Tautomeric Ratios Research Articles

13C nuclear magnetic resonance study of acid–base tautomeric equilibria

Acid–base tautomerisation equilibria are reported for o-, m-, and p-aminobenzoic acids, p-aminophenylacetic acid, pyridine-3-carboxylic (nicotinic) acid, and pyridine-4-carboxylic (isonicotinic) acid. These equilibria are determined by modelling the displacements of 13C n.m.r. chemical shifts of each carbon due to protonation of basic sites. The models are based on the corresponding displacement due to protonation of as many as twelve related compounds. Each such model compound yields several independent estimates of the tautomeric partitioning ratio corresponding to the several carbon resonances. It is observed that the mean tautomeric partitioning ratio and its estimated uncertainty are independent of the nature or the location of ring substituents and are unaffected by heteroatom substitution in the aromatic ring. This observation has led to the development of generic models for 13C n.m.r. chemical shift protonation displacements based on collections of similar model compounds. The use of generic models summarises and simplifies the determination of tautomeric equilibria.

An Ab initio study of the diadic prototropic tautomerism H3PX⇄H2PXH (X = O, NH, CH2)

The diadic prototropci tautomerisms of three model systems H3PX⇄H2PXH (X = O, NH, CH2) are investigated by ab initio methods. Stationary points corresponding to local minima and transition states are located using gradient techniques at HF/6-31G** level, and relative energies calculated at MP4/6-31G** including a zero-point energy (ZPE) correction, using the optimised geometries. Computation of vibrational frequencies shows a correlation between the norm of the transition-state imaginary frequencies and both the free energy of activation for hydrogen migration and the Hm–P–X angle. The intrinsic barriers ΔGo‡ calculated by the Marcus theory indicate that all three unimolecular hydrogen transfers belong to a homogenous series. The disparity between the calculated (gas-phase) stabilities and experimental observations based on solution studies is noted. However, it is shown that consideration of the computed net charges dipole moments and electric field gradients of the isolated species, in conjunction with a simple perturbation expression for the solvation energy, allows a successful rationalisation of tautomeric ratios measured in solution.

Kinetic studies of fast equilibrium by means of high-performance liquid chromatography. Part 11. Keto–enol tautomerism of some β-dicarbonyl compounds

The keto–enol tautomerism of some β-dicarbonyl compounds, including ethyl acetoacetate (1), acetylacetone (2), benzoylacetone (3), dibenzoylmethane (4), and 1,1,1-trifluoro-3-(2-thenoyl)acetone (5), has been investigated in various solvents at 25 °C, by using low-temperature (–20 to –50 °C) high-performance liquid chromatography (h.p.l.c.) for analytical measurements. Base-line separation of keto and enol tautomers of these compounds has been achieved on silica gel packings with a mobile phase composed of hexane and propan-1-ol containing a small amount of acetic acid. The ratios of keto and enol tautomers in different solvents have been determined; the percentages of enol tautomers have been found to be higher in polar solvents, in accord with n.m.r. data previously reported. In all solvents the proportions of enol tautomers of acetylacetone derivatives have been found to increase in the following order; (2) < (3) < (4); this has been attributed to the fact that enol tautomers of phenyl-substituted acetylacetones have been stabilized, owing to the formation of extended conjugated systems. For the ester (1), the presence of a very small amount of a third tautomer (0.1%) has been found, probably an unconjugated enol tautomer.

Studies of 6-fluoropyridoxal and 6-fluoropyridoxamine 5'-phosphates in cytosolic aspartate aminotransferase.

The chemical and spectroscopic properties of 6-fluoropyridoxal 5'-phosphate, of its Schiff base with valine, and of 6-fluoropyridoxamine 5'-phosphate have been investigated. The modified coenzymes have also been combined with the apo form of cytosolic aspartate aminotransferase, and the properties of the resulting enzymes and of their complexes with substrates and inhibitors have been recorded. Although the presence of the 6-fluoro substituent reduces the basicity of the ring nitrogen over 10 000-fold, the modified coenzymes bind predominately in their dipolar ionic ring forms as do the natural coenzymes. Enzyme containing the modified coenzymes binds substrates and dicarboxylate inhibitors normally and has about 42% of the catalytic activity of the native enzyme. The fluorine nucleus provides a convenient NMR probe that is sensitive to changes in the state of protonation of both the ring nitrogen and the imine or the -OH group of free enzyme and of complexes with substrates or inhibitors. The NMR measurements show that the ring nitrogen of bound 6-fluoropyridoxamine phosphate is protonated at pH 7 or below but becomes deprotonated at high pH around a pKa of 8.2. The bound 6-fluoropyridoxal phosphate, which exists as a Schiff base with a dipolar ionic ring at high pH, becomes protonated with a pKa of approximately 7.1, corresponding to the pKa of approximately 6.4 in the native enzyme. Below this pKa a single 19F resonance is seen, but there are two light absorption bands corresponding to ketoenamine and enolimine tautomers of the Schiff base. The tautomeric ratio is altered markedly upon binding of dicarboxylate inhibitors. From the chemical shift values, we conclude that during the rapid tautomerization a proton is synchronously moved from the ring nitrogen (in the ketoenamine) onto the aspartate-222 carboxylate (in the enolimine). The possible implications for catalysis are discussed.

Tautomeric ratio in 4-methylthiazol-2-ylguanidine, a model guanidinoheterocycle

The pKa values of fixed model tautomers have been used to assess tautomeric ratio in the title compound. Although pKT values of 3.62 and 5.32 between the major and the two minor tautomers result, reasons are given for believing that the method exaggerates both ratios. Nevertheless the arylimino tautomer is very dominant and evidence is adduced that this conclusion may be general for guanidinoheterocycles. If so, only this tautomer may be considered as the form that interacts with the histamine H2-receptor.

Conformational and tautomeric studies of acylguanidines. Part 1. Synthesis, ultraviolet spectroscopy, tautomeric preference, and site of protonation in some model compounds

Six of the ten possible acylguanidine conformers have been prepared as model compounds in fixed form; all protonate on the imino nitrogen. The expected close u.v. resemblance between the cations is found for the major band near 200 nm but there are complications origin to a second band at higher wavelength and of very variable intensity whose origin is traced to an electronic transition involving σ-resonance. There is a clear distinction between the u.v. spectra of the ‘through-conjugated’ and ‘cross-conjugated’ free base forms, but with unexpected complications in certain cases which are believed to be due to the co-existence of near-planar and severely twisted species. There is a large tautomeric preference for the ‘through-conjugated’ forms, the extent of which is complicated by perturbations due to the molecular scaffolding. An attempt to allow for this and to predict the pKa values for the nominal parent compounds leads to the conclusion that, for the anti-conformers, tautomeric ratio is ca. 300. Reasons are given for believing that these compounds are good models for related guanidinoheterocycles active as histamine H2-receptor antagonists.

Cornusiin A, a dimeric ellagitannin forming four tautomers, and accompanying new tannins in Cornus officinalis.

Cornusiin A (4), a dimeric ellagitannin which forms four anomeric isomers, have been isolated from Cornus officinalis fruits, along with cornusiin B (5) and related compounds. Their structures and the ratio of tautomers in the equilibrium mixture of 4 were determined. Oenothein C (8), which was produced by partial hydrolysis of 5, has been isolated also from Oenothera erythrosepala leaves.

1H-NMR study on the tautomerism of the imidazole ring of histidine residues : I. Microscopic p K values and molar ratios of tautomers in histidine-containing peptides

The 1H-NMR titration curves of chemical shifts versus pH were observed for imidazole, N 1-methylimidazole, l-histidine, N 1- methyl- l- histidine , N 3- methyl- l- histidine , and other related compounds. With these results, the macroscopic p K values of these compounds were determined by a computer curve-fitting for a simple dissociation sequence. From the p K values of imidazole and N 1-methylimidazole, the perturbation for the p K of the imidazole ring due to the substitution of a proton with a methyl group was estimated as -0.21 pH unit. The microscopic p K values of the individual tautomers of the imidazole ring were estimated with the p K values of N 1- methyl- l- histidine , N 3- methyl- l- histidine , and perturbation due to methyl substitution. The estimated p K values were 6.73 for the N 1-H tautomer and 6.12 for the N 3-H tautomer. These values were in good agreement with those obtained using carboxymethyl derivatives instead of methyl derivatives. Furthermore, the macroscopic p K value (6.02) calculated using the estimated microscopic p K values agreed with that (6.03) observed for the imidazole ring of l-histidine. Thus the method in this work was indicated to be self-consistent. The microscopic p K values of tautomers were also obtained for N α- acetyl- l- histidine and N α- acetyl- l- histidine methylamide. The molar ratios of tautomers were calculated on the basis of the microscopic p K values of tautomers. The intrinsic (or unperturbed) p K value of imidazole ring and perturbations due to the CO 2 − and NH 3 + were obtained for each of the N 1-H and N 3-H tautomers.

Solid-state structures of keto-disaccharides as probed by 13C cross-polarization, “magic-angle” spinning n.m.r. spectroscopy

The 13C cross-polarization, magic-angle spinning (CP-MAS) spectra of maltulose · H 2O and anhydrous lactulose were examined at different applied magnetic and proton-decoupling fields B 0 and B 1, respectively. Whereas the spectrum of lactulose was insensitive to these changes, that of maltulose showed significant responses. From selective-relaxation experiments, a mixture of three “forms” (based on the ratio of the C-2 resonances) of both lactulose and maltulose were shown to exist in the solid state. The 360-MHz 1H spectra of the unmutarotated disaccharides in dimethyl sulfoxide solution were used to establish the tautomeric composition of these crystalline solids. The tautomeric composition of lactulose, as determined by the 1H spectrum in dimethyl sulfoxide, correlated well with the CP-MAS data, but the spectrum of unmutarotated maltulose showed only a single, β-pyranose tautomer to be present. Based on the ratio of the lactulose tautomers determined from the 1H spectrum in dimethyl sulfoxide (referenced to fructose), the C-2 resonances corresponding to each anomeric form of lactulose were assigned in the CP-MAS spectrum. A “crossover” in chemical-shift positions of the anomeric resonances was observed in going from solution to the solid state. Furthermore, a pronounced increase in the proportion of the furanoid anomers was noted for crystalline lactulose relative to its solution.

The infra-red spectroscopy of some highly conjugated systems–V. Conformation and electron distribution in some heterocyclic anhydro-bases

Arguments based on isotopic substitution, solvent shifts, and intensity variation have been used to assign the double bond frequencies of some heterocyclic anhydro-bases. Their conformations have also been established, and on this basis the frequency and intensity of ν CO and ν CC are discussed. Departures of frequency from the predictions of the simple heterenone model [4] are shown to relate to the electronic character of the heterocyclic ring. By contrast, variations in ϵ A (CC) correlate with tautomeric ratio, being greater the more unfavourable this is, and with subsidiary factors such as the length of the overall conjugated system. Revised assignments are suggested for certain related compounds in the literature.

Isolation and Identification of 4-Hydroxy-2(or 5)-ethyl-5(or 2)-methyl-3(2H)-furanone, as a Flavor Component inShoyu(Soy Sauce)

Unstable compounds with intense sweet odor were isolated from shoyu and identified as 4-hydroxy-2-ethyl-5-methyl-3(2H)-furanone and 4-hydroxy-5-ethyl-2-methyI-3(2H)-furanone (tautomer ratio, about 3:2) on the basis of spectrometrical investigation. This may be the rare case of isolation of the tautomeric compounds from a natural product.They were concluded to be major and important flavor components in shoyu.

Studies on flavor components in Shoyu. II. Isolation and identification of 4-hydroxy-2(or 5)-ethyl-5(or 2)-methyl-3(2H)-furanone, as a flavor component in shoyu (soy sauce).

Unstable compounds with intense sweet odor were isolated from shoyu and identified as 4-hydroxy-2-ethyl-5-methyl-3 (2 H) -furanone and 4-hydroxy-5-ethyl-2-methyl-3 (2 H) -furanone (tautomer ratio, about 3:2) on the basis of spectrometrical investigation. This may be the rare case of isolation of the tautomeric compounds from a natural product. They were concluded to be major and important flavor components in shoyu.

Ultraviolet and nuclear magnetic resonance spectra of protonated benzamide and [15N]benzamide in concentrated sulphuric acid and pure fluorosulphuric acid

U.v. spectra of benzamide in concentrated sulphuric acid, which show a tautomeric change from N-protonated in 60% sulphuric acid to O-protonated in 100% sulphuric acid, are discussed and the estimated tautomeric ratios are shown to lead to reasonable activity coefficient ratios of the two types of cation. N.m.r. spectra of benzamide and [15N]benzamide in 100% sulphuric acid and in pure fluorosulphuric acid are reported. The effect of diluting sulphuric acid with water on the spectra is consistent with a conversion of the O-protonated cation into the N-protonated cation, although under conditions of rapid proton exchange n.m.r. spectra do not provide information on the position of the tautomeric equilibrium.

Oximes of azolecarbaldehydes and ?2-azolinecarbaldehydes and their derivatives

The parameters of the acid-base and tautomeric equilibria of the E (syn) isomers of 1-methyl-substituted benzimidazole-2-carbaldehyde oxime, imidazole-2-carbaldehyde oxime, Δ2-imidazoline-2-carbaldehyde oxime, and Δ2-pyrazoline-3-carbaldehyde oxime in aqueous solutions at 25°C have been determined by a potentiometric method using model compounds. The tautomeric ratio (zwitterion)/(uncharged form) rises in parallel with an increase in the basicity of the parent heterocycle. Unlike known heterocyclic aldoximes, 1-methyl-Δ2-imidazoline-2-carboxaldehyde oxime exists in the zwitterionic form.

Aromaticity and tautomerism. Part II. The 4-pyridone, 2-quinolone, and 1-isoquinolone series

Aromatic resonance energies for 4-pyridone and its analogues are similar to those for the 2-pyridone series. For the bicyclic compounds, the differences in aromaticity between the quinolinoid and the quinolonoid forms are significantly less than for the monocyclic derivatives. A new acidity function H–(q) is derived for the deprotonation of methyl quaternary salts to netural heterocyclic methide and imine bases. Quantitative tautomeric ratios are reported for the thioamide–thioimidate equilibrium in an Appendix.

The decarboxylation of some heterocyclic acetic acids. Part II. Direct and indirect evidence for the zwitterionic mechanism

Macroscopic and model pKa values for the 2- and 4-pyridylacetic acids in aqueous propan-2-ol have been measured and used to evaluate the microscopic pKa values as a function of solvent composition. From these are calculated the mole fractions of neutral from and zwitterion present, which in conjunction with the observed decarboxylation rate allows this to be re-expressed in terms of these sub-species (as kN and kZ respectively). The smoothness of the relation between kZ and solvent composition, and the contrast in this respect with kN, are used as evidence that decarboxylation goes through the zwitterion. Similar calculations applied to some of the heterocyclic acetic acids previously examined show that kZ, but not kN, is a linear or slightly curved function of Perrin's nuclear σ constants; this and other evidence is interpreted in favour of the zwitterionic route for the whole range of heterocycles examined. A refinement of the theory implicates tautomeric ratio, which has been measured for some heterocyclic acetates, as a further influence on decarboxylation rate. The β-keto-acids are discussed in the light of these results: it is concluded that they also are probably decarboxylated by the zwitterionic mechanism.

Some Metal Complexes Containing Ethyl Acetoacetate or Ethyl Malonate as a Neutral Ligand

Abstract The following metal complexes containing ethyl acetoacetate (etacH) or ethyl malonate (etmalH) as a neutral ligand have been prepared: CoCl2L, CoBr2L, ZnCl2L (L=etacH or etmalH), and MnBr2(etmalH). These compounds are all hygroscopic and unstable, releasing the ester ligand below 100°C in vacuo or in donor solvents. The infrared spectra show that the ratio of keto and enol tautomers in etacH complexes is similar to that for the free ligand. The ν(C=O) band attributable to the enol tautomer of etmalH is observed in the complexes, though not in the free ligand. The electronic spectra measured in Nujol suggest that the cobalt(II) complexes are tetrahedral. The structure of MnBr2(etmalH) is not yet certain.

Kinetics of α-hydrogen exchange of sodium α-[4-(4-bromophenyl)-thiazol-2-yl]propionate in D2O solutions

Data are reported for the rate of incorporation of deuterium into sodium α-[4-(4-bromophenyl)thiazol-2-yl]propionate in D2O solutions studied as a function of pD by use of a quantitative n.m.r. technique. The first-order rate coefficients measured over the pD range 6–14 pass through a minimum at pD = 10. The data, measured at 30°, fit the kinetic expression kobs=kD3O+[D3O+]+kOD–[OD–] where kD3O+ and kOD– have the values 4·1 × 102 and 7·3 × 10–3 l. mole–1 sec.–1 respectively. No catalysis could be detected by other acidic or basic species. A mechanism involving a change in the position of the tautomeric equilibrium of the acid and its anion is advanced to explain the observed pD profile. Quantitative support for this interpretation is given by data for the tautomeric ratios of related model compounds.

Potentially tautomeric pyridines. IV. Ethyl 2-, 3-, and 4-pyridyl acetate

Ultraviolet and infrared spectral comparisons of the isomeric pyridylacetic esters with the anhydro bases derived from the corresponding N-methyl derivatives show that the title compounds exist predominantly in the pyridine form. Basicity measurements provided a quantitative estimation of the tautomeric ratios.