Amine binding to water-soluble zinc porphyrins bearing a hydrophobic binding pocket

The binding of several amine ligands to water-soluble zinc porphyrins 3, 4, and 5 bearing a hydrophobic binding pocket was examined spectrophotometrically in water and in chloroform. In chloroform, substantially decreased binding constants (K) of these porphyrins compared to available data for synthetic zinc porphyrins were observed and this was ascribed to the tightly bound water molecule that must be released upon amine binding. In aqueous solution, the large K values of 3 among these complexes showed that a preorganized structure of the binding pocket is necessary for binding enhancements of the amine ligands. The positive entropy changes in aqueous solution were found to contribute largely to the amine binding to 3 and 4. These results suggested that hydrophobic interactions would dominantly affect the binding behaviour of these zinc porphyrins and apparently eclipse the direct non-polar attractions between the bound amines and the superstructures of the porphyrins.