Molecular Recognition of Aqueous Dipeptides at Multiple Hydrogen-Bonding Sites of Mixed Peptide Monolayers

Oligopeptide amphiphiles with different dipeptide moieties of −XYNH 2 (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 (2C 18 BCOOH) was investigated by π− A isotherm measurement, FT-IR spectroscopy, and XPS elemental analysis. For given GlyX 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 2C 18 BGly 2 NH 2 and 2C 18 BCOOH. Its binding constant of 475 M - 1 was 10 times larger than that observed for a single-component monolayer of 2C 18 BGly 2 NH 2 ( 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.