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
Tm. Bohanon et al., 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, LANGMUIR, 15(1), 1999, pp. 174-184
The phenomenon of molecular recognition inducing further function is common
in nature. However, there are few synthetic systems which achieve this cas
cade 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. Thi
s system involves amphiphiles forming monolayers at the air-water interface
where the headgroups are barbituric acid derivatives. It is subsequently s
een 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 bon
d which links the headgroup to the hydrophobic tail (retro-Knoevenagel reac
tion). This cleavage depends on four Factors which are (i) the self-organiz
ation of the amphiphiles, (ii) the insertion of TAP into the monolayer by t
he 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 d
ue to the hydrogen-bonding interactions, and (iv) the formation of a hydrop
hobic cleft, upon insertion, and the trapping of water molecules therein. T
hese 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 spe
ctroscopy in homogeneous organic solution. Finally, the X-ray crystal struc
ture 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 ac
id and TAP existing as ionic/hydrogen-bonded dimers in solution which can m
ove their equilibria such that TAP molecules are delivered to the monolayer
as neutral molecules.