Kinetic Studies of Molecular Recognition Based on Hydrogen Bonding at the Air−Water Interface by Using a Highly Sensitive Quartz-Crystal Microbalance

Abstract

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 (Δm), association constants (Ka), as well as binding and dissociation rate constants (k1 and k-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, k1 = 2.8 M-1 s-1, k-1 = 7.2 × 10-4 s-1, and Ka = 3900 M-1. The Ka 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 > 2-aminopyridine (C) forming two hydrogen bonds > pyridine (D) with only one hydrogen bond ≈ 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.