Temperature-dependent NMR and CD spectra of beta-peptides: On the thermal stability of beta-peptide helices - Is the folding process of beta-peptidesnon-cooperative?

Temperature-dependent NMR and CD spectra of methanol solutions of a beta-he xapeptide and of a beta-heptapeptide at temperatures between 298 and 393 K are reported. They establish the fact that the 3(14)-helical secondary stru ctures of the two beta-peptides, 1 and 2, do not 'melt' in the temperature range investigated. This is in sharp contrast to the behavior of the helice s of a-peptides and proteins which undergo cooperative unfolding ('denaturi ng') upon heating. A non-cooperative mechanism is proposed, with a stepwise , rather than an 'unzipping' opening of H-bonded rings (cf. Fig. 6). The ex perimental results are regarded as evidence that, of the three effects whic h have been identified as contributing to the stability of beta-peptide hel ices, ie., H-bonding, hydrophobic interactions, and ethane staggering, the latter one is predominant.