γ-Peptides Forming More Stable Secondary Structures than α-Peptides: Synthesis and Helical NMR-Solution Structure of the γ-Hexapeptide Analog of H-(Val-Ala-Leu)2-OH

Article abstract of DOI:10.1002/hlca.19980810514

For a comparison with the corresponding α- and β-hexapeptides H-(Val-Ala-Leu)₂-OH (A) and H-(β-HVal-β-HAla-β-HLeu)₂-OH (B), we have now prepared the corresponding γ-hexapeptide 1 built from the homochirally similar (S)-4-aminobutanoic acid, (R)-4-amino-5-methylhexanoic acid, and (R)-4-amino-6-methylheptanoic acid. The precursors were prepared either by double Arndt-Eistert homologation of the protected amino acids Boc-Val-OH, Boc-Ala-OH, and Boc-Leu-OH (Schemes 1 and 2), or by the superior route involving olefination/hydrogenation of the corresponding aldehydes (Boc-valinal, Boc-alaninal, and Boc-leucinal; Scheme 3). Conventional peptide-coupling methodology (EDC/HOBt) furnished the γ-hexapeptide 1 (through the intermediate γ-di- and γ-tripeptide derivatives 9-11). Analysis of NMR measurements in (D₅)pyridine and CD₃OH solution (COSY, TOCSY, HSQC, HMBC, ROESY) reveals that the γ-hexapeptide 1 adopts a right-handed helical structure ((P)-2.6₁ helix of ca. 5-A pitch, containing 14-membered H-bonded rings) which is to be compared with the left-handed helix of the corresponding β-peptide B ((M)-3₁ helix of 5-A pitch, 14-membered H-bonded rings) and with the familiar right-handed, so-called α-helix of α-peptides ((P)-3.6₁ helix of 5.4-A pitch, 13-membered rings). Like the helix sense, the helix dipole reverses when going from α- (N C) to β- (C N) to γ-peptides (N C). The surprising difference between the natural α-, and the analogous β- and γ-peptides is that the helix stability increases upon homologation of the residues.