Natural abundance carbon-13 nuclear magnetic resonance study of anthopleurin-A, a cardiac stimulant from the sea anemone Anthopleura xanthogrammica.

Natural-abundance 13C NMR spectra (at 15.04 MHz) of the polypeptide cardiac stimulant Anthopleurin-A are presented. The spectra contain many resolved one- and two-carbon resonances from carbonyl and aromatic carbons and a few resolved resonances from aliphatic carbons. Most of these have been assigned to individual carbons in the protein. The effect of pH on the 13C spectrum has been investigated. In conjunction with the resonance assignments, this yields estimates for the pK alpha values of the COOH-terminal and NH2-terminal residues, the side chain carboxylate of 1 of the 2 aspartic acid residues, and the imidazolium groups of the 2 histidine residues. The effects of the lanthanides La3+ and Gd3+ on the spectrum have also been studied. The results suggest that there are at least two binding sites, and further studies will be required to characterize these before they can be utilized as an aid in structural mapping. Finally, the results are discussed in relation to a postulated model for the mode of action of Anthopleurin-A.