10.1021/ja0607133.s001 Rebecca A. Jockusch Rebecca A. Jockusch Francis O. Talbot Francis O. Talbot Paul S. Rogers Paul S. Rogers Michela I. Simone Michela I. Simone George W. J. Fleet George W. J. Fleet John P. Simons John P. Simons Carbohydrate Amino Acids:  The Intrinsic Conformational Preference for a β-Turn-Type Structure in a Carbopeptoid Building Block American Chemical Society 2006 conformer alternative structures carbopeptoid units form C 10 sugar ring NH DFT ab initio calculations carbopeptoid dimer unit Intrinsic Conformational Preference amide preference gas phase 2006-12-27 00:00:00 Journal contribution https://acs.figshare.com/articles/journal_contribution/Carbohydrate_Amino_Acids_The_Intrinsic_Conformational_Preference_for_a_Turn_Type_Structure_in_a_Carbopeptoid_Building_Block/3037561 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 <i>cis</i>-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, C<sub>10</sub> turn. Despite the clear preference for C<sub>10</sub> 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.