Figure 2 - Structure elucidation of coelichelin.

(a) Assignment of the y ions (where the charge is retained in the COOH-terminal fragment of the peptide) observed in the product ion spectrum resulting from tandem mass spectrometry of (M + H)⁺ at m/z = 566 (a product ion with m/z = 250 is also observed corresponding to loss of both hfOrn residues). (b) Spin systems identified in Ga-coelichelin by TOCSY and COSY. (c) Correlations observed in the HMBC (solid lines) and ROESY (dashed lines) spectra of Ga-coelichelin that establish the sequence. Inter-residue CH-NH distance constraints calculated from the ROESY data are given in Å. The relative stereochemistry determined by modeling the Ga-coelichelin complex is also illustrated. (d) Calculated structure of the only diastereomer of Ga-coelichelin consistent with the inter-residue distances derived from the ROESY data and dihedral angles derived from the ¹H NMR spectrum. The inter-residue distances in the model corresponding to the experimental distances are illustrated in the same colors in c and d.