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 (2C( 18)BGly(2)NH(2)) with other functional amphiphiles bearing guanidinium, pyr idine, and alcoholic OH groups were prepared on water, and the binding of a queous dipeptides to these monolayers was investigated by pi-A. isotherm me asurement, FT-IR spectroscopy, and XPS elemental analysis. The binding beha vior of GlyLeu to the mixed monolayer of 2C(18)BGly(2)NH(2) and guanidinium amphiphile (2C(18)BGua) was analyzed by a Langmuir isotherm to give a satu ration guest/amphiphile ratio (alpha) 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 bind ing constant is much enhanced relative to those observed for the 2C(18)BGly (2)NH(2) single-component monolayer (35 M-1) and an equimolar mixed monolay er of 2C(18)BGly(2)NH(2) and benzoic acid amphiphile (2C(18)BCOOH) (475 M-1 ). When the second amphiphile was replaced with a pyridine amphiphile (2C(1 8)Py) or with an alcohol amphiphile (2C(18)OH), binding constants for GlyLe u 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 pep tide chains. The binding of a second dipeptide, LeuGly, to the mixed monola yer of 2C(18)BGly(2)NH(2)/2C(18)BGua gave a K value (2170 M-1) that is only one-third of that of GlyLeu. The difference is apparently related to the d isposition of the hydrophobic side chain of the Leu residue in the C-termin al insertion. Thus, size matching of side chains of amino acid residues in host and guest determines selectivity of binding. Guest dipeptides are boun d to the host most efficiently when the separation of host peptide chains i s suited for the formation of strong hydrogen bonds between host and guest.