Oxygen Radical Scavenger Activity, EPR, NMR, Molecular Mechanics and Extended-Hückel Molecular Orbital Investigation of the Bis(Piroxicam)Copper(II) Complex

摘要

The oxygen radical scavenger activity (ORSA) of [CuII(Pir)2] (HPir = Piroxicam = 4-hydroxy -2- methyl -N-2- pyridyl -2H- 1,2-benzothiazine -3- carboxamide 1,1-dioxide) was determined bychemiluminescence of samples obtained by mixing human neutrophils (from healthy subjects) and [CuII(Pir)2(DMF)2] (DMF = N,N -dimethylformammide) in DMSO/GLY/PBS (2:1:2, v/v) solution(DMSO = dimethylsulfoxide, GLY = 1,2,3-propantriol, PBS = Dulbecco’s buffer salt solution). The ratio of the residual radicals, for the HPir (1.02·10−4M) and [CuII(Pir)2(DMF)2] (1.08·10−5M)/HPir (8.01·10−−5M) systems was higher than 12 (not stimulated) [excess of piroxicam was added (Cu/Pir molar ratio ≈1:10) in order to have most of the metal complexed as bischelate]. In contrast, the ratioof residual radicals for the CuCl2 (1.00·10−5M) and [CuII(Pir)2(DMF)2] (1.08·10−5M)/Hpir (8.01·10−5M)system was 5. The [CuII(Pir)2] compound is therefore a stronger radical scavenger than eitherHPir or CuCl2. A molecular mechanics (MM) analysis of the gas phase structures of neutral HPir, itszwitterionic (HPir+-) and anionic (Pir-) forms, and some CuII-piroxicam complexes based on X-ray structures allowed calculation of force constants. The most stable structure for HPir has a ZZZ conformation similar to that found in the CuII (and CdII complexes) in the solid state as well as in the gas phase. The structure is stabilized by a strong H bond which involves the N(amide)-H and O(enolic) groups. The MM simulation for the [CuII(Pir)2(DMF)2] complex showed that two high repulsive intramolecular contacts exist between a pyridyl hydrogen atom of one Pir- molecule withthe O donor of the other ligand. These interactions activate a transition toward a pseudo-tetrahedralgeometry, in the case the apical ligands are removed. On refluxing a suspension of[CuII(Pir)2(DMF)2] in acetone a brown microcystalline solid with the Cu(Pir)2·0.5DMF stoichiometrywas in fact prepared. 13C spin-lattice relaxation rates of neutral, zwitterionic and anionic piroxicam,in DMSO solution are explained by the thermal equilibrium between the three most stablestructures of the three forms, thus confirming the high quality of the force field. The EPR spectrumof [CuII(Pir)2(DMF)2] (DMSO/GLY, 2:1, v/v, 298 and 110 K) agrees with a N2O2+O2 pseudo-octahedralcoordination geometry. The EPR spectrum of [CuII(Pir)2·0.5DMF agrees with a pseudo-tetrahedral coordination geometry. The parameters extracted from the room temperature spectra ofthe solution phases are in agreement with the data reported for powder and frozen solutions. Theextended-Hückel calculations on minimum energy structures of [CuII(Pir)2(DMF)2] and [CuII(Pir)2](square planar) revealed that the HOMOs have a relevant character of dx2−y2. On the other handthe HOMO of a computer generated structure for [CuII(Pir)2] (pseudo-tetrahedral) has a relevantcharacter of dxy atomic orbital. A dxy orbital is better suited to allow a dπ-pπ interaction to the O2- anion. Therefore this work shows that the anti-inflammatory activity of piroxicam could be due in partto the formation of [CuII(Pir)2] chelates, which can exert a SOD-like activity.