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, kH/kD= 5.6 at 55 °C. A mechanism for aldehyde–lactol tautomerization of gossypol is proposed.