An electron spin resonance study on the free radicals produced from aclacinomycin a and its derivatives: analysis of hyperfine structure of the spectra by means of molecular orbital method.

Quinone-containing carcinostatics, aclacinomycin A and its derivatives were investigated on the convertibility to free radical under a mild reducing condition. The hyperfine structures of electron spin resonance (ESR) spectra were satisfactorily reproduced by computer simulations, using the hyperfine coupling constants calculated by the Intermediate Neglect of Differential Overlap Molecular Orbital (INDO MO) method. This verifies the reliability of molecular orbital calculations and opens a way to analyze theoretically the correlation between chemical structures and carcinostatic activities. By analyzing hyperfine structures of ESR spectra, the free radical produced from aclacinomycin was identified as a neutral form of semiquinone radical of intact aclacinomycin. Taking into account the previous finding that 7-deoxyaklavinone (C1) is formed reductively by cytochrome P-450 reductase (EC 1.6.2.4; Komiyama et al., 1979), it is postulated that two types of semi-quinone radicals exist in vivo.