PREPARATION AND ESR SPECTROSCOPIC CHARACTERIZATION OF THE ZINC(II) AND CADMIUM(II) COMPLEXES OF STREPTONIGRIN SEMIQUINONE

Semiquinone metal complexes derived from the antitumor antibiotic stre ptonigrin have been prepared for the first time. They were obtained by reduction of the zinc(II) and cadmium(II) complexes of the parent ami noquinone ligand with sodium borohydride, followed by air oxidation of the intermediate dihydroquinones. Alternatively, N-benzyldihydronicot inamide reduction was used to produce the same Cd(II) complex of the p -semiquinone free radical. Electron spin resonance spectroscopic studi es showed that metal binding significantly changes the spin densities of the unpaired electron which is confined to the quinolinesemiquinone moiety of the complexed antibiotic. Complexation with both Cd(II) and Zn(II) perturbs the coupling constants of all atoms involved in deloc alization of the unpaired electron, shifting its distribution toward t he pyridine ring. The coupling constant of the pyridine ring-proton ad jacent to the semiquinone ring increases from 0.31 to 0.43 G in the Cd (II) complex in methanol, while the proton meta to the pyridine nitrog en increases from 1.76 to 1.96 G. Furthermore, the coupling constant o f the heterocyclic nitrogen increases from 0.46 to 0.61 G. A similar t rend is noted for the Zn(II) complex as well, including the observed d ecrease in splitting constant of the amino nitrogen from 1.34 to 1.08 G, and perturbation of the previously equivalent amino protons from 0. 89 and 0.89 to 1.09 and 0.95 G. The spectral parameters have been conf irmed by deuteration. Complexation studies using isotopically enriched Cd-113(II) revealed hyperfine coupling of the unpaired electron of th e p-semiquinone and the nuclear spin of Cd-113(II), indicating direct coordination between the metal and the complexing ligand. Although the metal complexes could readily be prepared in a series of different so lvent systems, they appear to have substantially shorter half lives th an the non complexed p-semiquinone radical (5 to 15 min vs. 2 to 3 wk in sealed ampoules). Formation of tight-binding p-semiquinone metal co mplexes as here described should provide useful leads for the design o f related systems to study p-quinone-metal interactions for mechanisti c elucidation of metal ion catalyzed quinone-dependent electron transf er reactions in biological oxidations. (C) 1997 Elsevier Science B.V.