Probing Protonation Sites of Isolated Flavins Using IR Spectroscopy: From Lumichrome to the Cofactor Flavin Mononucleotide

Infrared spectra of the isolated protonated flavin molecules lumichrome, lumiflavin, riboflavin (vitamin B₂), and the biologically important cofactor flavin mononucleotide are measured in the fingerprint region (600–1850 cm^?1) by means of IR multiple‐photon dissociation (IRMPD) spectroscopy. Using density functional theory calculations, the geometries, relative energies, and linear IR absorption spectra of several low‐energy isomers are calculated. Comparison of the calculated IR spectra with the measured IRMPD spectra reveals that the N10 substituent on the isoalloxazine ring influences the protonation site of the flavin. Lumichrome, with a hydrogen substituent, is only stable as the N1‐protonated tautomer and protonates at N5 of the pyrazine ring. The presence of the ribityl unit in riboflavin leads to protonation at N1 of the pyrimidinedione moiety, and methyl substitution in lumiflavin stabilizes the tautomer that is protonated at O2. In contrast, flavin mononucleotide exists as both the O2‐ and N1‐protonated tautomers. The frequencies and relative intensities of the two C?O stretch vibrations in protonated flavins serve as reliable indicators for their protonation site.     

δ Lactones from δ-ketoesters-II : Mechanism changes in alkylation reactions and substituent effects on stereoselection

Alkylation of methyl 4-methyl 5-oxo 5-phenyl (m and p-X substituted) pentanoates 1_a-b give cis and trans tetrahydro 5,6-dimethyl 6-phenyl 2H pyran-2-ones. LFER of isomer ratios as function of the X substituent on the phenyl ring is seen in MeLi-Et₂O. The lactone ratios of reactions in THF with MeMgCl are not affected by the X phenyl substituent, while a more complex situation is showed by reactions of MeMgl in Et₂O and benzene. Changes in the cis: trans-ratios with variations in reactant and solvent are discussed in terms of equilibrium between folded and unfolded conformations in transition states.     

Basicity of azoles. VII. Basicity of C-aminopyrazoles in relation to tautomeric and protonation studies

The pK_a values of five aminopyrazoles [3(5)-amino, 1-methyl-3-amino, 1-methyl-5-amino, 4-amino and 1-methyl-4-amino] were determined. The aqueous basicities are discussed in terms of tautomerism (72% of 3-amino tautomer), protonation site (only 4-aminopyrazoles protonate on the amino group) and amino substituent effects. The results of theoretical calculations, carried out at the semiempirical INDO level, indicate that in the gas phase 3- and 5-aminopyrazoles protonate on the pyrazolic nitrogen atom, whereas 4-aminopyrazoles possess similar proton affinities for both nitrogen atoms (pyrazolic and amino).     

Protonation of imidazo[2,1-b]thiazole and thiazolo[2,3-f]purine

The protonation of imidazo[2,1-b]thiazole and thiazolo[2,3-f]purine was investigated by PMR spectroscopy. In CF₃COOH and D₂SO₄ the imidazothiazole forms a monocation, the structure of which corresponds to the addition of a proton to the nitrogen atom of the imidazole ring. In aqueous D₂SO₄ solutions, the thiazolopurine forms mono- and dications. The first protonation occurs at the nitrogen atom of the pyrimidine ring, while the second protonation occurs at the nitrogen atom of the imidazole ring. The effect of delocalization of the positive charge in the cations of the investigated compounds was examined.     

A study of the structure of the cations of n-oxides of monosubstituted pyrazines and quinoxalines by the pmr method

The dependence of the chemical shifts of the protons on the concentration of D₂SO₄ in D₂O in a number of N-oxides of monosubstituted pyrazines and quinoxalines has been investigated, and the parameters of the PMR spectra of the neutral and the mono- and diprotonated forms of the compounds investigated have been determined. All the pyrazine and quinoxaline N-oxides considered protonate first at the unoxidized nitrogen atom (N₄). The first protonation of 2-aminopyrazine 1,4-di-N-oxide takes place at the oxygen atom of the N O group in position 1, and that of 2-methoxypyrazine at the oxygen atom of the N O group in position 4. The effect of the delocalization of the positive charge in the monocations of the compounds investigated has been considered.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1115–1123, August, 1973.     

An experimental and computational investigation on the fragmentation behavior of enaminones in electrospray ionization mass spectrometry

The dissociation pathways of protonated enaminones with different substituents were investigated by electrospray ionization tandem mass spectrometry (ESI‐MS/MS) in positive ion mode. In mass spectrometry of the enaminones, Ar? CO? CH?CH? N(CH₃)₂, the proton transfers from the thermodynamically favored site at the carbonyl oxygen to the dissociative protonation site at ipso‐position of the phenyl ring or the double bond carbon atom adjacent to the carbonyl leading to the loss of a benzene or elimination of C₄H₉N, respectively. And the hydrogen? deuterium (H/D) exchange between the added proton and the proton of the phenyl ring via a 1,4‐H shift followed by hydrogen ring‐walk was witnessed by the D‐labeling experiments. The elemental compositions of all the ions were confirmed by ultrahigh resolution Fourier transform ion cyclotron resonance tandem mass spectrometry (FTICR‐MS/MS). The enaminones studied here were para‐monosubstituted on the phenyl ring and the electron‐donating groups were in favor of losing the benzene, whereas the electron‐attracting groups strongly favored the competing proton transfer reaction leading to the loss of C₄H₉N to form a benzoyl cation, Ar‐CO⁺. The abundance ratios of the two competitive product ions were relatively well‐correlated with the σ_p⁺ substituent constants. The mechanisms of these reactions were further investigated by density functional theory (DFT) calculations. Copyright © 2010 John Wiley & Sons, Ltd.     

Deuteration of β-phenylpropionic acid in the presence of homogeneous potassium tetrachloroplatinate(II) catalyst

The kinetic of deuteration of -phenylpropionic acid with D₂O acidified with HCl in the presence of homogeneous K₂PtCl₄ catalyst has been investigated in the 100–130°C temperature interval. The quasi-unimolecular H/D exchange rate constants at 100 and at 130°C, corresponding to fast H/D exchange process, have been deduced by analysis of the composite integrated NMR signal of the substituted benzene ring hydrogens; the Arrhenius activation energy for this exchange was estimated.     

Intramolecular CO ⋯ H interaction in Arene(tricarbonyl)-chromium complexes

Unlike benzene(tricarbonyl)chromium which displays two carbonyl stretching vibrations bands in the IR spectrum, analogous tricarbonylchromium complexes of the general formula (C₆H₅ZMe)Cr(CO)₃ [Z = O, CH(OH), N(Pr), CH=CH] are characterized by three carbonyl bands, one of which is displaced to the low-frequency region. The appearance of that band was rationalized in terms of intramolecular interaction between hydrogen atoms in the substituent on the benzene ring and carbonyl groups.     

Substituent effect on the acid-promoted hydrolysis of 2-aryloxazolin-5-one: normal vs reverse

Computational investigations on the acid-promoted hydrolysis of 2-aryl-4,4-dimethyloxazolin-5-one (AMO) and its seven para- and meta-substituted derivatives (with electron-donating groups R = OH, OCH(3), CH(3) and electron-withdrawing groups R = Cl, m-Cl, CF(3), NO(2)) were presented by the density functional theory (B3LYP) method. Two types of reaction mechanism, N-path and O-path, are taken into account, in which the attacks by water molecules at the C2 and C5 are accelerated after the protonation on N3 and carbonyl oxygen atoms, respectively. Our computational results clearly manifest that the hydrolysis of AMOs has an obvious substituent effect at the para and meta positions of the benzene ring by correlating the barrier heights with the Hammett constants of substituents. Furthermore, the N-path shows a normal substituent effect, while the favorable O-path shows a reverse substituent effect. The observed reverse substituent effect in experiment actually springs from the reverse substituent effect of the O-path, not the N-path. The substituent effect of the N-path and O-path can be explained by the electrostatic potential at nuclei (EPN) values and Fukui function, respectively. Our theoretical data provided will allow for a better understanding of the acid-promoted hydrolysis mechanism and the observed reverse substituent effect of the AMOs, in nice agreement with the available experimental conclusion.     

Studies on the series of azoles and azines. 65. Mass spectra of 5-arylidene-, 5-ethoxycarbonylmethylene-hydantoins and their derivatives

The mass spectra of 5-m- and p-substituted benzylidenehydantoins, their thio analogs and 5-carbethoxymethylenehydantoins with a substituent at the -carbon atom of the side chain were studied. The fragmentation of the molecular ions of 5-arylidenehydantoins proceeds in one direction, splitting of the X=C-NR-C=O fragment, irrespective of the substituent in the benzene ring. The peak intensity of the fragmentary ions formed from the molecular ions is linearly dependent on the -constants of the substituent. The direction of the fragmentation of 5-ethoxycarbonylmethylenehydantoins markedly depends on the substituent at the -carbon atom in the side chain that determines the stability of the hydantoin ring and the carboethoxyl group. The fragmentation of these compounds under electron impact proceeds by five paths, related to splitting of fragments O=C₍₂₎NCH₍₄₎=O, C₂H₄, C₂H₅O, C₂H₅OH, and COOC₂H₅.See [1] for Communication 64.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 625–631, May, 1988.     

Monomolecular sheets of propeller‐shaped triethyl 4,4′,4′′‐[benzene‐1,3,5‐triyltris(ethyne‐2,1‐diyl)]tribenzoate deuterochloroform monosolvate

The title compound, C₃₉H₃₀O₆·CDCl₃, has a chemical threefold axis and an approximately planar structure, with an ethoxycarbonyl substituent on each of the terminal benzenes oriented in the same direction, thus forming a propeller‐shaped molecule. This molecule is of particular interest in the field of metal–organic frameworks (MOFs), where its hydrolyzed analogue forms MOF structures with high surface areas. The benzene ring which occupies the centre of the molecule forms π–π interactions to the equivalent benzene ring at a perpendicular distance of 3.32 (1) Å. Centrosymmetric dimers formed in this way are interconnected by intermolecular C—H...π interactions with a rather short H...CgA distance of 2.51 Å (CgA is the centroid of the central benzene ring). The molecules are arranged in regular parallel sheets. Within a sheet, molecules are interconnected via C—H...O interactions where all carbonyl O atoms participate in weak hydrogen bonds as hydrogen‐bond acceptors. Neighbouring sheets are connected through the above‐mentioned π–π and C—H...π interactions.     

Deuteriation of benzoic acid in the presence of homogeneous potassium tetrachloroplatinate(II) catalyst

The kinetic behavior of deuteriation of benzoic acid with D₂O acidified with HCl in the presence of homogeneous K₂PtCl₄ catalyst has been investigated in the 100–130°C temperature interval. The quasiunimolecular H/D rate constants at 100 and 130°C corresponding to an exchange process in ortho positions of the substituted benzene ring hydrogens were determined by¹H NMR integration signal. These same constants for meta and para positions have been deduced by analysis of the composite¹H NMR signal, and the Arrhenius activation energies for these exchange reactions were estimated.     

Direct Correlation Between the H/D Exchange Reaction and Conformational Changes of the Cation in Imidazolium-Based Ionic Liquid–D2O Mixtures

We present a study on the effects of deuterated water on the conformational equilibria of the imidazolium cation in aqueous mixtures of imidazolium-based ionic liquids at room temperature. We provide spectroscopic evidence that the conformational dynamics of the imidazolium cation in D₂O are directly related to the H/D exchange reaction of the C–H group at position 2 on the imidazolium ring. The relation is supported by comparing Raman spectra obtained from solutions prepared with H₂O and D₂O.     

Electrophilic substitution in the benzene ring of indole compounds (Review)

Data on the orientation of electrophilic substitution reactions (nitration, halogenation, acylation, and sulfonation) in the benzene ring of indoles are systematized. The effect of protonation on the direction of substitution is examined. The effects of a pyrrole ring and a substituent in the benzene ring on the orientation reactions are compared.Deceased.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1181–1199, September, 1980.     

Electrophilic bromination of gaseous aromatic compounds: Mechanism and linear free energy effects on reaction rates

Electrophilic bromination of monosubstituted aromatic compounds is effected in a pentaquadrupole mass spectrometer using BrCO⁺ and CH₃NH₂Br⁺ as mass-selected reagent ions. Reaction normally occurs at the ring and the brominated product can be mass selected in turn and caused to dissociate by Br˙ loss upon collision-induced dissociation. Linear free energy correlations with Brown substituent σ⁺ constants describe the extent of gas-phase bromine cation addition under the non-equilibrium, low-pressure and solvent-free conditions which pertain in quadruple collision cells. The electrophilic addition reaction proceeds via a σ-complex to the ring as suggested by MS³ spectra, except in the case of nitrobenzene, where substituent bromination is suggested by the occurrence of a competitive process in which the nitrosubstituent is displaced by bromine. The reactivity parameters ρ are ?0.23 and ?0.56 for the gaseous reagents, BrCO⁺ and CH₃NH₂Br⁺, respectively. Both values are much less negative than corresponding values for bromination in solution. The greater reactivity of BrCO⁺ is evident by the fact that it reacts even with the strongly deactivated substrates and this is consistent with a weak Br? CO bond. Competitive protonation occurs in the case of CH₃NH₂Br⁺ and, unlike bromination, the rate of this reaction does not correlate with σ⁺ values. This is suggested to be a consequence of protonation at the ring in some cases and at the substituent in others, including acetophenone and benzonitrile. Evidence for this is that, in contrast to its lack of correlation with substituent constants, the rate of protonation correlates linearly with proton affinity.     

Studies of solution dynamics of poly(N-vinyl pyrrolidone) and its iodine adduct

Carbon-13 NMR spin-lattice relaxation times T₁ of poly(N-vinyl pyrrolidone) (PVP) and PVP-iodine have been studied in several solvents and at different temperatures. Three kinds of motion can be identified from the T₁ data: segmental motion, ring rotation, and ring puckering. The effective correlation time for segmental motion is calculated to be 1 × 10^?9s, in good agreement with published proton NMR data. Another solvent, 1,1,2,2-tetrachloroethane, behaves like D₂O, the segmental correlation time being 3 × 10^?9s. In benzene, however, the linewidths are very broad and tend to narrow with increasing temperature, but the T₁s are not very different from those of PVP in D₂O. The results suggest association of pyrrolidone rings in benzene that reduces chain dimensions and also restricts chain mobility. As for PVP-iodine in water, again broad resonances are observed which sharpen considerably at higher temperatures. The result agrees with previous suggestions of specific interactions between the pyrrolidone group and iodine.     

Chemical ionization mass spectra of mononitroarenes

The H₂ and CH₄ chemical ionization mass spectra of a selection of substituted nitrobenzenes have been determined. It is shown that reduction of the nitro group to the amine is favoured by high source temperatures and the presence of water in the ion source. The H₂ chemical ionization mass spectra are much more useful for distinguishing between isomeric compounds than the CH₄ CI mass spectra because of the more extensive fragmentation. For ortho substituents bearing a labile hydrogen abundant [MH ? H₂O]⁺ fragments are observed. When the substituent is electron-releasing both ortho and para substituted nitrobenzenes show abundant [MH? OH]⁺ fragment ions while meta substituted compounds show abundant loss of NO and NO₂ from [MH]⁺. The latter fragmentation is interpreted in terms of protonation para to the substituent or ortho to the vitro function, while the first two fragmentation routes arise from protonation at the nitro group. When the substituent is electron-attracting the chemical ionization mass spectra of isomers are very similar except for the H₂O loss reaction for ortho compounds.     

Investigation of acid deuterium exchange in a number of isoquinoline and 4-hydroxyisoquinoline derivatives

The acid deuterium exchange of isoquinoline and 4-hydroxy- and 3-methyl-4-hydroxyisoquinolines at 145°C in 94% D₂SO₄ was investigated by PMR spectroscopy. The sequence of substitution and the rate constants for deuterium exchange of the protons of the isoquinoline ring were determined. The most reactive protons in isoquinoline and 3-methyl-4-hydroxyisoquinoline are those of the benzene ring, while the proton in the 3 position of the -pyridol ring is the most reactive in 4-hydroxyisoquinoline.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1647–1650, December, 1972.     

Dissociative protonation and proton transfers: fragmentation of alpha, beta-unsaturated aromatic ketones in mass spectrometry

In mass spectrometry of the alpha,beta-unsaturated aromatic ketones, Ph-CO-CH=CH-Ph', losses of a benzene from the two ends and elimination of a styrene are the three major fragmentation reactions of the protonated molecules. When the ketones are substituted on the right phenyl ring, the electron-donating groups are in favor of losing a styrene to form the benzoyl cation, PhCO(+), whereas the electron-withdrawing groups strongly favor loss of benzene of the left side to form a cinnamoyl cation, Ph'CH=CHCO(+). When the ketones are substituted on the left phenyl ring, the substituent effects on the reactions are reversed. In both cases, the ratios of the two competitive product ions are well-correlated with the sigma p(+) substituent constants. Theoretical calculations indicate that the carbonyl oxygen is the most favorable site for protonation, and the olefinic carbon adjacent to the carbonyl is also favorable especially when a strong electron-releasing group is present on the right phenyl ring. The energy barrier to the interconversion between the ions formed from protonation at these two sites regulates the overall reactions. Transfer of a proton from the carbonyl oxygen to the ipso position on either phenyl ring, which is dissociative, triggers loss of benzene.     

(±)‐(4‐Oxo‐4H‐chromen‐2‐yl)(phenyl)methyl acetate

The title compound, C₁₈H₁₄O₄, forms a supramolecular structure viaπ–π stacking and weak C—H⋯O and C—H⋯π interactions. The benzo­pyran moiety is almost planar. The benzene ring of the phenyl­methyl acetate substituent is nearly perpendicular to the fused benzene and pyran rings and also to the methyl acetate group.