Assignment of aromatic resonances in the 1H nuclear magnetic resonance spectra of cardioactive polypeptides from sea anemones.

High-resolution 1H NMR spectroscopy at 300 MHz has been used to investigate the aromatic residues of a series of homologous polypeptides from sea anemones: anthopleurin-A from Anthopleura xanthogrammica and toxins I and II from Anemonia sulcata. Using two-dimensional NMR techniques, specific assignments to individual protons have been made for all aromatic resonances in the spectra of these molecules. In all three polypeptides the resonances from the two conserved Trp residues, 23 and 33, are shifted significantly from their random coil values, and the indole NH resonance of Trp-23 is not observed. These shift perturbations are due in part to a mutual interaction of the two indole rings, which is also indicated by the observation of nuclear Overhauser enhancements between protons of the two rings. Several other nonpolar side chains also interact with these two Trp residues, forming a hydrophobic region, the overall structure of which is conserved throughout the series. The other aromatic residues in these polypeptides appear not to participate in this structural region.