SPECTROSCOPIC AND KINETIC-STUDIES OF THE ALDEHYDE LACTOL TAUTOMERIZATION OF GOSSYPOL IN SOLUTION

H-1 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 less-than-or-equal-to 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 half-arrow-right-over-half-arrow-left 2 equilibrium is studied by means of UV-VIS absorption spectroscopy and the results are compared with those of H-1 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 less-than-or-equal-to 3): k298 = (2.14 +/- 0.03) 10(-2) dm3 mol-1 s-1; DELTA-H not-equal-to = 64.0 +/- 3.4 kJ mol-1; DELTA-S not-equal-to = -63 +/- 11 J mol-1 K-1; and the value of kinetic isotope effects, k(H)/k(D) = 5.6 at 55-degrees-C. A mechanism for aldehyde-lactol tautomerization of gossypol is proposed.