Oligomers of beta(2)- and of beta(3)-homoproline: What are the secondary structures of beta-peptides lacking H-bonds?

To study the role of H-bonds in stabilizing beta-peptidic secondary structu res, we have synthesized beta-oligopeptides (up to the octadecamer 12) cons isting beta(2)- and beta(3)-homoproline, i.e., beta-peptides lacking amide protons. The enantiomer purity of the building block beta(2)-homoproline (n ipecotic acid, 4) was determined by HPLC analysis of the N-(2,4-dinitrophen yl) derivative 5 on a Chiralcel-OD column (cf: Fig. 2). The CD spectra of t he all-(S)-beta(2)- and all-(S)-beta(3)-HPro-containing beta-peptides displ ay novel and intensive CD patterns which may be indicative of a secondary s tructure (cf: Fig. 3). It is noteworthy that a distinct CD pattern was obse rved with the beta(3)-HPro derivatives containing as few as three residues (7a). The crystal structure of a N-deprotected beta(3)-HPro-tripeptide 7c i s presented (cf. Figs. 4 and 5), and a model for the structure of beta-pept ides consisting of beta(3)-HPro is discussed (cf: Figs. 6 and 7).