Molecular Recognition of Aqueous Dipeptides by Noncovalently Aligned Oligoglycine Units at the Air/Water Interface

Binding of aqueous dipeptides, GlyX and X’Gly (X = Leu, Phe, Pro, and Ala; X’ = Leu and Phe), onto monolayers of dialkyl oligoglycyl amphiphiles has been investigated by π-A isotherm measurement, FT-IR spectroscopy, and XPS elemental analysis. Dipeptides with the N-terminal glycine residue (GlyX) were selectively bound onto monolayers of an amphiphile in which dioctadecylamine moiety was connected with the glycylglycinamide head group via the terephthaloyl unit. When the dipeptide (GlyLeu) concentration in the subphase was varied, the Langmuir-type saturation behavior was observed with the equimolar binding and the binding constant of 35 M^–1. The binding efficiency increased in the order of GlyPhe > GlyLeu > GlyPro > GlyAla, implying that the binding is promoted by hydrophobic interaction. The binding was not detected when either of the terephthaloyl and glycylglycinamide units were absent in the monolayer component. Dipeptides with the C-terminal glycine residue (X′Gly) were not bound at all. These results are satisfied by a molecular model in which guest peptides were inserted into the monolayer from the C-terminal. The observed binding selectivity is explained by hydrophobic interaction between the side chain of the C-terminal residue and the hydrophobic cavity in the monolayer and formation of stable antiparallel hydrogen bonding between guest dipeptides and host diglycine chains. The formation of a specific binding site by noncovalent self-assembly leads to a new thinking in the monolayer research.