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Chiral Selectivity in the Achiral Amino Acid Glycine

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posted on 12.12.2019, 13:33 by Henri Brunner, Takashi Tsuno, Gábor Balázs
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.