Carbohydrate Amino Acids:  The Intrinsic Conformational Preference for a β-Turn-Type Structure in a Carbopeptoid Building Block

Infrared ion-dip spectroscopy coupled with DFT and ab initio calculations are used to establish the intrinsic conformational preference of the basic structural unit of a peptide mimic, a cis-tetrahydrofuran-based “carbopeptoid” (amide−sugar−amide), isolated at low temperature in the gas phase. The carbopeptoid units form a β-turn-type structure, stabilized by an intramolecular NH → OC hydrogen bond across the sugar ring, thus forming a 10-membered, C₁₀ turn. Despite the clear preference for C₁₀ β-turn structures in the basic unit, however, the presence of multiple hydrogen-bond donating and accepting groups also generates a rich conformational landscape, and alternative structures may be populated in related molecules. Calculations on an extended, carbopeptoid dimer unit, which includes an alternating amide−sugar−amide−sugar−amide chain, identify conformers exhibiting alternative hydrogen-bonding arrangements that are somewhat more stable than the lowest-energy double β-turn forming conformer.