Acid-base properties for each protonation site of six corticotropin fragments.

The individual basicity of each protonation site for six different fragments of aH-ACTH (corticotropin) have been characterized in terms of group constants. Microconstants have also been estimated for the N- and C-terminal moieties of ACTH (15-32) and ACTH (1-28), respectively. In these cases the bifunctional termini contain adjacent protonation sites of similar basicity in close interaction. For fragments consisting of 14 or fewer amino acids the acid-base properties of all the protonation sites can be well interpreted by taking into account only the primary peptide structure. The group constant values for ACTH (15-32), ACTH (1-28) and ACTH (1-32) provide evidence that the functional groups of these fragments are influenced by several intramolecular interactions such as H-bonding and macromolecular hydrophobic effects. By means of the above constants the concentration of any arbitrarily chosen microspecies can be calculated.