Temperature-dependent NMR and CD spectra of β-peptides. On the thermal stability of β-peptide helixes. Is the folding process of β-peptides non-cooperative?
Temp.-dependent NMR and CD spectra of MeOH solns. of a beta-hexapeptide and of a beta-heptapeptide between 298 and 393 K are reported. They establish the fact that the 314-helical secondary structures of the 2 beta-peptides do not "melt" in the temp. range investigated. This is in sharp contrast to the behavior of the helixes of alpha-peptides and proteins which undergo cooperative unfolding (denaturing) upon heating. A non-cooperative mechanism is proposed, with a stepwise, rather than an unzipping opening of H-bonded rings. The exptl. results are regarded as evidence that, of the 3 effects which were identified as contributing to the stability of beta-peptide helixes, i.e., H-bonding, hydrophobic interactions, and ethane staggering, the latter one is predominant.