Kinetic studies of molecular recognition based on hydrogen bonding at the air-water interface by using a highly sensitive quartz-crystal microbalance

The complementary guest binding process onto the cyanurate lipid 1 monolayer at the air-water interface was observed by using a highly sensitive 27 MHz quartz-crystal microbalance (QCM), which was attached horizontally on the monolayer from the air phase. Binding amount (Delta m), association constants (K-a), as well as binding and dissociation rate constants (k(1) and h(-1)) could be obtained from time courses of the frequency decrease (mass increase) of the QCM. A highly sensitive 27 MHz QCM was used to detect small mass changes of Langmuir adsorption of small guest molecules instead of our conventional 9 MHz QCM. Guests such as 2,4,6-triaminopyrimidine (A) possessing three complementary hydrogen bonding sites were selectively bound to the monolayer 1 showing a 1:2 host-guest binding ratio, k(1) = 2.8 M(-1) s(-1), k(-1) = 7.2 x 10(-4) s(-1), and K-a = 3900 M(-1). The K-a value was consistent with that obtained by NMR spectra in bulk CDCl3. Guest molecules of A hardly bound to monolayers 2-4 with -NH2, -CONH2, and -OH head groups. Binding constants of guest molecules to the monolayer 1 were of the following order: 2,4,6-triaminopyrimidine (A) > 2,6-diaminopyridine (B) possessing three hydrogen bonds > a-aminopyridine (C) forming two hydrogen bonds > pyridine (D) with only one hydrogen bond approximate to barbituric acid (E) having no complementary hydrogen binding sites. The obtained binding kinetics at the water interface were compared with molecular mechanics calculations of binding energy in vacuum.