Ion-pairing of anionic DTPA-based cyclophanes with diaminoalkanes and methylated amino acids, lysine and arginine, in their dicationic form

Complex formation of three anionic cyclo- phanes with the dications of diaminoalkanes and basic amino acid esters was studied by 1 H NMR titrations in aqueous media. Every cyclophane bearing six CH2CO2 - arms consists of two diethylenetriaminepentaacetic (DTPA) units linked by two aromatic diamine units through amide bonds; the interlinking amine is p-xylen- ediamine (abbreviated as px), 4,4 0 -methylenedianiline (ma) or 4,4 0 -diaminobibenzyl (dab). The cyclophanes that exist in the anionic form cyH2 4- (with two acidic protons) have multi-point electrostatic interaction with diaminoalkane dications ? H3N(CH2)nNH3 ? , so as to construct ion pairs stabilized by hydrogen-bonding between CH2CO2 - and NH3 ? . The formation constants vary from 31 to 2,477 M -1 depending on the cation-anion combinations; the highest stability is found for the complex of the px-derived cyclophane, cy(px)H2 4- , with diaminobutane, and the lowest for diaminobutane-cy(dab)H2 4- pair. The stability is correlated with the chain length of the diaminoalkanes as well as the size of the cyclophanes. These geometrical factors are also crucial for ion-pairing with the dications of methylated lysine and arginine; the formation constant is 2,884 M -1 for (lys)H2 2? -cy(px)H2 4- , and 1,688 M -1 for (arg)H2 2? -cy(px)H2 4- , while the constants of the cy(ma)H2 4- and cy(dab)H2 4- complexes are in a much smaller range of 39-230 M -1 .