Uncharged water-soluble porphyrin tweezers as a supramolecular sensor forα-amino acids

Abstract

The binding between uncharged cobalt porphyrin tweezers and L-amino acids in aqueoussolutions is studied by means of UV–vis and circular dichroism spectroscopy.By varying the length of the aliphatic bridge between the two porphyrin units,the number of cobalt ions in the porphyrin cores and the pH of the solution,the chirality induction phenomenon has been investigated. The binding of theamino acid to the porphyrin seems to occur via a coordination mechanism betweenthe metal and the nitrogen of the amino group; the steric, hydrophobic andπ–π interactions operate to stabilize the complexes. The chirogenesis displays an oppositebehaviour in the presence of aromatic guests with respect to the non-aromatic ones.Moreover, the UV–vis and the induced circular dichroism spectral changes suggest that theamino acid arrangement in the tweezers is determined by many factors, so that, unlike inorganic solvent, the porphyrin tweezers in aqueous solution allow for two differentarrangements of the same aromatic amino acid. The experimental findings indicate that theporphyrins tweezers reported in the paper are promising in opening perspectives towardtheir application as a selective molecular sensor in aqueous solutions directly.