Structural and NMR investigations of the ternary adducts of twenty α-amino acids and selected dipeptides with a chiral, diaqua–ytterbium complex

A detailed investigation of the nature of the binding of each of the 20 common α-amino acids and various selected dipeptides to a chiral, diaqua–ytterbium complex in aqueous solution has been carried out. Analysis of the dipolar ¹H NMR paramagnetic shifts suggests that the α-amino acids form a common chelated structure within a nine-coordinate mono-capped square antiprismatic coordination environment, with the amine N axially disposed. Crystal structures of nine chelated YbL¹–amino acid adducts (Gly, Ala, Ser, Thr, Met) confirm this. The ternary complexes with dipeptides (e.g. Gly-Ala, Gly-Ser, Gly-Met, Gly-Asp, Gly-Asn, Gly-His, Ser-Met, Asp-Phe, His-Gly) also favour the terminal amine as the axial donor with the proximate amide group binding to generate a five-ring chelate. Evidence for chelation through side-chain functionality was found only in the case of N-terminal Asp. The chiral environment about the ytterbium ion upon amino acid binding has also been probed using near-IR circular dichroism spectroscopy.