Study of the relationship between nitrogen basicity and receptor affinity in a substituted quinoline series

In buffered water:methanol media, the pKa values of acid-base equilibria involved in the protonation of substituted quinolines are determined by UV-visible absorption spectrometry. The observed behavior depends on the nature of R2: the N-CH3 derivatives exist as quinoline, the N-H derivatives as 4-iminoquinoline tautomers. The iminoquinoline zwitterionic species present at physiological pH may have high receptor affinity. A linear relationship between pKa of the endocyclic nitrogen and log IC50 is discussed.