INFLUENCE OF ALKALI-EARTH-METAL AND ALKALINE-EARTH-METAL CATIONS ON THE SALT-INDUCED PEPTIDE FORMATION REACTION

The reaction mechanism of the salt-induced peptide formation from amin o acids has been investigated by variation of the inorganic salt deliv ering Cl- ions and providing the dehydrating effect. Chloride anions p roved to be essential to prevent chelate complexation of the second am ino acid. Upon exchange of sodium by other alkali- or alkaline-earth e lements, peptide formation is still observed. The dipeptide yields are mainly determined by two factors: on the one hand the pH of the solut ion should be below 3 to prevent Cu(II)-catalysed peptide hydrolysis a nd give an optimum species distribution for peptide formation, and abo ve 2 to keep proton-catalysed peptide hydrolysis as low as possible; o n the other hand by the concentration of the inorganic salt for removi ng water from the reaction and thus shifting the equilibrium towards t he peptide side. The hydration enthalpies of the cations are the deter mining factor for the initial rate of peptide formation and lead to th e series Mg2+ > Ca2+ > Ba2+> Na+ > NH4+ > K+ > Cs+. In the long run th e initial advantage of divalent cations is overruled by stronger hydro lysis due to the lower pH of their solutions. The ion NH4+ is a typica l, apparently due to its buffering ability.