H N.M.R. study of the conformation of [Glu4] oxytocin and its lanthanide complexes in aqueous solution

At 400 MHz the 1H chemical shifts, peptide NH-C alpha H coupling constants (JN alpha) and peptide hydrogen exchange rates of [Glu4] oxytocin in aqueous solution closely resemble those previously reported for oxytocin under comparable conditions, indicating that both the parent hormone and its analogue have similar conformations in this solvent. The hydrogen exchange data suggest a dynamic equilibrium between conformation(s) in which the peptide NH's of Asn5 and Cys6 are internally hydrogen bonded and conformation(s) in which these hydrogens are bonded to the solvent. [Glu4] oxytocin forms 1:1 complexes with lanthanide metal ions. The diamagnetic La3+ complex exhibits values of JN alpha very similar to those of the metal free hormone analogue, suggesting that coordination of the metal is accompanied by minimal perturbation of the peptide backbone. Specific average proton-metal distances estimated from Gd3+ induced paramagnetic relaxation effects indicate that the metal is probably coordinated to the Glu4 carboxyl group and the sidechain carbonyl of Asn5. Limiting shifts induced by binding of paramagnetic Yb3+ are also reported.