Conformational restriction through Cαi ←→ Cαi cyclization: 1-aminocycloheptane-1-carboxylic acid (Ac7c)
A complete series of N- and C-blocked, monodispersed homo-oligopeptides to the pentamer level from 1-aminocycloheptane-1-carboxylic acid (Ac7c), an α-amino acid conformationally restricted through Ciα ←→ Ciα cyclization, and three tripeptides with Ac7c combined with Ala, Leu, and Val residues have been synthesized by solution methods and fully characterized. The solution conformational preferences have been determined by IR absorption and 1H NMR spectroscopy. In addition, the molecular structures of three derivatives (Ac7c hydantoin, ClCH2CO-Ac7c-OH, and Z-Ac7c-OH; Z = benzyloxycarbonyl) and four peptides [the dipeptide Z-Ac7c-L-Ala-OMe, the tripeptides Z-Ac7c-(L-Ala)2-OMe and Z-(Ac7c)3-OBut, the tetrapeptide Z-(Ac7c)4-OBut, and the pentapeptide Z-(Ac7c)5-OBut] have been assessed in the crystal state by X-ray diffraction. The results obtained confirm the tentative conclusions put forward on the basis of our previous preliminary study, namely that β-bends and 310-helices are preferentially adopted by Ac7c-based peptides. A comparison with the structural tendencies extracted from published work on peptides from α-aminoisobutyric acid, the prototype of Cα,α-dialkylated glycines, and the other extensively investigated members of the class of 1-aminocycloalkane-1-carboxylic acids (Acnc, with n = 3–6, 8, 9) is made and the implications for the use of the Ac7c residue in conformationally constrained analogues of bioactive peptides are briefly discussed.