A Flavin Analogue with Improved Solubility in Organic Solvents

Aminolactonization of Unactivated Alkenes Catalyzed by Aryl Iodine

A one-step protocol of the aryl iodine-catalyzed aminolactonization of unactivated alkenes under oxidation conditions was first reported to efficiently construct diverse amino lactones in a short time using HNTs₂ as the compatible nitrogen source. In addition, we investigated the influence of the reaction rate based on the structure of the iodoarene precatalyst, which revealed the selective adjustment effect on aminolactonization and oxylactonization. Finally, preliminary experiments verified the feasibility of asymmetric aminolactonization catalyzed by a chiral iodoarene precatalyst.

Solid-state NMR of flavins and flavoproteins

Why apply solid-state NMR (SSNMR) to flavins and flavoproteins? NMR provides information on an atom-specific basis about chemical functionality, structure, proximity to other groups, and dynamics of the system. Thus, it has become indispensable to the study of chemicals, materials, catalysts, and biomolecules. It is no surprise then that NMR has a great deal to offer in the study of flavins and flavoenzymes. In general, their catalytic or electron-transfer activity resides essentially in the flavin, a molecule eminently accessible by NMR. However, the specific reactivity displayed depends on a host of subtle interactions whereby the protein biases and reshapes the flavin's propensities to activate it for one reaction while suppressing other aspects of this cofactor's prodigious repertoire (Massey et al., J Biol Chem 244:3999-4006, 1969; Müller, Z Naturforsch 27B:1023-1026, 1972; Joosten and van Berkel, Curr Opin Struct Biol 11:195-202, 2007). Thus, we are fascinated to learn about how the flavin cofactor of one enzyme is, and is not, like the flavin cofactor of another. In what follows, we describe how the capabilities of SSNMR can help and are beginning to bear fruit in this exciting endeavor.

NMR spectroscopy on flavins and flavoproteins

(1)H-, (11)B-, (13)C-, (15)N-, (17)O-, (19)F-, and (31)P-NMR chemical shifts of flavocoenzymes and derivatives of it, as well as of alloxazines and isoalloxazinium salts, from NMR experiments performed under various experimental conditions (e.g., dependence of the chemical shifts on temperature, concentration, solvent polarity, and pH) are reported. Also solid-state (13)C- and (15)N-NMR experiments are described revealing the anisotropic values of corresponding chemical shifts. These data, in combination with a number of coupling constants, led to a detailed description of the electronic structure of oxidized and reduced flavins. The data also demonstrate that the structure of oxidized flavin can assume a configuration deviating from coplanarity, depending on substitutions in the isoalloxazine ring, while that of reduced flavin exhibits several configurations, from almost planar to quite bended. The complexes formed between oxidized flavin and metal ions or organic molecules revealed three coordination sites with metal ions (depending on the chemical nature of the ion), and specific interactions between the pyrimidine moiety of flavin and organic molecules, mimicking specific interactions between apoflavoproteins and their coenzymes. Most NMR studies on flavoproteins were performed using (13)C- and (15)N-substituted coenzymes, either specifically enriched in the pterin moiety of flavin or uniformly labeled flavins. The chemical shifts of free flavins are used as a guide in the interpretation of the chemical shifts observed in flavoproteins. Although the hydrogen-bonding pattern in oxidized and reduced flavoproteins varies considerably, no correlation is obvious between these patterns and the corresponding redox potentials. In all reduced flavoproteins the N(1)H group of the flavocoenzyme is deprotonated, an exception is thioredoxin reductase. Three-dimensional structures of only a few flavoproteins, mostly belonging to the family of flavodoxins, have been solved. Also the kinetics of unfolding and refolding of flavodoxins has been investigated by NMR techniques. In addition, (31)P-NMR data of all so far studied flavoproteins and some (19)F-NMR spectra are discussed.

Use of Ethyl (Benzothiazol-2-ylsulfonyl)acetate for Malonic Ester-type Syntheses of Carboxylic Acids and Esters

A new methodology for the synthesis of substituted carboxylic acids is described. Alkylation of either ethyl (benzothiazol-2-ylsulfonyl)acetate or ethyl 2-(benzothiazol-2-ylsulfonyl)propionate was achieved with alkyl halides and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in dichloromethane solution. These products were then desulfinated and hydrolysed in one-pot under mild conditions to give substituted acetic acids in good-to-excellent yields.

Manipulating Reduction Potentials in an Artificial Safranin Cofactor

Safranines hold great promise as artificial flavin-like electron transfer cofactors with tunable properties. We report the design and chemical synthesis of the p-methoxy derivative of safranine O using a new synthetic route based on the Ulmann condensation. Spectroelectrochemical comparison of the purified parent safranine and this derivative demonstrates that the modification increases its two-electron reduction potential by 125 mV, or 5.75 kcal/mol. This modification also causes redshifts in the absorbance and fluorescence spectra of the cofactor, suggesting that it may find future utility in arrayed sensor applications.

15N solid-state NMR as a probe of flavin H-bonding

Flavins mediate a wide variety of chemical reactions in biology. To learn how one cofactor can be made to execute different reactions in different enzymes, we are developing solid-state NMR (SSNMR) to probe the flavin electronic structure, via the (15)N chemical shift tensor principal values (δ(ii)). We find that SSNMR has superior responsiveness to H-bonds, compared to solution NMR. H-bonding to a model of the flavodoxin active site produced an increase of 10 ppm in the δ(11) of N5, although none of the H-bonds directly engage N5, and solution NMR detected only a 4 ppm increase in the isotropic chemical shift (δ(iso)). Moreover SSNMR responded differently to different H-bonding environments, as H-bonding with water caused δ(11) to decrease by 6 ppm, whereas δ(iso) increased by less than 1 ppm. Our density functional theoretical (DFT) calculations reproduce the observations, validating the use of computed electronic structures to understand how H-bonds modulate the flavin's reactivity.

Hydrogen bond-free flavin redox properties: managing flavins in extreme aprotic solvents

We report a simple, single step reaction that transforms riboflavin, which is insoluble in benzene, to tetraphenylacetyl riboflavin (TPARF), which is soluble in this solvent to over 250 mM. Electrochemical analysis of TPARF both alone and in complexes with two benzene-soluble flavin receptors: dibenzylamidopyridine (DBAP) and monobenzylamidopyridine (MBAP), prove that this model system behaves similarly to other previously studied flavin model systems which were soluble only in more polar solvents such as dichloromethane. Binding titrations in both benzene and dichloromethane show that the association constants of TPARF with its ligands are over an order of magnitude higher in benzene than dichloromethane, a consequence of the fact that benzene does not compete for H-bonds, but dichloromethane does. The alteration induced in the visible spectrum of TPARF in benzene upon the addition of DBAP is very similar to the difference produced by transfer to dichloromethane, further indicating that the flavin head group engages in a similar degree of hydrogen bonding with dichloromethane as with its ligands. This work underlines the importance of using a truly nonpolar solvent for the analysis of the effects of hydrogen bonding on the energetics of any biomimetic host-guest model system.