Opposed Aromatic Surfaces Behave as Independent Binding Sites for Carbohydrate Stacking: Analysis of Sandwich-like CH/π/CH Complexes.

Abstract

CH/π bonds are versatile elements for the construction of complex molecular architectures, thus playing key roles in many biomolecular recognition processes. Although seldom acknowledged, aromatic units are inherently bivalent and can participate in CH/π bonds through either face simultaneously, leading to the formation of ternary stacking complexes. This sandwich-like arrangement is by far the most common in natural complexes and could potentially lead to negative cooperativity due to unfavorable polarization or electrostatic effects, especially when polarized CH fragments are involved. To evaluate the energetics of such interaction modes, we selected a biologically relevant model, carbohydrate/aromatic stacking, and conducted an experimental analysis comparing binary CH/π interactions to ternary CH/π/CH stacking. Our approach utilized a dynamic combinatorial strategy, which is well-suited to reveal minor stability differences among aromatic complexes. Our results showed that carbohydrate/aromatic stacking is relatively insensitive to molecular recognition events occurring on the opposite side of the aromatic platform, whether exposed to water or involved in additional CH/π contacts, with free energy fluctuations lower than 10%. Based on these data, for all practical purposes, the two opposing aromatic surfaces can be considered independent, noninteracting binding sites, making aromatic platforms optimal connecting elements for supramolecular cross-linking.