β‐Peptides: Synthesis by Arndt‐Eistert homologation with concomitant peptide coupling. Structure determination by NMR and CD spectroscopy and by X‐ray crystallography. Helical secondary structure of a β‐hexapeptide in solution and its stability towards pepsin

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Abstract
The β‐hexapeptide (H‐β‐HVal‐β‐HAla‐β‐HLeu)2‐OH (2) was prepared from the component L‐β‐amino acids by conventional peptide synthesis, including fragment coupling. A cyclo‐β‐tri‐ and a cyclo‐β‐hexapeptide were also prepared. The β‐amino acids were obtained from α‐amino acids by Arndt‐Eistert homologation. All reactions leading to the β‐peptides occur smoothly and in high yields. The β‐peptides were characterized by their CD and NMR spectra (COSY, ROESY, TOCSY, and NOE‐restricted modelling), and by an X‐ray crystal‐structure analysis. β‐Sheet‐type structures (in the solid state) and a compact, left‐handed or (M) 31 helix of 5‐Å pitch (in solution) were discovered. Comparison with the analogous secondary structures of α‐peptides shows fundamental differences, the most surprising one at this point being the greater stability of β‐peptide helices. There are structural relationships of β‐peptides with oligomers of β‐hydroxyalkanoic acids, and dissimilarities between the two classes of compounds are a demonstration of the power of H‐bonding. The β‐hexapeptide 2 is stable to cleavage by pepsin at pH 2 in H2O for at least 60 h at 37°, while the corresponding α‐peptide H‐(Val‐Ala‐Leu)2‐OH is cleaved instantaneously under these conditions. The implication of the described results are discussed.