Acid-base properties and microspeciation of six angiotensin-type octapeptides

The acid-base chemistry of six angiotensin II analogues (A II) was analyzed by determining two types of terms. Macroconstants were used to characterize the molecular proton-binding and dissociating ability of the octapeptides, and group constants to quantitate the submolecular basicity of each individual protonation site of the derivatives. The group constant values indicated that some sites (Arg-guanidino, Tyr-phenolate, C-terminal carboxylate) were of similar basicity in the different analogues, while others (His-imidazole, N-terminal amino) were significantly different. The group constant values are interpreted by taking into consideration the intramolecular effects of the adjoining moieties and are used for microspeciation.