# Conformations of \beta-Amino Acid Residues in Peptides: X-Ray Diffraction Studies of Peptides Containing the Achiral Residue 1-Aminocyclohexaneacetic Acid, $\beta^{3,3}Ac_6c$

Vasudev, Prema G and Rai, Rajkishor and Shamala, Narayanaswamy and Balaram, Padmanabhan (2008) Conformations of \beta-Amino Acid Residues in Peptides: X-Ray Diffraction Studies of Peptides Containing the Achiral Residue 1-Aminocyclohexaneacetic Acid, $\beta^{3,3}Ac_6c$. In: Peptide Science, 90 (2). pp. 138-150.

The conformational preferences of the 3,3-disubstituted \beta-amino acid residue, 1-aminocyclohexaneacetic acid $(\beta^{3,3}Ac_6c)$ 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 $(\pm60^o)$. Relatively few intramolecularly hydrogen bonded conformations are observed. In the dipeptide $Boc-\beta^{3,3}Ac_6c-\beta^{3,3}$ $Ac_6c-NHMe$ a $C_6$ hydrogen bond is observed. In Piv-Pro-$\beta^{3,3}Ac_6c-NHMe$ 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 $b^{2,2}Ac_6c$.