Dispersive interactions in supramolecular porphyrin complexes

Lipophilic interactions with several water-soluble synthetic porphyrins are reviewed, including the design of new artificial receptors for peptides and nucleotides. The binding free energies ΔG with a large number of substrates RX with substituents X show, that hydrophobic contributions found with saturated frameworks are negligible compared to dispersive forces; stacking with aromatic compounds is essentially a function of the number of participating π-electrons. The dispersive forces become apparent in the strong ΔG increase from X=F to Cl to Br to I, and in the high ΔG found also e.g. for X=NO2. Even amides form relatively strong complexes, showing additive behavior with some oligopeptides. Additivity of rather constant ΔΔG values for many substituents is generally observed and will allow the construction of a parameter scale for dispersive interactions. With charged substrates the ion pair contribution at the applied medium ionic strength is invariably about 5 kJ/mol. The presence of anions at usual buffer concentrations diminish in particular the stacking/dispersive interactions.