Abstract
Continuing with our interest in the guanidinium group and the different interactions than can establish, we have carried out a theoretical study of the complexes formed by this cation and the aromatic amino acids (phenylalanine, histidine, tryptophan and tyrosine) using DFT methods and PCM-water solvation. Both hydrogen bonds and cation-π interactions have been found upon complexation. These interactions have been characterized by means of the analysis of the molecular electron density using the Atoms-in-Molecules approach as well as the orbital interactions using the Natural Bond Orbital methodology. Finally, the effect that the cation-π and hydrogen bond interactions exert on the aromaticity of the corresponding amino acids has been evaluated by calculating the theoretical NICS values, finding that the aromatic character was not heavily modified upon complexation.
Highlights
Cation-π interactions have been the objective of a vast number of experimental and computational studies since Kerbarle’s seminal publication in 1981 [1]
Complexes type [a] which are formed by a bifurcated hydrogen bonds (HBs) plus an additional cation-π interaction
The complexes established by the guanidinium cation and the aromatic amino acids, Phe, His, Trp and Tyr by means of cation-π and hydrogen bonding interactions have been computationally studied using PCM–water at the M06-2X/6-311++G(d,p) level
Summary
Cation-π interactions have been the objective of a vast number of experimental and computational studies since Kerbarle’s seminal publication in 1981 [1]. Molecules 2015, 20 has been made by Dougherty and co-workers who showed, for example, that even in water phenyl hosts bind to cationic guests stronger than to neutral or charged molecules [2] They carried out a protein database assessment showing that cation-stabilization is fundamental in protein structure and function and that arginine (Arg) in particular is the residue that most often [3] binds. They reported the importance of these interactions for protein engineering [4,5]. This group have revisited the controversial proposal that substituent effects in cation-π interactions can be attributed mainly to electrostatic effects by analyzing 171 aromatic systems interacting with Na+; they found that both electrostatic and π-polarization effects describe cation-π interactions [10]
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