NMR titration and molecular model MOPAC calculation of protonation processes of two MRI ligands

The protonation process of two DTPA bis(amide) derivatives,DTPA-BDMA and DTPA-BDEA,was studied by using 1H NMR titration and MOPAC calculation.Their protonation process was proposed in the order of the central amine,the terminal amines,the central carboxyl,the terminal carboxyl,the other terminal carboxyl and central amine.During the protonation of the terminal amine,there existed a large fraction of proton transfer from the central amine to the other terminal amine.