Molecular recognition-induced function and competitive replacement by hydrogen-bonding interactions:: Amphiphilic barbituric acid derivatives, 2,4,6-triaminopyrimidine, and related structures at the air-water interface

The phenomenon of molecular recognition inducing further function is common in nature. However, there are few synthetic systems which achieve this cascade type mechanism, and those are generally carried out in noncompetitive solvents. Here a synthetic system is described that partakes in recognition events at an aqueous interface, which subsequently induces a reaction. This system involves amphiphiles forming monolayers at the air-water interface where the headgroups are barbituric acid derivatives. It is subsequently seen when 2,4,6-triaminopyrimidine (TAP) is present in the subphase that the barbituric acid headgroup is cleaved by the hydrolysis of a C=C double bond which links the headgroup to the hydrophobic tail (retro-Knoevenagel reaction). This cleavage depends on four Factors which are (i) the self-organization of the amphiphiles, (ii) the insertion of TAP into the monolayer by the formation of six hydrogen bonds to two adjacent barbituric acid groups (This insertion is discussed in ter ms of a linear ( coplanar) and a zigzag type (crinkled) geometry.), (iii) the polarization of the C=C double bond due to the hydrogen-bonding interactions, and (iv) the formation of a hydrophobic cleft, upon insertion, and the trapping of water molecules therein. These studies involve the use of surface pressure-area isotherms, UV/vis and FTIR reflection spectroscopy at the air-water interface and of H-1 NMR spectroscopy in homogeneous organic solution. Finally, the X-ray crystal structure of a barbituric acid TAP salt is reported in which ionic and hydrogen-bonding interactions are shown to hold the dimer pair together in the solid state. Competition experiments in the monolayer point toward barbituric acid and TAP existing as ionic/hydrogen-bonded dimers in solution which can move their equilibria such that TAP molecules are delivered to the monolayer as neutral molecules.