Cation binding cyclic peptides composed of imino acid residues

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Abstract
Cyclo(L‐Pro‐Sar)n (n = 2–4) with moderate flexibility and hydrophobicity of molecular structure was synthesized, and the characteristics of these cyclic peptides and their metal complexes in acetonitrile were investigated in connection with the residual properties using 13C‐nmr measurements. The cyclic tetrapeptide cyclo(L‐Pro‐Sar)2 showed a sterically hindered phenomenon in acetonitrile in which the amide backbone adopted a cis‐trans‐cis‐trans sequence. The cyclic hexapeptide cyclo(L‐Pro‐Sar)3 existed as a mixture of several conformers whose interconversion is slow on the nmr time scale, including cis‐cis‐trans and/or cis‐trans‐trans arrangement of the Sar‐Pro bond. Finally, it was demonstrated that the cyclic octapeptide cyclo(L‐Pro‐Sar)4 behaved as a mixture of multiple conformers which allowed for cis‐trans isomerism about the Pro‐Sar peptide bond, of which 20–30% had the all‐cis Sar‐Pro bond isomer and the remaining 70–80% had one (or more) cis Sar‐Pro bond isomer. 13C‐nmr spectra also demonstrated that cyclo(L‐Pro‐Sar)n (n = 3,4) formed a 1:1 ion complex whose conformation was characterized by an all‐trans peptide bond in the presence of excess metal salt. Cation binding studies, using CD measurements, established that the ion selectivity of cyclo(L‐Pro‐Sar)4 in acetonitrile decreased in the order, Ba2+ > Ca2+ > Na+ > Mg2+ > Li+.