Solid-state acid-base interactions in complexes of heterocyclic bases with dicarboxylic acids: crystallography, hydrogen bond analysis, and 15N NMR spectroscopy.
A cancer candidate, compound 1, is a weak base with two heterocyclic basic nitrogens and five hydrogen-bonding functional groups, and is sparingly soluble in water rendering it unsuitable for pharmaceutical development. The crystalline acid-base pairs of 1, collectively termed solid acid-base complexes, provide significant increases in the solubility and bioavailability compared to the free base, 1. Three dicarboxylic acid-base complexes, sesquisuccinate 2, dimalonate 3, and dimaleate 4, show the most favorable physicochemical profiles and are studied in greater detail. The structural analyses of the three complexes using crystal structure and solid-state NMR reveal that the proton-transfer behavior in these organic acid-base complexes vary successively correlating with Delta pKa. As a result, 2 is a neutral complex, 3 is a mixed ionic and zwitterionic complex and 4 is an ionic salt. The addition of the acidic components leads to maximized hydrogen bond interactions forming extended three-dimensional networks. Although structurally similar, the packing arrangements of the three complexes are considerably different due to the presence of multiple functional groups and the flexible backbone of 1. The findings in this study provide insight into the structural characteristics of complexes involving heterocyclic bases and carboxylic acids, and demonstrate that X-ray crystallography and 15N solid-state NMR are truly complementary in elucidating hydrogen bonding interactions and the degree of proton transfer of these complexes.
Li
Z Jane
ZJ
Pfizer Global Research and Development, Pfizer Inc., Eastern Point Road, Groton, Connecticut 06340, USA. jane.li@pfizer.com
Abramov
Yuriy
Y
Bordner
Jon
J
Leonard
Jason
J
Medek
Ales
A
Trask
Andrew V
AV
D000588:Amines / Q000737:chemistry*; D018360:Crystallography, X-Ray; D003998:Dicarboxylic Acids / Q000737:chemistry*; D015195:Drug Design; D006571:Heterocyclic Compounds / Q000737:chemistry*; D006860:Hydrogen Bonding; D006863:Hydrogen-Ion Concentration; D046911:Macromolecular Substances / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D009587:Nitrogen Isotopes; D018719:Erb-b2 Receptor Tyrosine Kinases / Q000037:antagonists & inhibitors; D012995:Solubility
D000588:Amines; D003998:Dicarboxylic Acids; D006571:Heterocyclic Compounds; D046911:Macromolecular Substances; D009587:Nitrogen Isotopes; D018719:Erb-b2 Receptor Tyrosine Kinases