Microscopic Protonation Deprotonation Equilibria of the Antiinflammatory Agent Piroxicam

The microscopic ionization behavior of piroxicam was investigated using two different approaches, i.e., direct UV spectroscopy and an indirect analogue approach (deductive method). The best microscopic pK(a) values (PKa12 = 4.60, pK(a21) = 5.40, pK(a22) = 2.72, and pK(a11) = 1.92) were obtained by the deductive method using as pK(a22) the pK(a) of the enolic O-methylated piroxicam 2. The results show remarkable electrostatic effects in the protonation/deprotonation equilibria, a marked increase in the acidity of the enolic function (2.68 pK(a) units) being caused by the pyridinium group. The electronic structure of piroxicam was studied based on H-1-NMR chemical shifts at various ionization states, indicating an extended electron conjugation through the molecule. The partition measurements in octan-1-ol/H2O of zwitterionic compound 3 (the pyridyl N-methyl derivative of piroxicam (1)) suggest that the two opposite charges in zwitterionic piroxicam are indeed in a close intramolecular proximit