Stereochemistry of Peptides Containing 1-Aminocyclopentanecarboxylic Acid $({Acc}^5)$ : Solution and Solid-state Conformations of $Boc-{Acc}^5-{Acc}^5-NHMe$

The conformational analysis of a protected homodipeptide of 1-aminocyclopentanecarboxylic acid $({Acc}^5)$ has been carried out. 'H-nmr studies establish a \beta-turn conformation for $Boc-{Acc}^5-{Acc}^5-NHMe$ in chloroform and dimethylsulfoxide solutions involving the methylamide NH in an intramolecular hydrogen bond. Supportive evidence for the formation of an intramolecular hydrogen bond is obtained from ir studies. X-ray diffraction studies reveal a type III \beta-turn conformation in the solid state stabilized by a 4 \rightarrow 1 hydrogen bond between the Boc CO and methylamide NH groups. The \phi, \psi values for both ${Acc}^5$ residues are close to those expected for an ideal $3_{10}$-helical conformation (\phi = \pm 60 deg, \psi = \pm 30 deg).