Studies on ligand-ligand interaction - Formation and stability of proton dipeptide amino acid complexes

Potentiometric titration method was used to determine the stabilities of proton complexes forming between phenylalanyl- L - leucine and six amino acids(glycine, alanine, valine, leucine, isoleucine and phenylalanine) at ionic strength of 0.1mol/L (NaNO3) and 25 degreesC. The results show that the order of stability of the two forms of complexes is valine > alanine > leucine > isoleucine > phenylalanine > glycine. For further understanding of these results, semiempirical calculations at PM3 level and molecular mechanic (AMBER force field) method were used to optimize the conformation and the energy of proton complexes. In the view of weak interligand interaction, the recognitions of amino acids by phenylalanyl - L - leucine are discussed. The results show that the electrostatic forces and hydrogen bond give main contributions to the stabilities of the complexes. The van der Waals interactions and the bonding interactions also play a very important role in the complexation.