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Two copper(II) complexes were synthesized from tridentate ligands involving a (Nquinoline,Nazo,Ophenol) donor set. The copper(II) ion is tetracoordinate with a chloride ion occupying the fourth position of the coordination sphere. Both X-ray diffraction and electron paramagnetic resonance (EPR) spectroscopy reveal that the copper(II) ion geometry is square planar in 1. In contrast, significant tetrahedral distortions are observed in 2, as a result of the steric clash between the hydrogen atoms of the methyl substituent of the quinoline group and the chloride ion. Cyclic voltammetry curves of 1 and 2 in CH2Cl2 display a reversible oxidation wave at E1/2 = 0.59 V and 0.56 V versus ferrocenium/ferrocene, respectively, which was assigned to the phenoxyl/phenolate redox couple. Compounds 1+ and 2+ were generated and characterized by UV/Vis and EPR spectroscopy. Their reactivity with benzyl alcohol was investigated by kinetic measurements. [ABSTRACT FROM AUTHOR]

In human, Tyrosinase enzyme (TyH) is involved in the key steps of protective pigments biosynthesis (in skin, eyes and hair). The use of molecules targeting its binuclear copper active site represents a relevant strategy to regulate TyH activities. In this work, we targeted 2-Hydroxypyridine- N -oxide analogs (HOPNO, an established chelating group for the tyrosinase dicopper active site) with the aim to combine effects induced by combination with a reference inhibitor (kojic acid) or natural substrate (tyrosine). The HOPNO–MeOH (3) and the racemic amino acid HOPNO-AA compounds (11) were tested on purified tyrosinases from different sources (fungal, bacterial and human) for comparison purposes. Both compounds have more potent inhibitory activities than the parent HOPNO moiety and display strictly competitive inhibition constant, in particular with human tyrosinase. Furthermore, 11 appears to be the most active on the B16–F1 mammal melanoma cells. The investigations were completed by stereospecificity analysis. Racemic mixture of the fully protected amino acid 10 was separated by chiral HPLC into the corresponding enantiomers. Assignment of the absolute configuration of the deprotected compounds was completed, based on X-ray crystallography. The inhibition activities on melanin production were tested on lysates and whole human melanoma MNT-1 cells. Results showed significant enhancement of the inhibitory effects for the (S) enantiomer compared to the (R) enantiomer. Computational studies led to an explanation of this difference of activity based for both enantiomers on the respective position of the amino acid group versus the HOPNO plane. [Display omitted] • Hydroxypyridine- N -oxide analogs prepared including a non-canonic amino acid. • Inhibition on tyrosinase enzymes from mushroom, bacterial and human sources tested. • Inhibition stereoselectivity demonstrated (enzymes and cell-based assays). [ABSTRACT FROM AUTHOR]