Microscopic Protonation/Deprotonation Equilibria of the Anti-Inflammatory Agent Piroxicam |
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Authors: | Krisztina Takcs-Novk Jzsef Kksi Benjmin Podnyi Bla Noszl Ruey-Shiuan Tsai Giuseppe Lisa Pierre-Alain Carrupt Bernard Testa |
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Institution: | Krisztina Takács-Novák,József Kökösi,Benjámin Podányi,Béla Noszál,Ruey-Shiuan Tsai,Giuseppe Lisa,Pierre-Alain Carrupt,Bernard Testa |
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Abstract: | 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 pKa values (pKa12 = 4.60, pKa21 = 5.40, pKa22 = 2.72, and pKa11 = 1.92) were obtained by the deductive method using as pKa22 the pKa 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 pKa units) being caused by the pyridinium group. The electronic structure of piroxicam was studied based on 1H-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 proximity. |
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