A Spectroscopic Study on p-Hydroxyphenylpyruvic Acid. Keto-Enol Tautomerism and Stability of Its Complex with Fe⁺³ Ions

Abstract

Abstract1H-, 13C -NMR , IR, UV-Vis, and MS spectra of p-hydroxyphenylpyruvic acid (pH PPA) have been recorded and fully interpreted.pHPPA exists in solution as a mixture of interconverting forms: keto, hydrated keto and only one enol tautomer to which the Z configuration has been assigned according to the value (3.7 Hz) of the vicinal IH-C = C-13COOH coupling constant.In organic solvents the enol isomer is far more stable whereas in aqueous solutions the keto form predominates. The tautomeric equilibrium is pH-dependent and the anion is present as keto form not only in aqueous solution but also in H2O-DMSO mixtures with a high content of DMSO.The Z enol tautomer does form a coloured complex (λmax = 680 nm) with Fe+3 ions. The complex decomposes rapidly in all considered solvents except DMSO.In view of a possible use of H2O-DMSO mixtures for clinical analysis purpose, the enol fraction of pHPPA and the stability of the pH PPA -FeCl3 com plex in H2O-DMSÒ mixtures have been examined. Our results suggest that a solvent com position containing at least 80 vol.% in DM SO could be an appropriate solvent for pH PPA determination by FeCl3 method.

Novobiocin biosynthesis in Streptomyces spheroides: identification of a dimethylallyl diphosphate:4-hydroxyphenylpyruvate dimethylallyl transferase

“…3A and B). The co-occurrence of such tautomeric forms has indeed been reported for 4HPP [13,15]. In organic solvents, hence in our I H-NMR analysis, the enol form is far more stable, whereas in aqueous solution the keto form predominates [13].…”

“…Both isolated compounds showed molecular ions at m/z 247 (M-1) as expected for prenylated 4HPP (Mr 248). Fragmentation was similar to that of 4HPP [13]: m/z (relative intensity): 247 (M-1, 100), 203 (7), 201 (3), 175 (69).…”

Temperature-variable NMR Study of the keto-enol Tautomerism of Phenylpyruvic Acid

“…On the other hand, a protophilic solvent, such as water and HCl, would reduce the enol formation due to the ionization of PPA. [3][4][5][6]10 Upon considering the influence of a solvent on the keto-enol tautomerism on the basis of the Onsager model, 14 the tautomer should favor its existence in the more polar isomer, i.e., the keto one as the polarity of a solvent increases. 15 It can thereore be understood that the keto form is more stable than the enol one in another solvent, such as water and HCl, [3][4][5][6]10 while the enol form becomes predominant in the DMSO solvent.…”

“…Many spectroscopic studies, ultra violet (UV), infrared (IR), Raman and nuclear magnetic resonance (NMR) studies, have been reported on the keto-enol tautomerism [3][4][5][6][7][8][9][10][11][12] and conformation 6,9,10,12,13 of PA and PPA. These many investigations have revealed that these compounds exist as equilibrium mixtures consisting of the keto and enol forms.…”

“…3 Sciacovelli et al also investigated the tautomerism of PPA by NMR, UV, and IR spectroscopy, and found that the keto isomer increases in a water-DMSO solvent, depending upon the water content. [4][5][6] Furthermore, PPA was found to predominantly take the keto form in the solid state and in an aqueous solution. Ray et al analyzed the complete IR and Raman spectra of liquid and crystalline PA, and found that PA takes the cyclic hydrogen-bonded dimer structure in which the two carbonyl groups are in a trans configuration in the pure liquid and crystalline phase.…”