Molecular Recognition of Nucleolipid Monolayers of 1-(2-Octadecyloxycarbonylethyl)cytosine to Guanosine at the Air−Water Interface and Langmuir−Blodgett Films

Abstract

Molecular recognition of 1-(2-octadecyloxycarbonylethyl)cytosine monolayers to guanosine at the air−water interface and Langmuir−Blodgett films has been studied in detail using surface pressure−area isotherm, UV−vis, FTIR, and surface-enhanced Raman scattering spectroscopy techniques. The direct FTIR spectroscopic evidence for the molecular recognition of Watson−Crick complementary base pairs is observed. It is shown that the molecular recognition occurs between the cytosine and guanosine bases through triple hydrogen bonds at the expense of the intermolecular hydrogen-bonding network between the adjacent cytosine moieties in the monolayers. The cytosine rings undergo a change in orientation and aromatic stacking from the flat-on geometry without stacking interaction before molecular recognition to an end-on one in the J-aggregate fashion after molecular recognition. The triple hydrogen bonds between the cytosine and guanosine base pairs is stable below 60 °C and is completely dissociated above 120 °C, while the intermolecular hydrogen-bonding interaction between the adjacent cytosine moieties in the monolayers significantly strengthens the interaction between the corresponding hydrocarbon chains, so that the phase transition temperature of the LB films is increased to 145 °C. The process of molecular recognition is revealed in all directions using various molecular spectroscopic techniques.