Interaction of aromatic units of amino acids with guanidinium cation: The interplay of π···π, X-H···π, and M+ ···π contacts.

Complexes formed by guanidinium cation and a pair of aromatic molecules among benzene, phenol, or indole have been computationally studied to determine the characteristics of the cation···π interaction in ternary systems modeling amino acid side chains. Guanidinium coordinates to the aromatic units preferentially in the following order: indole, phenol, and benzene. Complexes containing two different aromatic units show an intermediate behavior between that observed for complexes with only one kind of aromatic unit. Most stable structures correspond to doubly-T shaped arrangements with the two aromatic units coordinating guanidinium by its NH2 groups. Other structures with only one aromatic unit coordinated to guanidinium, such as T-shaped or parallel-stacked ones, are less favorable but still showing significant stabilization. In indole and phenol complexes, the formation of hydrogen bonds between the aromatic molecules introduces extra stabilization in T-shaped structures. Three body effects are small and repulsive in doubly T-shaped minima. Only when hydrogen bonds involving the aromatic molecules are formed in T-shaped structures a cooperative effect can be observed. In most complexes the interaction is controlled by electrostatics, with induction and dispersion also contributing significantly depending on the nature and orientation of the aromatic species forming the complex. Although the stability in these systems is mainly controlled by the intensity of the interaction between guanidinium and the aromatic molecules coordinated to it, interactions between aromatic molecules can modulate the characteristics of the complex, especially when hydrogen bonds are formed.