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roughly constant above 1 (Fig. 5,b). Similar behavior is seen when the shift of the
wavelength of the maximum of each curve is plotted vs. the number of equiv. of Zn2: a
shift of ca. 10 nm occurs between 0 and 1.0 equiv., and no more change above 1 equiv. of
Zn2. According to the molar-ratio method [32], these two plots are evidence for the
existence of a 1:1 complex between the b-peptide 2 and the Zn2 ion.
We have prepared for the first time b-peptides containing b3hCys and b3hHis
building blocks. The CD spectrum of the b-peptide 3 can be attributed to a 314-helical
conformation, both in MeOH and in H2O solution, and the pH dependence of the CD
spectrum is compatible with salt-bridge stabilization; also, Zn2 complexation appears
to stabilize this helix. We cannot draw any conclusions about the nature of the
prevailing secondary structure(s) of 2 from the CD measurements. It is clear that a
conformational change occurs upon complexation with Zn2, and that a 1:1 complex is
formed.
It is important to point out that CD spectra can give hints about the secondary
structures that these b-peptides may predominantly have in solution [17][27][31][33];
only after the detailed structure analysis by the high-resolution techniques that NMR
spectroscopy5) or X-ray diffraction can provide will it be possible to confirm or dismiss
the conclusions we have drawn from CD measurements [34] about the predominant
conformations of the b-peptides 2 and 3.
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