The correlation between
β2‐,
β3‐, and
β2,3‐amino acid‐residue configuration and stability of helix and hairpin‐turn secondary structures of peptides consisting of homologated proteinogenic amino acids is analyzed (
Figs. 1–3). To test the power of Zn
2+ ions in fortifying and/or enforcing secondary structures of
β‐peptides, a
β‐decapeptide, 1 , four
β‐octapeptides, 2 – 5 , and a
β‐hexadecapeptide, 10 , have been devised and synthesized. The design was such that the peptides would
a) fold to a
14‐helix ( 1 and 3 ) or a hairpin turn ( 2 and 4 ), or form neither of these two secondary structures (
i.e., 5 ), and
b) carry the side chains of cysteine and histidine in positions, which will allow Zn
2+ ions to use their extraordinary affinity for RS
? and the imidazole N‐atoms for stabilizing or destabilizing the intrinsic secondary structures of the peptides. The
β‐hexadecapeptide 10 was designed to
a) fold to a turn, to which a
14‐helical structure is attached through a
β‐dipeptide spacer, and
b) contain two cysteine and two histidine side chains for Zn complexation, in order to possibly mimic a Zn‐finger motif. While CD spectra (
Figs. 6–8 and
17) and ESI mass spectra (
Figs. 9 and
18) are compatible with the expected effects of Zn
2+ ions in all cases, it was shown by detailed NMR analyses of three of the peptides,
i.e., 2, 3, 5 , in the absence and presence of ZnCl
2, that
i)
β‐peptide 2 forms a hairpin turn in H
2O, even without Zn complexation to the terminal
β3hHis and
β3hCys side chains (
Fig. 11),
ii)
β‐peptide 3 , which is present as a
14‐helix in MeOH, is forced to a hairpin‐turn structure by Zn complexation in H
2O (
Fig. 12), and
iii)
β‐peptide 5 is poorly ordered in CD
3OH (
Fig. 13) and in H
2O (
Fig. 14), with far‐remote
β3hCys and
β3hHis residues, and has a distorted turn structure in the presence of Zn
2+ ions in H
2O, with proximate terminal Cys and His side chains (
Fig. 15).
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