Several beta-lapachone analogues, the o-naphthoquinones CG 10-248, CG
9-442 and CG 8-935, were reduced to their semiquinones by sodium boroh
ydride, the liver NADPH-P450 reductase system and Crithidia fasciculat
a cells, in anaerobic solutions. ESR spectra of the radical anions sho
wed hyperfine spin couplings located at protons of the naphthalene rin
g. Borohydride reduction of another o-naphthoquinone, mansonone E, yie
lded spin couplings located at the naphthalene and methyl groups proto
ns. The symmetrical polarized carbonyl groups were essential for elect
ron capture and semiquinone production. These observations support the
idea that quinones are capable of redox-cycling and oxygen radical ge
neration.