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

摘要

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 beta-turn-type structure,stabilized by an intramolecular NH -> O=C hydrogen bond across the sugar ring,thus forming a 10-membered,C_(10) turn.Despite the clear preference for C_(10) beta-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 beta-turn forming conformer.