Conformations of beta-amino acid residues in peptides: X-ray diffraction studies of peptides containing the achiral residue 1-aminocyclohexaneacetic acid, beta(3,3)Ac(6)c

The conformational preferences of the 3,3-disubstituted I beta-amino acid residue, 1-aminoeyclohexaneacetie acid ((beta(3,3)Ac(6)c) have been investigated by determining the crystal structures of the parent amino acid, the, hydrochloride derivative, 10 protected derivatives and di and tripeptides. The symmetrical cyclohexyl substituent at the beta-position restricts the values of the torsion angles phi (N-C-beta) and theta (C-beta-C-alpha) to approximately gauche values (+/- 60 degrees). Relatively few intramolecularly hydrogen bonded conformations are observed. In the dipeptide Boc-beta(3,3)'Ac(6)c-beta(3,3)Ac(6)c-NHMe a C-6 hydrogen bond is observed. In Piv-Pro-beta(3,3)Ac(6)c-NHMe a C-11 hydrogen bonded hybrid alpha beta turn is characterized. In a majority of cases the amino group occupies the axial position in the cyclohexane ring. The conformations observed are compared with crystallographically observed structures for other beta-residues, including beta(2,2)Ac(6)c. (c) 2008 Wiley Periodicals, Inc.