Molecular Recognition between 2,4,6-Triaminopyrimidine Lipid Monolayers and Complementary Barbituric Molecules at the Air/Water Interface:  Effects of Hydrophilic Spacer, Ionic Strength, and pH

Previously it was shown that molecular recognition occurs between complementary vesicles containing either a lipid with a 2,4,6-triaminopyrimidine (TAP) or with a barbituric acid (BAR) head group separated from the alkyl chains by an tetraoxyethylenic spacer. In view of better understanding the nature of the interaction, Langmuir monolayers and LB films of these complementary lipids are investigated. The selective binding of the TAP−Lipid with various barbituric acids is studied by π/ A isotherms, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy, and fluorescence microscopy. The effects of the subphase pH and ionic strength on this molecular recognition point out an electrostatic contribution due to the acido−basic properties of TAP and BAR in addition to an hydrogen bonding interaction. The comparison with a melamine compound (diC l2 mela) shows that this recognition is still efficient in the presence of the tetraoxyethylenic spacer.