Recognition of aqueous flavin mononucleotide on the surface of binary monolayers of guanidinium and melamine amphiphiles

Recognition of aqueous flavin mononucleotide (FMN) on the surface of binary monolayers of guanidinium amphiphiles (monoalkyl derivative, C₁₈Gua, or dialkyl derivative, 2C₁₈Gua) and the melamine amphiphile (2C₁₈mela-NN) has been investigated by π-A isotherms, FTIR spectroscopy, and XPS measurements. π-A Isotherms and FTIR spectra of C₁₈Gua-2C₁₈mela-NN (1:1) monolayers show that there is no direct hydrogen bonding and/or coulombic interactions between C₁₈Gua and 2C₁₈mela-NN on pure water and that the C₁₈Gua component is dissolved into the subphase upon compression. In contrast, the presence of aqueous FMN prevented C₁₈Gua molecules from dissolving into the subphase. Maximum condensation was observed at a 1:1 ratio in C₁₈Gua-2C₁₈mela-NN mixed monolayers on aqueous FMN. XPS analyses revealed that one FMN molecule was bound to one C₁₈Gua-2C₁₈mela-NN (1:1) unit and binding was saturated at 5 × 10^-6 mol dm^-3FMN. Peak shifts observed in FTIR spectra indicated that the isoalloxiazine ring in FMN formed hydrogen bonds with 2C₁₈mela-NN. These results support a model that the isoalloxazine and phosphate functions in FMN are bound via hydrogen bonding to melamine in 2C₁₈mela-NN and guanidinium in C₁₈Gua, respectively. Similar binding behaviour of FMN was observed for mixed monolayers of 2C₁₈Gua-2C₁₈mela-NN.