Electron spin resonance and electron nuclear double resonance studies of cation radicals derived from tocopherol...

Electron spin resonance (ESR) and electron nuclear double resonance (ENDOR) measurements were performed for the cation radicals obtained from the model compounds of alpha-, beta-, gamma- and delta-tocopherol (vitamin E) by oxidizing the tocopherol precursors in an AlCl3-CH2Cl2 solution. The proton hyperfine coupling constants and g-values were precisely determined. The ENDOR spectra of the cation radicals of alpha-, beta-, gamma- and delta-tocopherol models in CH2Cl2 at -100 degrees C clearly show 10, 6, 6 and 12 different proton hyperfine couplings, respectively. By varying the temperature, the ESR spectra of the alpha- and delta-tocopherol model cations exhibit line-width alternation phenomena characteristic of the hindered rotation of the OH group. However, neither the beta- nor the gamma-tocopherol model cation radical ESR spectra show any sign of an alternating line-width effect. These results are interpreted by assuming that the beta- and gamma- tocopherol model cations are stabilized in the trans and cis conformations, respectively. On the other hand, both the alpha- and delta-tocopherol model cations exist as cis and trans isomers.