Protonation equilibria of quinolone antibacterials

Area of Science:

• Medicinal Chemistry
• Analytical Chemistry
• Spectroscopy

Background:

• Antibacterial 7-piperazinyl fluoroquinolone derivatives are crucial in medicine.
• Understanding their acid-base properties is essential for drug design and efficacy.
• These compounds possess multiple proton-binding sites influencing their behavior in biological systems.

Purpose of the Study:

• To investigate the acid-base properties of seven antibacterial 7-piperazinyl fluoroquinolone derivatives.
• To quantitate basicities at both molecular (macroconstants) and submolecular (microconstants) levels.
• To determine the microspeciation of these compounds.

Main Methods:

• Potentiometry
• UV spectroscopy
• Nuclear Magnetic Resonance (NMR) spectroscopy, specifically 1H NMR

Main Results:

• Identified two proton-binding sites of similar basicity: piperazine amino and carboxylate groups.
• Quantitated macroconstants and microconstants for the fluoroquinolone derivatives.
• Calculated the concentration of positive, zwitterionic, neutral, and negatively charged species.
• Observed that zwitterionic forms consistently predominate over neutral protonation isomers.
• Found considerable variation in the zwitterionic:neutral concentration ratio across the derivatives.

Conclusions:

• The study provides a detailed understanding of the acid-base behavior of these antibacterial agents.
• Microspeciation analysis reveals distinct charge distributions influenced by molecular structure.
• The differing zwitterionic:neutral ratios highlight structural nuances affecting drug properties and potential interactions.