title: Acid dissociation of 2-mercaptohistamine and its related compounds.
abstract: The acid dissociation constants of 2-mercaptohistamine hydrochloride (2-MH) and its related compounds ergothioneine (Erg), 2-mercaptoimidazole (MI), N-methyl-2-mercaptoimidazole (MMI) and histamine dihydrochloride were determined by potentiometry at 15 degrees , 25 degrees and 35 degrees and mu = 0.1 (NaClO(4)). The values of DeltaH and DeltaS for the ionization equilibria were calculated. The magnitude of DeltaH (9.5 kcal/mole) for pk(a1) of 2-MH is similar to the value DeltaH = 9-10 kcal/mole found for the ionization of a number of amino groups. The value DeltaH = 7.4 kcal/mole for pk(a2) of 2-MH is similar to the values for the ionization of a variety of thiols (DeltaH = 5-7 kcal/mole). Such values were not obtained for Erg, MI and MMI, which suggests that 2-MH exists mainly in the thiol form, whereas Erg, MI and MMI are in the thione form.
authors.affiliation: Faculty of Pharmaceutical Sciences, University of Tokushima, Tokushima 770, Japan.
authors.forename: H
authors.initials: H
authors.lastname: Sakurai
authors.forename: S
authors.initials: S
authors.lastname: Takeshima