Induced circular dichroism active complexes of synthetic gadolinium(III) porphyrinates with chiral amino acids

Synthetic gadolinium(III) meso-tetrakis(3,5-di-tert-butylphenyl)porphyrinates extracted zwitterionic l-phenylglycine from an aqueous solution to give a 1:1 complex in benzene through synergistic binding of both amino and carboxylic groups of the amino acids. The resulting highly coordinated complex afforded intense circular dichroism (CD) bands of reversed S-shape type at the Soret region (400–450 nm). In the case of d-phenylglycine, a complete mirror image of the CD spectrum was recorded after extraction. While the intensity of the induced CD peak was affected by the nature of the organic solvent, meso-aryl substituent and Cα-residue, its sign was well related to the stereochemistry of the bound amino acid. A combination of biphasic extraction and CD spectrometry provides a promising method for determination of the absolute configuration of unprotected α-amino acids.