Uncharged Water-soluble porphyrin Tweezers as a Supramolecular Sensor for a-Amino Acids

The binding between uncharged cobalt porphyrin tweezers and L-amino acids in aq. solns. is studied by means of UV-vis and CD spectroscopy. By varying the length of the aliph. bridge between the two porphyrin units, the no. of cobalt ions in the porphyrin cores and the pH of the soln., the chirality induction phenomenon has been investigated. The binding of the amino acid to the porphyrin seems to occur via a coordination mechanism between the metal and the nitrogen of the amino group; the steric, hydrophobic and À-À interactions operate to stabilize the complexes. The chirogenesis displays an opposite behavior in the presence of arom. guests with respect to the non-arom. ones. Moreover, the UV-vis and the induced CD spectral changes suggest that the amino acid arrangement in the tweezers is detd. by many factors, so that, unlike in org. solvent, the porphyrin tweezers in aq. soln. allow for two different arrangements of the same arom. amino acid. The exptl. findings indicate that the porphyrins tweezers reported in the paper are promising in opening perspectives toward their application as a selective mol. sensor in aq. solns. directly.