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A generally applicable synthetic approach to dipeptide-DOTAM conjugates has been developed which is based on the peralkylation of 1,4,7,10-tetraazacyclododecane (cyclen) with protected N-iodoacetyl dipeptides. Standardized procedures were used for the alkylation, metalation, and purification of the resultant lanthanide complexes. Using this approach, we have been able to rapidly and reliably prepare and screen five different ligands each with up to six lanthanide ions. This preliminary investigation has identified several paramagnetic compounds with strong chemical exchange saturation transfer (PARACEST) properties in water at physiological temperature and pH. Extension of the synthetic approach to a wide variety of amino acids is possible.

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Potentiometric equilibrium measurements have been performed at (25.0 ( 0.1)°C and ionic strength I ) 0.1 mol dm -3 KNO 3 for the interaction of the purine nucleotides adenosine 5′-mono, 5′-di, and 5′-triphosphate and Cu(II), Co(II), Ni(II), Mn(II), Zn(II), Ca(II), and Mg(II) with the biologically important secondary ligand zwitterionic buffers 3-(N-morpholino)propanesulfonic acid, 3-[(1,1-dimethyl-2-hydroxyethyl)amino]-2-hydroxypropanesulfonic acid, N-(2-hydroxyethyl)piperazine-N′-2-hydroxypropanesulfonic acid, piperazine-N,N′-bis(2-ethanesulfonic acid), and piperazine-N,N′-bis(2-hydroxypropanesulfonic acid) in a 1:1:1 ratio and the formation of various 1:1:1 normal and protonated mixed-ligand complex species was inferred from the potentiometric pH titration curves. The experimental conditions were selected such that self-association of the purine nucleotides and their complexes was negligibly small; i.e., the monomeric normal and protonated ternary complexes were studied. Initial estimates of the formation constants of the resulting species and the acid dissociation constants of adenosine 5′-mono-, 5′-di-, and 5′-triphosphate and the zwitterionic buffer secondary ligands have been refined with the SUPERQUAD computer program. In some M(II) mixed-ligand systems, the interligand interactions between the coordinate ligands, possibly H bond formation, have been found to be most effective in deciding the stability of the mixed-ligand complexes formed in solutions.

Potentiometric equilibrium measurements have been performed at (25.0 ± 0.1) °C and ionic strength I = 0.1 mol dm-3 (KNO3) for the interaction of glycine (aminoethanoic acid), serine (2-amino-3-hydroxypropanoic acid), methionine (2-amino-4-(methylthio)butanoic acid), aspartic acid (aminobutanedioic acid), glutamic acid (2-aminopropanedioic acid), and histidine (α-amino-1H-imidazole-4-propanoic acid) and Cu(II), Co(II), Ni(II), Mn(II), and Zn(II) with the biologically important secondary ligand zwitterionic buffers β-hydroxy-4-morpholinepropanesulfonic acid (MOPSO), 4-morpholinepropanesulfonic acid (MOPS), 3-[bis(2-hydroxyethyl)amino]-2-hydroxy-1-propanesulfonic acid (DIPSO), and 3-[N-tris(hydroxymethyl)methyl)amino]-2-hydroxypropanesulfonic acid (TAPSO) in 1:1:1 and 1:1:2 ratios, and the formation of various 1:1:1 ternary complexes and 1:1:2 quaternary complex species was inferred from the potentiometric pH titration curves. Initial estimates of the formation constants of the resulting species and the acid dissociation constants of the different amino acids and secondary ligands studied have been refined with the SUPERQUAD computer program. The order of stability of the different normal ternary complexes in the systems under investigation in terms of metal ion follows generally the trend Cu(II) > Ni(II) > Co(II) > Zn(II) > Mn(II).