Structural, Luminescence, and NMR Studies of the Reversible Binding of Acetate, Lactate, Citrate, and Selected Amino Acids to Chiral Diaqua Ytterbium, Gadolinium, and Europium Complexes

The nature of the ternary complexes formed in aqueous media at ambient pH on reversible
binding of acetate, lactate, citrate, and selected amino acids and peptides to chiral diaqua europium,
gadolinium, or ytterbium cationic complexes has been examined. Crystal structures of the chelated ytterbium
acetate and lactate complexes have been defined in which the carboxylate oxygen occupies an “equatorial”
site in the nine-coordinate adduct. The zwitterionic adduct of the citrate anion with [EuL1] was similar to the
chelated lactate structure, with a 5-ring chelate involving the apical 3-hydroxy group and the α-carboxylate.
Analysis of Eu and Yb emission CD spectra and lifetimes (H2O and D2O) for each ternary complex, in
conjunction with 1H NMR analyses of Eu/Yb systems and 17O NMR and relaxometric studies of the Gd
analogues, suggests that carbonate, oxalate, and malonate each form a chelated (q = 0) square-antiprismatic complex in which the dipolar NMR paramagnetic shift (Yb, Eu) and the emission circular
polarization (gem for Eu) are primarily determined by the polarizability of the axial ligand. The ternary
complexes with hydrogen phosphate, with fluoride, and with Phe, His, and Ser at pH 6 are suggested to
be monoaqua systems with Eu/Gd with an apical bound water molecule. However, for the ternary complexes
of simple amino acids with [YbL1]3+, the enhanced charge demand favors a chelate structure with the
amine N in an apical position. Crystal structures of the Gly and Ser adducts confirm this. In peptides and
proteins (e.g. albumin) containing Glu or Asp residues, the more basic side chain carboxylate may chelate
to the Ln ion, displacing both waters.