Does a peptide bound to a monoclonal antibody always adopt a unique conformation?

The conformation of a synthetic undecapeptide derived from the Escherichia coli tryptophan synthase beta2 subunit was studied by NMR spectroscopy when bound to a monoclonal antibody (mAb 164-2) Fab' fragment directed against the native protein. The peptide 1(H-G-R-V-G-I-Y-F-G-M-K)11, peptide 11, was recognized by the antibody and its corresponding Fab' fragments with high affinity (K(D) = 1.1+/-0.2* 10(-8) M). Peptide 11 was labelled with 15N and its structure at the binding site of the Fab' 164-2 fragment was studied by isotope-editing techniques. 1H-15N heteronuclear spectra indicated the presence of two Fab'-peptide 11 complexes with two different conformations in slow chemical exchange on the chemical shift time scale.