Bioinspired chemical inversion of L-amino acids to D-amino acids.

L-Amino acids dominate the natural world almost to the exclusion of D-amino acids. However, some important D-amino acids1 have been found in biology, notably as neurotransmitters (D-serine)2 and as building blocks (D-alanine) for bacterial cell wall synthesis.3 There has been considerable interest in understanding the mechanism of the so-called “L to D conversion” of amino acids4 since enzymes involved in these processes are often targeted for development of therapeutic agents (e.g., antibiotics and antidepressants). In nature, L-amino acids are converted to D-amino acids by pyridoxal phosphate dependent enzymes that racemize amino acids (Scheme 1a).4 The first step of the racemization reaction involves the formation of a special type of imines with internal resonanceassisted H-bonds (RAHB).5 We developed a chiral analogue (1) of pyridoxal that binds amino acids by formation of imines with RAHBs. The strong H-bonds can be used to activate the bound amino acids and convert L-amino acids (L-aa) to D-amino acids (Daa) by epimerization of the imines (1-L-aa) formed between 1 and L-aa (Scheme 1b). Receptor 1 was prepared as previously described.6 Stereoselective recognition of amino acids by 1 can be studied in two different ways. In the first procedure, at least 2-fold excess of a racemic mixture of amino acid is added to 1 and allowed to equilibrate by imine exchange.6 In the second procedure, 1 equiv of an amino acid (racemic or either enantiomer) is added to 1 and the mixture is allowed to equilibrate by triethylamine catalyzed epimerization (Scheme 1b). Without the base, epimerization is not significant even after weeks. Epimerization of the imines (1-L-aa) formed between 1 and L-amino acids (Scheme 1b) were monitored by 1H NMR. In a typical experiment, L-serine (10 mM) and 1 (10 mM) were mixed in a DMSOd6 solution to give the corresponding imine (1-L-Ser) within minutes. Triethylamine (50 mM) was then added to the reaction mixture and the epimerization reaction was monitored for 2 days. Figure 1 shows the time dependent change in the 1H NMR spectrum due to the epimerization of three imines ((a) 1-L-Ser to 1-D-Ser; (b) 1-L-Ala to 1-D-Ala, (c) 1-L-His to 1-D-His). In all cases (Figure 1), the signals due to the two urea N-H and the imine C-H can be conveniently monitored free from other signals. A clear pattern in the 1H NMR for the epimerization reaction emerges regardless of the amino acid used. The two urea N-H signals shift dramatically downfield and the imine C-H signal shifts upfield with the epimerization reaction. Figure 1 shows that the reactions are remarkably clean and complete for all amino acids. The epimerization reaction takes place by proton exchange at the R position of the amino acid as evidenced by deuteration at this position (solvent: 90/10 v/v DMSO-d6/D2O). (In principle, epimerization could also take place by rotation about the single bond connecting the two naphthyl groups of the receptor. This is unlikely since the energy barrier to such a rotation is expected to be high.7 Epimerization by bond rotation or imine exchange would not be accompanied by deuteration). The ratios of [1-D-aa] to [1-L-aa] at equilibrium (Scheme 1b) for thirteen different amino acids are listed in Table 1 (Figures S1S13, Supporting Information). It is clear from the table that the receptor binds all amino acids with the same sense of stereoselectivity. Although the sense of the receptor stereoselectivity for binding R-amino acids is the same as that for it binding 1,2-amino alcohols,6 the magnitude of the receptor stereoselectivity is considerably greater for binding R-amino acids (>10:1) than for binding 1,2-amino alcohols (∼4:1). The stronger H-bonding † Ewha Womans University. ‡ Sookmyung Women’s University. § University of Toronto. Figure 1. Partial 1H NMR spectra showing the epimerization of (a) 1-LSer to 1-D-Ser, (b) 1-L-Ala to 1-D-Ala, and (c) 1-L-His to 1-D-His.