Hs. Soedjak et al., PREPARATION AND ESR SPECTROSCOPIC CHARACTERIZATION OF THE ZINC(II) AND CADMIUM(II) COMPLEXES OF STREPTONIGRIN SEMIQUINONE, Biochimica et biophysica acta (G). General subjects, 1335(3), 1997, pp. 305-314
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.