Assessing the protonation state of drug molecules: the case of aztreonam.
Herein we examine the viability of physicochemical approaches based on standard computational chemistry tools to characterize the structure and energetics of flexible drug molecules with various titratable sites. We focus on the case of the monobactam antibiotic aztreonam, whose structure and physicochemical properties have been ascribed to several tautomeric forms, although it is still unclear which protonation states are responsible for its biological activity. First, we experimentally determined the pKa values for aztreonam over the pH range 0.8-7.0 using both 1H NMR and 13C NMR spectroscopy. Second, we carried out quantum chemical calculations on snapshots extracted from classical molecular dynamics simulations. Various levels of approximation were used in the energy calculations: ONIOM(HF/3-21G*:AMBER) for geometry relaxation, B3LYP/6-31+G** for electronic and electrostatic solvation energies, and molecular mechanics for attractive dispersion energy. The value of the free energy of solvation of a proton was treated as a parameter and chosen to give the best match between calculated and experimental pKa values for small molecules. Overall, this computational scheme can give satisfactory results in the pKa calculations for drug molecules.
Díaz
Natalia
N
Departamento de Química Física y Analítica, Universidad de Oviedo, Julian Clavería 8, 33006 Oviedo (Asturias), Spain.
Sordo
Tomás L
TL
Suárez
Dimas
D
Méndez
Rosa
R
Villacorta
Javier Martín
JM
Simón
Luis
L
Rico
Manuel
M
Jiménez
M Angeles
MA
D000900:Anti-Bacterial Agents / Q000737:chemistry*; D001398:Aztreonam / Q000737:chemistry*; D003198:Computer Simulation; D006863:Hydrogen-Ion Concentration; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D011522:Protons; D011789:Quantum Theory; D013816:Thermodynamics
D000900:Anti-Bacterial Agents; D011522:Protons; D001398:Aztreonam