Conformational difference between diastereomers of Dnp-Val-Aib-Gly-Leu-pNA studied by x-ray crystal analyses

In order to investigate the conformational change of the alpha-aminoisobutyric acid (Aib) containing peptide by the D/L replacement of an amino acid residue, single crystals of two diastereomers, Dnp-L-Val-Aib-Gly-L-Leu-pNA (L-L isomer) and Dnp-D-Val-Aib-Gly-L-Leu-pNA (D-L isomer), were prepared from aqueous methanol solutions as CH3OH and CH3OH.H2O solvates, respectively, and were analyzed by the x-ray diffraction method. Molecular conformation of L-L isomer adopts consecutive two different types of beta-turns, a type II' beta-turn bent at Aib-Gly, and a type III beta-turn bent at Gly-Leu, stabilized by two intramolecular (Leu)NH...O = C(Val) and (pNA)NH...O = C(Aib) hydrogen bonds. In contrast, these two intramolecular hydrogen bonds lead the D-L isomer to a distorted 3(10)-helix conformation consisting of consecutive two type-III beta-turn of Aib-Gly-Leu sequence. The most significant structural difference between these diastereomers is the mutual orientation between the Dnp and pNA chromophores. While the extensive stacking of both the chromophores is intramolecularly formed for the folded conformation of L-L isomer, they are oriented toward an opposite direction in the open conformation of D-L isomer and are intermolecularly stacked with each other. The large separation between these diastereomers observed in the chromatography is discussed in the relation with their conformational differences.