Chiral Space Formed by (+)‐(1S)‐1,1′‐Binaphthalene‐2,2′‐diyl Phosphate: Recognition of Aliphatic L‐α‐Amino Acids

Abstract

(+)-(1S)-1,1′-Binaphthalene-2,2′-diyl hydrogen phosphate (bnppa) is one of the useful optical selectors. To disclose the molecular mechanism by which bnppa recognizes aliphatic L-α-amino acids and separates them by fractional crystallization, X-ray analyses of bnppa and of its salts with L-alanine, L-valine, L-norvaline, and L-norleucine have been undertaken. All the amino acids adopt energetically favorable conformations in the crystal structures. The conformations and the packing patterns of bnppa in these crystal structures are very similar. The bnppa molecules are packed in a specific way to form hydrophobic and hydrophilic layers that are well separated. Between bnppa molecules, at the interface of these hydrophobic and hydrophilic layers, a space with chirality is formed. This space, designated as chiral space, recognizes the optically active amino acids. The packing of bnppa is mainly governed by intermolecular CH⋅⋅⋅π interactions between naphthalene moieties. The chiral space is responsible for the molecular recognition by bnppa allowing fractional crystallization of the L-α-amino acids.