Synthesis of an ion-binding peptide (DECYL-2) more selective for calcium than ionophore A23187.

The synthesis of the cation-binding cyclic octapeptide, cyclo(Glu-Sar-Gly-(N-decyl)Gly)₂ is reported. This peptide, containing two ionizable Glu carboxyl side chain protons per molecule, can form neutral cation complexes with divalent ions via protonmetal exchange. Solubilized in chloroform solution, the peptide has been found to extract calcium from an aqueous phase (pH 8.5, 100 mM Tris) generally on a $\text{1}\text{1}$ molar basis. By contrast, under comparable conditions but with other metal chlorides, the peptide does not extract magnesium, sodium, or potassium. That the extraction proceeds via proton-metal exchange was demonstrated by the absence of (radioactive) chloride ion from the organic phase. Parallel sets of experiments performed with the naturally-occurring ionophore A23187 reaffirmed that the latter substance extracts calcium and magnesium with nearly equal propensity.