Multisite recognition of aqueous dipeptides by oligoglycine arrays mixed with guanidinium and other receptor units at the air-water interface

Equimolar mixed monolayers of dioctadecyl glycylglycinamide amphiphile (2C₁₈BGly₂NH₂) with other functional amphiphiles beating guanidinium, pyridine, and alcoholic OH groups were prepared on water, and the binding of aqueous dipeptides to these monolayers was investigated by π-Aisotherm measurement, FT-IR spectroscopy, and XPS elemental analysis. The binding behavior of GlyLeu to the mixed monolayer of 2C₁₈BGly₂NH₂ and guanidinium amphiphile (2C₁₈BGua) was analyzed by a Langmuir isotherm to give a saturation guest/amphiphile ratio (α) of 0.46 and a binding constant (K) of 6400 M^-1. The former value indicates that the binding site for one GlyLeu molecule was formed cooperatively by the two monolayer components. The binding constant is much enhanced relative to those observed for the 2C₁₈BGly₂NH₂ single-component monolayer (35 M^-1) and an equimolar mixed monolayer of 2C₁₈BGly₂NH₂ and benzoic acid amphiphile (2C₁₈BCOOH) (475 M^-1). When the second amphiphile was replaced with a pyridine amphiphile (2C₁₈Py) or with an alcohol amphiphile (2C₁₈OH), binding constants for GlyLeu were lowered to 124 and 43 M^-1, respectively. The enhanced binding in the former is attributed to strong guanidinium-carboxylate interaction upon C-terminal guest insertion and stable antiparallel hydrogen bonding among peptide chains. The binding of a second dipeptide, LeuGly, to the mixed monolayer of 2C₁₈BGly₂NH₂/2C₁₈BGua gave a K value (2170 M^-1) that is only one-third of that of GlyLeu. The difference is apparently related to the disposition of the hydrophobic side chain of the Leu residue in the C-terminal insertion. Thus, size matching of side chains of amino acid residues in host and guest determines selectivity of binding. Guest dipeptides are bound to the host most efficiently when the separation of host peptide chains is suited for the formation of strong hydrogen bonds between host and guest.