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The acid-base chemistry of a tetraprotic, ampholyte ACE-inhibitor, lisinopril was studied by different methods. Potentiometry in aqueous medium and co-solvent technique in methanol-water mixtures as well as 1 H NMR-pH titration were applied for the highly precise measurement of protonation macroconstants. The logK values of lisinopril (at 25.0 °C and 0.15 M ionic strength) were found: logK 1 = 10.75 ± 0.01, logK 2 = 7.13 ± 0.01, logK 3 = 3.13 ± 0.01, logK 4 = 1.63 ± 0.01, calculated as an average of the best two values obtained by independent methods. NMR-pH titration was used to assign the constants to the functional groups and for the examination of site-specific, submolecular basicities of the molecule (determination of protonation microconstants). In the first two well separated protonation steps the macro- and microconstants are identical and assigned to the primary amino group (logK 1 = logk A ) and to the secondary amine basicity (logK 2 = logk B A ), respectively. The two carboxylates exhibit overlapping protonation and characterized first here by microconstants (logk D AB = 2.15 log k C AB = 3.10) revealing that the carboxylate on proline ring has 9 times greater intrinsic basicity than the carboxylate in the side chain. The distribution of protonation species (Lis 2- ; HLis - ; H 2 Lis; H 3 Lis + ; H 4 Lis 2+ ) and microspecies (ABC; ABD) against the pH was calculated and used to interpret the pharmacokinetic and pharmacodynamic properties of lisinopril.

Physico-chemical Profiling of ACE-inhibitor Lisinopril: Acid-base Properties