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 by
chemiluminescence 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 ratio
of 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 either
HPir or CuCl2. A molecular mechanics (MM) analysis of the gas phase structures of neutral HPir, its
zwitterionic (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 with
the O donor of the other ligand. These interactions activate a transition toward a pseudo-tetrahedral
geometry, 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
stoichiometry
was in fact prepared. C13
spin-lattice relaxation rates of neutral, zwitterionic and anionic piroxicam,
in DMSO solution are explained by the thermal equilibrium between the three most stable
structures of the three forms, thus confirming the high quality of the force field. The EPR spectrum
of [CuII(Pir)2(DMF)2] (DMSO/GLY, 2:1, v/v, 298 and 110 K) agrees with a N2O2+O2 pseudo-octahedral
coordination 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 of
the solution phases are in agreement with the data reported for powder and frozen solutions. The
extended-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 hand
the HOMO of a computer generated structure for [CuII(Pir)2] (pseudo-tetrahedral) has a relevant
character 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 part
to the formation of [CuII(Pir)2]
chelates, which can exert a SOD-like activity.
