A multinuclear NMR relaxometry study of ternary adducts formed between heptadentate Gd^III chelates and L-lactate

Dramatic relaxation enhancements of L-lactate resonances have been observed upon formation of ternary adducts with Gd^III complexes of heptadentate DO3A and DO3A-like ligands (DO3A = 1,4,7,10-tetraazaciclododecane- 1,4,7-triacetic acid). Detailed ¹H and ¹⁷O NMR relaxometry investigations allow us to obtain structural, dynamic and thermodynamic information on the ternary complexes in which L-lactate acts as a bidentate ligand replacing two water molecules in the inner coordination sphere of the Gd^III ion. It has been found that the exchange rate of the coordinated L-lactate is modulated by the structural and electronic properties of the parent Gd-heptacoordinated macrocyclic chelate. In addition to the characterisation of the relaxation behaviour of the ¹H methyl resonance of L-lactate, this study has been extended to its ¹³C isomer (fully enriched at the three positions) and to the trifluoro-L-lactate. The obtained results may be relevant to the development of relaxation agents able to promote the relaxation enhancement of specific substrates detectable by in vivo magnetic resonance spectroscopy.