Microequilibrium of drug molecules]

Protonation macro-, micro-, and submicro constants are physico-chemical parameters of crucial importance, concerning the fate of bio-, drug, and narcotic drug molecules in the body and in protic solvents. The most important characteristics, relationships, applications and biological significance of these parameters are reviewed, using acetylcysteine and cysteine, as examples. The mucolytic effect of acetylcysteine, an active principle in numerous drugs, is interpreted in terms of protonation state of the molecule and its thiolate site. Microscopic protonation constants of acetylcysteine, data that have not previously appeared, are also reported.