Dear Snehasis,

Cation–π interaction is a noncovalent molecular interaction between the face of an electron-rich π system and an adjacent cation. This interaction is an example of noncovalent bonding between a monopole (cation) and a quadrupole (π system). Bonding energies are significant, with solution-phase values falling within the same order of magnitude as hydrogen bonds and salt bridges. Similar to these other non-covalent bonds, cation–π interactions play an important role in nature, particularly in protein structure, molecular recognition and enzyme catalysis. 

Examples for such phenomenon with proteins:

1-Protein Sci. 2004 Aug; 13(8): 2244–2251.

doi:  10.1110/ps.04702104

PMCID: PMC2279832

Cation–π interactions studied in a model coiled-coil peptide

Morris M. Slutsky and E. Neil G. Marsh

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Abstract

Cation–π interactions between aromatic amino acids and the positively charged residues lysine and arginine have been proposed to play an important role in stabilizing protein structure. We have used a peptide that adopts a coiled coil structure as a model system to evaluate the energetic contribution of cation–π interactions to protein folding. Peptides were designed in which phenylalanine, tyrosine, and tryptophan were placed at a solvent-exposed position of the helix, one turn removed from an arginine residue that could provide a favorable cation–π interaction. Only the arginine–phenylalanine pairing provided significant stabilization of the peptide structure and it appears that hydrophobic packing, rather than the cation–π effect, is more likely to be responsible for the stability of this peptide. We conclude that any stabilizing effect of cation–π interactions in these peptides is much smaller than that predicted from computational studies.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2279832/

2-Acc Chem Res. Author manuscript; available in PMC 2014 Mar 18.

Published in final edited form as:

Acc Chem Res. 2013 Apr 16; 46(4): 885–893.

Published online 2012 Dec 7. doi: 10.1021/ar300265y

PMCID: PMC3957424

NIHMSID: NIHMS561562

The Cation-π Interaction

DENNIS A. DOUGHERTY

The chemistry community now recognizes the cation-π interaction as a major force for molecular recognition, joining the hydrophobic effect, the hydrogen bond, and the ion pair in determining macromolecular structure and drug-receptor interactions. This Account provides the author’s perspective on the intellectual origins and fundamental nature of the cation-π interaction.

Early studies on cyclophanes established that water-soluble, cationic molecules would forgo aqueous solvation to enter a hydrophobic cavity if that cavity was lined with π systems. Important gas phase studies established the fundamental nature of the cation-π interaction. The strength of the cation-π interaction – Li+binds to benzene with 38 kcal/mol of binding energy; NH4+ with 19 kcal/mol– distinguishes it from the weaker polar-π interactions observed in the benzene dimer or water-benzene complexes. In addition to the substantial intrinsic strength of the cation-π interaction in gas phase studies, the cation-π interaction remains energetically significant in aqueous media and under biological conditions. Many studies have shown that cation-π interactions can enhance binding energies by 2 – 5 kcal/mol, making them competitive with hydrogen bonds and ion pairs in drug-receptor and protein-protein interactions.

As with other noncovalent interactions involving aromatic systems, the cation-π interaction includes a substantial electrostatic component. The six (four) Cδ−–Hδ+ bond dipoles of a molecule like benzene (ethylene) combine to produce a region of negative electrostatic potential on the face of the π system. Simple electrostatics facilitate a natural attraction of cations to the surface. The trend for (gas phase) binding energies is Li+>Na+>K+>Rb+: as the ion gets larger the charge is dispersed over a larger sphere and binding interactions weaken, a classical electrostatic effect. On other hand, polarizability does not define these interactions. Cyclohexane is more polarizable than benzene, but a decidedly poorer cation binder.

Many studies have documented cation-π interactions in protein structures, where Lys or Arg side chains interact with Phe, Tyr, or Trp. In addition, countless studies have established the importance of cation-π interaction in a range of biological processes. Our work has focused on molecular neurobiology, and we have shown that neurotransmitters generally use a cation-π interaction to bind to their receptors. We have also shown that many drug-receptor interactions involve cation-π interactions. A cation-π interaction plays a critical role in the binding of nicotine to ACh receptors in the brain, an especially significant case. Other researchers have established important cation-π interactions in the recognition of the “histone code,” in terpene biosynthesis, in chemical catalysis, and in many other systems.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3957424/

As you can see the binding energy for such interactions is significant and your calculations (energy and orbitals) should show that.

Hoping this will be helpful,

Rafik

Thank you, Rafik Karaman Sir.

It is helpful to my current research work.