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A multinuclear NMR study of the interaction between phenylboronic acid (PBA) and sialic acid (Neu5 Ac) has been performed. The latter compound is known to be overexpressed on the cell surface of tumor cells. The results of this investigation suggest that the binding of PBA to sialic acid is pH dependent. 17O NMR experiments with glycolic acid as the model compound prove that an interaction at the alpha-hydroxycarboxylate occurs at pH < 9, while a study with threonic and erythronic acids shows that the PBA group interacts selectively with the vicinal diol functions at higher pH. Similarly, Neu5 Ac binds PBA through its alpha-hydroxycarboxylate at low pH (< 9) and through its glycerol side chain at higher pH values. The conditional stability constant of the phenylboronate ester at pH 7.4 is 11.4. On cell surfaces, sialic acid is connected to the neighboring sugar unit through the 2-hydroxy group. To mimic this the 2-alpha-O-methyl derivative of Neu5 Ac was included in this study. The erythro configuration of the hydroxy substituents prevents stable-complex formation at positions C7 and C8 and, consequently, the strongest interaction is observed at positions C8 and C9, leading to a five-membered 2-boron-1,3-dioxalate. In addition, a relatively small amount of the C7-C9 six-membered complex was observed. Molecular modeling studies confirm that the C8-C9 boronate complex has the lowest energy.

Four new ligands for lanthanide ions based on the H3do3a (=1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid) structure and bearing one N-sulfonylacetamide arm were synthesized, i.e., H4dota-NHSO2R=10-{2-[(R)sulfonylamino]-2-oxoethyl}-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acids 1a–e. A 15N-NMR study of the 15N-labelled Eu3+ complex of one such ligands, 1d, showed that the coordination of the N-sulfonylacetamide arm involves the carbonyl O-atom rather than the N-atom. The relaxometric properties of the corresponding Gd3+ complexes were investigated as a function of pH and temperature. These complexes have relaxivities in the range 4.5–5.3 mM−1 s−1, at 20 MHz and 25°, and are characterized by a single H2O molecule in their inner coordination sphere. The mean residence lifetime of this molecule is relatively long (500–700 ns) compared to other anionic complexes. The slow rate of H2O exchange can be justified by the extensive delocalization of the negative charge on the N-sulfonylacetamide arm. The long residence time of the coordinated H2O allowed the observation of the effect of the prototropic exchange on the relaxivity. The study of the interaction between the complex [Gd(1e)]- and HSA revealed a weak affinity constant highlighting the importance of a localized negative charge on the complex to promote a strong interaction with the protein.

The formation of a macromolecular adduct between a suitably functionalized GdIII complex and an exogeneous substance such as a poly-β-cyclodextrin (see picture) forms the basis of the novel route to contrast agents for magnetic resonance imaging endowed with high relaxivity discussed herein.

The development of gadolinium chelators that can be easily and readily linked to various substrates is of primary importance for the development high relaxation efficiency and/or targeted magnetic resonance imaging (MRI) contrast agents. Over the last 25 years a large number of bifunctional chelators have been prepared. For the most part, these compounds are based on ligands that are already used in clinically approved contrast agents. More recently, new bifunctional chelators have been reported based on complexes that show a more potent relaxation effect, faster complexation kinetics and in some cases simpler synthetic procedures. This review provides an overview of the synthetic strategies used for the preparation of bifunctional chelators for MRI applications.