Evaluation of the pKa values and ionization sequence of bumetanide using 1H and 13C NMR and UV spectroscopy

✍ Scribed by Benben Song; Amit K. Galande; Krishna Kodukula; Walter H. Moos; Susan M. Miller

• Publisher: John Wiley and Sons
• Year: 2011
• Tongue: English
• Weight: 317 KB
• Volume: 72
• Category: Article
• ISSN: 0272-4391
• DOI: 10.1002/ddr.20443

✦ Synopsis

Non-thiazide loop diuretics such as bumetanide and furosemide are well-established medicaments, with varying substituents that alter their chemical properties in ways that affect biological parameters, including biodistribution, efficacy, and safety. However, literature data supporting the assignment of pK a values and ionization sequence of bumetanide are limited. The present study summarizes available literature data and then characterizes nuclear magnetic resonance (NMR) and ultraviolet (UV) spectral changes over a range of pH values to delineate the apparent sequence of deprotonations. Three macroscopic pK a values, 1.44, 3.74, and 10.37, were determined in water via pH titration experiments monitored by 1 H NMR. Both 1 H and 13 C analyses support assignment of the lowest pK a value to the anilinium ion, as described for another diuretic, piretanide. However, UV data suggest that initial deprotonation of the benzoic acid competes to some extent with deprotonation of the anilinium ion, such that both pathways may operate simultaneously in an aqueous environment. Thus, the observed pK a values likely represent the macroscopic averages of the two competing deprotonation sequences. Calculated pK a values for bumetanide, piretanide, and model compounds are contrary to the deprotonation sequence we and others have determined experimentally, which is not surprising given the complicated factors driving the acid-base properties of these molecules. Drug Dev Res 72: 416-426, 2011.