Influence of alkali- and alkaline-earth-metal cations on the ‘salt-induced peptide formation’ reaction

Abstract

The reaction mechanism of the salt-induced peptide formation from amino acids has been investigated by variation of the inorganic salt delivering Cl^– ions and providing the dehydrating effect. Chloride anions proved to be essential to prevent chelate complexation of the second amino acid. Upon exchange of sodium by other alkali- or alkaline-earth elements, peptide formation is still observed. The dipeptide yields are mainly determined by two factors: on the one hand the pH of the solution should be below 3 to prevent Cu^II-catalysed peptide hydrolysis and give an optimum species distribution for peptide formation, and above 2 to keep proton-catalysed peptide hydrolysis as low as possible; on the other hand by the concentration of the inorganic salt for removing water from the reaction and thus shifting the equilibrium towards the peptide side. The hydration enthalpies of the cations are the determining facto for the initial rate of peptide formation and lead to the series Mg²⁺ > Ca²⁺ > Ba²⁺ > Na⁺ > NH₄⁺ > K⁺ > Cs⁺. In the long run the initial advantage of divalent cations is overruled by stronger hydrolysis due to the lower pH of their solutions. The ion NH₄⁺ is atypical, apparently due to its buffering ability