| Abstract
| - Interaction between microperoxidase-8 (MP8), a water-soluble hemeprotein model, and a widerange of N-aryl and N-alkyl N‘-hydroxyguanidines and related compounds has been investigated usingUV−visible, EPR, and resonance Raman spectroscopies. All the N-hydroxyguanidines studied bind tothe ferric form of MP8 with formation of stable low-spin iron(III) complexes characterized by absorptionmaxima at 405, 535, and 560 nm. The complex obtained with N-(4-methoxyphenyl) N‘-hydroxyguanidineexhibits EPR g-values at 2.55, 2.26, and 1.86. The resonance Raman (RR) spectrum of this complex isalso in agreement with an hexacoordinated low-spin iron(III) structure. The dissociation constants (Ks) ofthe MP8 complexes with mono- and disubstituted N-hydroxyguanidines vary between 15 and 160 μM atpH 7.4. Amidoximes also form low-spin iron(III) complexes of MP8, although with much larger dissociationconstants. Under the same conditions, ketoximes, aldoximes, methoxyguanidines, and guanidines completelyfail to form such complexes with MP8. The Ks values of the MP8−N-hydroxyguanidine complexes decreaseas the pH of the solution is increased, and the affinity of the N-hydroxyguanidines toward MP8 increaseswith the pKa of these ligands. Altogether these results show that compounds involving a -C(NHR)NOHmoiety act as good ligands of MP8−Fe(III) with an affinity that depends on the electron-richness of thismoiety. The analysis of the EPR spectrum of the MP8−N-hydroxyguanidine complexes according toTaylor's equations shows a strong axial distortion of the iron, typical of those observed for hexacoordinatedheme−Fe(III) complexes with at least one π donor axial ligand (HO-, RO-, or RS-). These data stronglysuggest that N-hydroxyguanidines bind to MP8 iron via their oxygen atom after deprotonation or weakeningof their O−H bond. It thus seems that N-hydroxyguanidines could constitute a new class of strong ligandsfor hemeproteins and iron(III)-porphyrins.
|
