Aryl phosphate complexation by cationic cyclodextrins. An enthalpic advantage for guanidinium over ammonium and unusual enthalpy-entropy compensation.

The enthalpies and entropies for phosphate complexation by ammonium and guanidinium groups have been compared using cationic cyclodextrins. Aryl phosphate binding by guanidinium hosts is associated with more favorable enthalpies and less favorable entropies, consistent with the idea that guanidinium-phosphate interactions are stronger but more directional than ammonium-phosphate interactions. The slope of an enthalpy-entropy plot suggests that complexation occurs with surprisingly small changes in the order of these systems.