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

[GRAPHICS] The enthalpies and entropies for phosphate complexation by ammonium and gua nidinium groups have been compared using cationic cyclodextrins. Aryl phosp hate binding by guanidinium hosts is associated with more favorable enthalp ies and less favorable entropies, consistent with the idea that guanidinium -phosphate interactions are stronger but more directional than ammonium-pho sphate interactions. The slope of an enthalpy-entropy plot suggests that co mplexation occurs with surprisingly small changes in the order of these sys tems.