KINETIC-STUDIES OF MOLECULAR RECOGNITION BASED ON HYDROGEN-BONDING ATTHE AIR-WATER-INTERFACE BY USING A HIGHLY SENSITIVE QUARTZ-CRYSTAL MICROBALANCE

The complementary guest binding process onto the cyanurate lipid 1 mon olayer at the air-water interface was observed by using a highly sensi tive 27 MHz quartz-crystal microbalance (QCM), which was attached hori zontally on the monolayer from the air phase. Binding amount (Delta m) , association constants (K-a), as well as binding and dissociation rat e constants (k(1) and h(-1)) could be obtained from time courses of th e 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 h ydrogen bonding sites were selectively bound to the monolayer 1 showin g 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 wit h that obtained by NMR spectra in bulk CDCl3. Guest molecules of A har dly bound to monolayers 2-4 with -NH2, -CONH2, and -OH head groups. Bi nding constants of guest molecules to the monolayer 1 were of the foll owing order: 2,4,6-triaminopyrimidine (A) > 2,6-diaminopyridine (B) po ssessing three hydrogen bonds > a-aminopyridine (C) forming two hydrog en bonds > pyridine (D) with only one hydrogen bond approximate to bar bituric acid (E) having no complementary hydrogen binding sites. The o btained binding kinetics at the water interface were compared with mol ecular mechanics calculations of binding energy in vacuum.