A straightforward and versatile method for the determination of the absolute configuration of amino acids is presented. The proposed method involves the in situ formation of chiral complexes of optically active amino acids with the achiral dimolybdenum tetraacetate [Mo2(OAc)4] acting as an auxiliary chromophore. Optically active amino acids in reaction with [Mo2(OAc)4] in DMSO exchange in situ its acetate ligand to form chiral Mo-complexes. These complexes display several CD bands in the spectral range from 600 to 250 nm. The resulting CD spectra are suitable for the assignment of absolute configuration, since the observed sign of Cotton effects arising within the d-d absorption bands of the metal core depends solely upon the chirality of the amino acid ligands. It is shown that two Cotton effects at around 300 nm and 400 nm are especially useful for the correlation between their sign and the stereostructure of amino acids by means of the hexadecant rule. The present study demonstrates that the hexadecant rule can be extended to differently N-protected amino acids. The dimolybdenum method can also be applied for determination of the absolute configuration of cyclic β9-amino acids representing precursors of peptidomimetics.
Keywords: Circular dichroism, dimolybdenum tetraacetate, α-amino acids, β-amino acids, chiral Mo-complexes
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