N-hydroxyguanidines as new heme ligands: UV-visible, EPR, and resonance Raman studies of the interaction of various compounds bearing a C=NOH function with microperoxidase-8
D. Lefevre-groboillot et al., N-hydroxyguanidines as new heme ligands: UV-visible, EPR, and resonance Raman studies of the interaction of various compounds bearing a C=NOH function with microperoxidase-8, BIOCHEM, 40(33), 2001, pp. 9909-9917
Interaction between microperoxidase-8 (MP8), a water-soluble hemeprotein mo
del, and a wide range of N-aryl and N-alkyl N ' -hydroxyguanidines and rela
ted compounds has been investigated using UV-visible, EPR, and resonance Ra
man spectroscopies. All the N-hydroxyguanidines studied bind to the ferric
form of MPS with formation of stable low-spin iron(IH) complexes characteri
zed by absorption maxima at 405, 535, and 560 nm. The complex obtained with
N-(4-methoxyphenyl) N ' -hydroxyguanidine exhibits EPR g-values at 2.55, 2
.26, and 1.86. The resonance Raman (RR) spectrum of this complex is also in
agreement with an hexacoordinated low-spin iron(III) structure. The dissoc
iation constants (K-s) of the MP8 complexes with mono- and disubstituted N-
hydroxyguanidines vary between 15 and 160 muM at pH 7.4. Amidoximes also fo
rm low-spin iron(HI) complexes of MP8, although with much larger dissociati
on constants. Under the same conditions, ketoximes, aldoximes, methoxyguani
dines, and guanidines completely fail to form such complexes with MP8. The
K-s values of the MP8-N-hydroxyguanidine complexes decrease as the pH of th
e solution is increased, and the affinity of the N-hydroxyguanidines toward
MP8 increases with the pK(a) of these ligands. Altogether these results sh
ow that compounds involving a -C(NHR)=NOH moiety act as good ligands of MP8
-Fe(III) with an affinity that depends on the electron-richness of this moi
ety. The analysis of the EPR spectrum of the MP8-N-hydroxyguanidine complex
es according to Taylor's equations shows a strong axial distortion of the i
ron, typical of those observed for hexacoordinated heme-Fe(III) complexes w
ith at least one pi donor axial ligand (HO-, RO-, or RS-). These data stron
gly suggest that N-hydroxyguanidines bind to MP8 iron via their oxygen atom
after deprotonation or weakening of their O-H bond. It thus seems that N-h
ydroxyguanidines could constitute a new class of strong ligands for hemepro
teins and iron(III)-porphyrins.