Spectroscopic and kinetic studies of the aldehyde–lactol tautomerization of gossypol in solution

Abstract

¹ H NMR spectra of gossypol in C₂H₅OD solution show that an equilibrium between aldehyde 1 and lactol 2 tautomers is formed and that the aldehyde tautomer is predominant, whereas in C₂H₅OD–DCl solution (pH ⩽ 1) only the lactol form is observed. UV–VIS absorption spectroscopy studies demonstrate that in ethanol and ethanol–HCl solutions gossypol shows different spectra which can be attributed to both tautomers 1 and 2. The influence of various solvents on the 1⇌2 equilibrium is studied by means of UV–VIS absorption spectroscopy and the results are compared with those of ¹H NMR studies. Furthermore, the decrease of the absorbance at the long wavelength band in the spectrum of 1 is used to determine the kinetic parameters of aldehyde–lactol tautomerization in ethanol–HCl solution (pH ⩽ 3): k₂₉₈=(2.14 ± 0.03) 10^–2 dm³ mol^–1 s^–1; ΔH^≠= 64.0 ± 3.4 kJ mol^–1; ΔS^≠=–63 ± 11 J mol^–1 K^–1; and the value of kinetic isotope effects, k_H/k_D= 5.6 at 55 °C. A mechanism for aldehyde–lactol tautomerization of gossypol is proposed.