Chiral Selectivity in the Achiral Amino Acid Glycine.

A Cambridge Structural Database (CSD) search for the zwitterion of glycine, the only natural amino acid which does not have a chiral center at Cα, and its derivatives H3N-CαH2-C'(Ocis)Otrans(H/R/M) gave 425 hits, 420 of which are chiral because of the ψ conformation with regard to the central C'-Cα bond (ψ = OcisC'CαN ≠ 0 or 180°). Chiral ψ conformations distort the planar carboxylic group CαC'(Ocis)Otrans group stereoselectively to an asymmetric flat tetrahedron with four different corners, specified by (Rθ)/(Sθ). Negative rotation angles ψ predominantly induce pyramidalization angles θ = OtransC'CαOcis below -180°, resulting in a high diastereoselectivity for (Mψ,Sθ)-glycine. Positive rotation angles ψ preferentially give (Pψ,Rθ)-glycine. Density functional theory calculations confirm the results of the data analysis, indicating an increase of pyramidalization θ for increasing rotation angles ψ. This is corroborated by the analysis of section averages of the structural data, showing an increase of the ψ/θ selectivity in the series 1.5, 2.8, 3.5, and 6.0 for increasing rotation angles ψ. Without any chiral bias, 419 glycine derivatives crystallize in the ratio racemate/conglomerate 359:65.