 2-[bis(2-hydroxyethyl)aminolethyl side chain in place of the usual D-ribityl side chain at position 10 have been synthesized and evaluated for riboflavin antagnostic activity in the rat. Two flavins of those synthesized are potent antagonists of the vitamin.

Anti-riboflavin activity of 8N-alkyl analogues of roseoflavin in some Gram-positive bacteria

Three new 8N-alkyl analogues of roseoflavin (MM), i.e., 8-ethylamino- (EH), 8-methylethylamino- (ME), 8-diethylamino-8-demethyl-D-riboflavin (EE), their tetraacetates, and 8-amino-8-demethyl-D-riboflavin (HH) tetraacetate, were synthesized. A relation between the anti-riboflavin activity and the chemical structure of 8N-alkyl analogues (8N-methyl, ethyl) was studied by a restoration by riboflavin (RF) of inhibitory effect of the analogues on a growth of Gram-positive bacteria, i.e., Sarcina lutea, Bacillus cereus, and Staphylococcus aureus. The inhibitory effect of most of the analogues was restored by RF. But in some cases, i.e., 8-methylamino-8-demethyl-D-riboflavin (MH) in Sar. lutea and MM in Staph. aureus, the effect was not completely restored. Apparently, the inhibition in early phase of growth was restored, but the maximum growth was still suppressed. The non-alkylated amino analogue (HH) showed only unrestorable suppression of maximum growth in Sar. lutea. Of restorable effect by RF of N-alkyl analogues, approximate decreasing orders of anti-RF activity were as follows. Dialkylated analogue greater than monoalkylated. HH showed insignificant anti-RF activity. In each group, methylated analogue greater than ethylated. In B. cereus monoalkylated analogues, and in Staph. aureus monoalkylated and EE showed no significant inhibitory effect. Redox potentials of the N-alkyl analogues were measured, and a definite relation between the chemical structure and the potential was found (RF = EE greater than ME greater than MM greater than HH greater than EH greater than MH). But the anti-RF activity of the analogues was not completely explained by the difference of the redox potential from RF.

4-Alkyldihydroflavin: coenzyme synthesis and modification of flavodoxin

A stable (i.e. non-oxidisable), reduced flavocoenzyme, 4a-(n-propyl)-4a,5-dihydro-riboflavin 5'-monophosphate, has been synthesized. The method of choice was selective photo-allylation in position 4a. Stabilization of this substituent was achieved by hydrogenation after N(5) protection, while hydrogenation of the unprotected compound would have led to removal of the allyl substituent. The mode of this substituent mobility has been checked with deuterated photosubstrate. Upon hydrogenation, the allyl substituent becomes firmly and irreversibly fixed to the flavin surface. The diastereomers thus formed have been separated and characterized by their ultraviolet, circular dichroic and nuclear magnetic resonance spectra. After subsequent side chain phosphorylation it has been shown that only one of the diastereomeric dihydro-flavocoenzymes is firmly bound by Peptostreptococcus elsdenii apoflavodoxin. This leads to the hypothesis that (a) the non-planar 4a,5-dihydro-flavin isomer and, therefore, C(4a) as active flavin center, may have biological relevance, while (b) the flavin site in flavodoxin is accessible to some extent for a stacking interaction with a redox-partner group.

Simple synthesis of a 4a-hydroperoxy adduct of a 1,5-dihydroflavine: preliminary studies of a model for bacterial luciferase

The solution chemistry of N(5)-alkyl flavinium cations and radical species formed by their le- reduction are discussed. Previously unknown, the 4a-flavine hydroperoxides are established to be formed on reaction of N(5)-alkyl flavinium cations with H2O2 or on reaction of N(5)-alkyl-1, 5-dihydroflavines with 3O2. The stability of the 4a-flavine hydroperoxide species is exemplified in the isolation and characterization of 4a-hydroperoxy-N(5)-ethyl-3-methyl-lumiflavine. 4a-Flavine hydroperoxide compounds are shown to be stronger oxidants than H2O2, and to undergo a chemiluminescent reaction in the presence of an aldehyde. Preliminary observations on the chemiluminescent reaction of 4a-flavine hydroperoxides + RCHO are provided, and these are compared to those in the literature dealing with the bioluminescence of bacterial luciferase in the presence of 3O2 and RCHO.

Identification and properties of 8-hydroxyflavin--adenine dinucleotide in electron-transferring flavoprotein from Peptostreptococcus elsdenii

1. A new flavin prosthetic group has been isolated in pure form from the electron-transferring flavoprotein of Peptostreptococcus elsdenni. Its structure has been established as the FAD derivative of 7-methyl-8-hydroxyisoalloxazine: (see article). Proof of this structure has been obtained by chemical syntehsis of 7-methyl-8-hydroxyisoalloxazine models, and by stepwise degradation of the native compound to 7-methy-8-hydroxyalloxazine. The orange chromophore is characterized by a strong absorption band with a maximum at 472 nm (xi = 41 000 M-1 CM-1) and a pK at 4.8 due to the ionisation of the C(8)-OH group. 2. The properties of a series of functionally substituted derivatives of 8-hydroxy flavins and lumichromes have been investigated to provide a basis for interpreting the effects of pH on the spectroscopic properties of the 8-hydroxy derivatives of FAD and FMN. 3. The 8-hydroxy derivative of FAD is bound by apo-D-amino acid oxidase; the complex shows no catalytic activity. The 8-hydroxy derivative of FMN is bound by apoflavodoxin to give a complex which has catalytic activity similar to that of native flavodoxin. The complex is reversibly reduced by dithionite, first to a relatively stable semiquinone and further to the dihydroflavin form.