Uncharged Water-Soluble Co(II)-Porphyrin: A Receptor for Aromatic alfa-Amino Acids

Changes in the UV-vis spectra and induced CD (ICD) signals obsd., in correspondence with the porphyrin Soret region, for aq. solns. of achiral 5,10,15,20-tetrakis{p-[?-methoxy poly(oxy-ethylene)]phenyl}porphyrin cobalt (II) (Co-P) and arom. a-L-amino acids (Trp and Phe) give direct evidence for the coordination between the Co-P and amino acids. Considering that Co-P, besides the Co atom (one-fixation-point system), does not contain in the mol. active ligand groups and that no ICD signals have been obsd. in the case of Co-P/Ala, it has been concluded that hydrophobic interactions or stacking interactions between the arom. rings of the porphyrin and those of Trp or Phe, acting as further amino acid (AA) fixation points, can strongly reduce the mobility of the chiral guest, thus permitting the generation of ICD signals. The effects of changes of both pH (in the range 2-9) and amino acid structure on the ICD phenomenon have also been investigated. In particular, the following have been obsd.: (i) strong ICD signals for all of the Co-P/N-acetyl amino acid aq. solns. at pH 7, (ii) an unexpected ICD band with a bisignate form for the Co-P/Ala soln. at pH 9 after long aging, and (iii) an opposite ICD signal when a-D-Phe and a-D-Trp enantiomers have been used. The data reported in this paper show how the binding mechanism between receptor and AAs changes by modulating properly the pH or the mol. structures and indicate that in these aq. solns. the coordination Co-N is not the fundamental mechanism giving rise to the formation of the complexes and that the binding can be driven by hydrophobic interactions. These occurrences, through the anal. of the spectroscopic response (and, in particular, the form of the ICD band), can allow the recognition of AAs.