Effects of divalent metal cations on circular dichroism and 1H nuclear magnetic resonance spectra of linear and cyclic peptides having side-chain imidazolyl and acetamido groups.

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A linear peptide, N-t-butoxycarbonyl-S-acetamidomethyl-L-cysteinyl-D-leucyl-L-histidyl-S-acetamidomethyl-L-cysteinyl-D-leucyl-L-histidine methyl ester (1) was synthesized by the solid phase method. Subsequent cyclization by the azide method offered a cyclic peptide, cyclo-S-acetamidomethyl-L-cysteinyl-D-leucyl-L-histidyl-S-acetamidomethyl-L-cysteinyl-D-leucyl-L-histidyl (2). The circular dichroism (CD) and 1H NMR were observed for these peptides 1 and 2, in the absence and presence of metal cations, Zn(II), Ca(II), Mg(II). Spectral results for Zn(II) complexes indicated striking contrasts to those of Ca(II) and Mg(II) complexes. Thus, in H2O, the addition of Zn(II) caused CD spectral changes for both peptides, while Ca(II) and Mg(II) did not cause changes in any CD spectral region. In methanol, the additions of Zn(II) and Ca(II) caused CD changes, while the addition of Mg(II) did not cause any CD changes. The changes caused by adding Zn(II) were quite different from those by Ca(II), suggesting that Zn(II) is coordinated with the peptides in a mode different from that of Ca(II). 1H NMR data also suggested the different coordination mode between these two metal complexes in methanol.
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