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, C(18)Gua, or dialkyl derivative, 2C(18)Gua) and the melamine amphiphile (2C(18)mela-NN) has been investigated by pi-A isotherms, FTIR spectroscopy, and XPS measurements. pi-A Isotherms and FTIR spectra of C(18)Gua-2C(18)mela-NN(1:1) monolayers show that there is no direct hydrogen bonding and/or coulombic interactions between C(18)Gua and 2C(18)mela-NN on pure water and that the C(18)Gua component is dissolved into the subphase upon compression. In contrast, the presence of aqueous FMN prevented C(18)Gua molecules from dissolving into the subphase. Maximum condensation was observed at a 1:1 ratio in C(18)Gua-2C(18)mela-NN mixed monolayers on aqueous FMN. XPS analyses revealed that one FMN molecule was bound to one C(18)Gua-2C(18)mela-NN(1:1) unit and binding was saturated at 5 x 10(-6) mol dm(-3) FMN. Peak shifts observed in FTIR spectra indicated that the isoalloxiazine ring in FMN formed hydrogen bonds with 2C(18)mela-NN. These results support a model that the isoalloxazine and phosphate functions in FMN are bound via hydrogen bonding to melamine in 2C(18)mela-NN and guanidinium in C(18)Gua, respectively. Similar binding behaviour of FMN was observed for mixed monolayers of 2C(18)Gua-2C(18)mela-NN.