6533b874fe1ef96bd12d61d8

RESEARCH PRODUCT

Conformational investigation of α,β-dehydropeptides. X. Molecular and crystal structure of Ac-ΔAla-NMe2 compared with those of Ac-L-Ala-NMe2, Ac-DL-Ala-NMe2 and other dimethylamides

subject

description

A series of three homologous dimethyldiamides Ac-DeltaAla-NMe2, Ac-L-Ala-NMe2 and Ac-DL-Ala-NMe2 has been synthesized and the structures of these amides determined from single-crystal X-ray diffraction data. To learn more about the conformational preferences of compounds studied, the fully relaxed (phi-psi) conformational energy maps in vacuo (AM1) of Ac-DeltaAla-NMe2 and Ac-L-Ala-NMe2 were obtained, and the calculated minima reoptimized with the DFT/B3LYP/6-31G** method. The crystal-state results have been compared with the literature data. Ac-DeltaAla-NMe2 and other alpha,beta-dehydroamino acid dimethyldiamides, Ac-DeltaXaa-NMe2 adopt the conservative conformation of the torsion angles phi, psi = approximately -45 degrees, approximately 130 degrees, which are located in the high-energy region (region H) of Ramachandran diagram. Ac-L-Ala-NMe2 and Ac-DL-Ala-NMe2, as well as other saturated amino acid dimethylamides Ac-L/DL-Xaa-NMe2, present common peptide structures, and no conformational preferences are observed. Molecular packing of the amides analysed reveals two general hydrogen-bonded motifs. Dehydro and DL-species are paired into centrosymmetric dimers, and L-compounds form catemers. However, Ac-DeltaAla-NMe2 and Ac-DL-Ala-NMe2 constitute exceptions: their molecules also link into catemers.