Molecular recognition of aqueous dipeptides at multiple hydrogen-bonding sites of mixed peptide monolayers

Oligopeptide amphiphiles with different dipeptide moieties of -XYNH₂ (X = Gly and Ala, Y = GLy, Ala, Val, Leu, and Phe) were synthesized. Binding of aqueous dipeptides onto monolayers of equimolar mixtures of these amphiphiles with a benzoic acid amphiphile (2C₁₈BCOOH) was investigated by π-A isotherm measurement, FT-IR spectroscopy, and XPS elemental analysis. For given GIyX dipeptides (X = neutral and hydrophobic residues), the binding ratio was lessened with increasing sizes of the side chain of the Y residue in the GlyY dipeptide moiety of the host amphiphiles. The Langmuir-type saturation behavior was observed for binding of GlyLeu to an equimolar monolayer of 2C₁₈BGly₂NH₂ and 2C₁₈BCOOH. Its binding constant of 475 M^-1 was 10 times larger than that observed for a single-component monolayer of 2C₁₈BGly₂NH₂ (K = 35 M^-1). The saturation guest/host ratio was 0.47. The mode of substrate insertion into the binding site was examined by FT-IR spectroscopy. When the hydrophobic residue was on the C-terminal of a guest dipeptide (GlyX), the C-terminal insertion was selected with accompanying formation of cyclic carboxylic acid dimers at the interface. In the case of XGly guests, the N-terminal insertion with salt bridge formation with the host was observed. When the two residues of a dipeptide had close hydrophobicities, both C- and N-terminal insertions were observed. Formation of these binding sites is apparently induced by dipeptide binding.