Molecular Recognition at the Air-Water Interface. Specific Binding of Nitrogen Aromatics and Amino Acids by Monolayers of Long-Chain Derivatives of Kemp’s Acid

Long-chain derivatives of Kemp’s acid formed stable monolayers at the air-water interface, where the carboxylic acid groups produced the cyclic dimer species and served as a molecular cleft for specific binding of nitrogen aromatics and amino acids. The structure of the long-chain substituents was crucial for forming the cyclic dimer. Combinations of FT-IR, XPS, and UV spectroscopies of LB films and surface pressure-area isotherms revealed that substrates of complementary shape and functionality bound to the cleft mainly by hydrogen bonding. Phthalazine formed the 1:2 (substrate/amphiphile) complex, and enhanced binding of phthalazine (binding constant, 30 M⁻¹) compared to that of quinazoline, quinoxaline, and pyridazine was ascribable to the proper location of nitrogen atoms within the molecule as well as smaller solubility in water. A more basic substrate, benzimidazole, bound to the monolayer 5 times more strongly probably in a form of the 1:1 complex. It is remarkable that significant substrate binding was attained even when the host monolayer and the substrates remained in exposure to the aqueous microenvironment. The monolayer of octadecanoic acid was not an effective receptor, implying that the convergent carboxylic acids were the intrinsic element of the molecular recognition.