Metallation of protoporphyrins used as fluorescent chemo sensor for imidazole recognition

Metalloporphyrin Complexes play significant roles in many biological and catalytic systems. The diversity of their functions is due in part to the variety of metals that bind in the pocket of the porphyrin ring system. Two kinds of metalloporphyrin derivatives, Cu (II) and Zn (II) protoporphyrins (PP) were microscale synthesized and characterized by spectroscopic methods and magnetic measurements. A PP ligand bound to each metal center in a tetradentate fashion including four amine nitrogen atoms in the equatorial planes. These complexes were found to recognize imidazolyl groups of histidine and histamine derivatives as guest molecules by coordination and additional non-covalent interactions. These added analytes displace the selective fluorescent indicator, which when released to the solution displays its full fluorescence. Thus, analyte recognition is signaled by the sharp appearance of the fluorescence of the indicators. The binding affinities to histidine and histamine were investigated and accounted for different complexation properties. Moreover, we demonstrated that careful choice of a fluorescent indicator with tuned affinity toward the receptor can provide discrimination in sensing of a desired substrate and the role that the metal coordination plays on the hypochromic shift and loss of fluorescence distincted characteristics of hypsoporphyrins were also discussed. (author)