Lanthanide ethylenediaminetetra-acetate chelates as aqueous shift reagents: evidence for effective axial symmetry in bidentate cytidine 5′-monophosphate and alanine complexes

Abstract

Lanthanide-induced shift (l.i.s.) data are presented for cytidine 5′-monophosphate (cydmp) and L-alanine binding to a series of lanthanide–edta chelates at high pH (edta = ethylenediaminetetra-acetate). Slow exchange of these substrates between their bound and free environments precludes the use of the heavier Ln(edta) chelates at 25 °C. The cydmp proton l.i.s. ratios at 90 °C are independent of the Ln(edta) used, indicative of effective axial symmetry for this substrate. The alanine l.i.s. values contain large contact shift components which once removed yield corrected pseudo-contact shifts which also are adequately described by an effective axial symmetry model. The results indicate that chemically reasonable structures may be determined from l.i.s. data for substrates acting as bidentate chelators of the lanthanide, providing rapid exchange conditions are met and contact shift contributions are extracted from experimental l.i.s. values for nuclei near the co-ordination site(s). Therefore the fact that the substrate acts as a bidentate chelator does not necessarily exclude the possibility that the axially symmetric model for the l.i.s. is a satisfactory model.