Cation-pi interaction
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[[Image:cationpi.png|thumbnail|150px|Cation-<math>\pi</math> interaction between [[benzene]] and a sodium [[cation]].]]
'''Cation-<math>\pi</math> interaction''' is a noncovalent [[chemical bond|molecular interaction]] between the face of an electron-rich [[aromaticity|<math>\pi</math> system]] (e.g. [[benzene]], [[ethylene]]) with an adjacent [[cation]] (e.g. Li<sup>+</sup>, Na<sup>+</sup>). This unusual interaction is an example of noncovalent bonding between a monopole (cation) and a [[Quadrupole#Electric_quadrupole|quadrupole]] (<math>\pi</math> system). Cation-<math>\pi</math> interaction energies are of the same order of magnitude as [[Hydrogen_bond|hydrogen bonds]] or [[Salt_bridge_%28protein%29|salt bridges]] and play an important role in [[molecular recognition]].<ref>{{cite book|author=Eric V. Anslyn, Dennis A. Dougherty |publisher=University Science Books |title=Modern Physical Organic Chemistry |year= 2004}} ISBN 9-78-891389-31-3</ref>
[[Image:Benzene_Quadrupole.png|thumbnail|250px|The <math>\pi</math> system above and below the benzene ring leads to a [[Quadrupole#Electric_quadrupole|quadrupole]] charge distribution.]]
==Origin of the Effect==
[[aromaticity|Benzene]], the model <math>\pi</math> system has no permanent [[dipole moment]], as the contributions of the weakly polar carbon-hydrogen bonds cancel due to [[molecular symmetry]]. However, the electron-rich <math>\pi</math> system above and below the benzene ring hosts a partial negative charge. In order to counterbalance this sandwiching negative charge, a positive charge is associated within the plane in which all benzene atoms lie. An [[Quadrupole#Electric_quadrupole|(electric) quadrupole]] (a pair of dipoles, which do not cancel each other) results. The negatively charged <math>\pi</math> system can then interact favorably with positively charged ions.
==Influences on the Strength of the Cation-<math>\pi</math> Interaction==
The cation-<math>\pi</math> interaction is comparable in strength to hydrogen bonding and can in some cases be a decisive [[intermolecular force]]. Several criteria influence the strength of the bonding: the nature of the cation, the [[substituent|subsitutents]] on the <math>\pi</math> system, as well as the solvent.
===Nature of the Cation===
From [[electrostatics]] ([[Coulomb_law|Coulomb's law]]), smaller and more positively charged cations lead to larger electrostatic attraction. The following table shows a series of [[Gibbs_free_energy|Gibbs free energy]] changes for the interaction of benzene with several [[Alkaline_metal|alkaline metals]] in the gas phase.<ref>{{cite journal|author=J. C. Amicangelo, and P. B. Armentrout |journal= [[J. Phys. Chem. A]] |title=Absolute Binding Energies of Alkali-Metal Cation Complexes with Benzene Determined by Threshold Collision-Induced Dissociation Experiments and ab Initio Theory |year= 2000 |volume=104 |issue=48 |pages=11420 |doi=10.1021/jp002652f}}</ref> The influence of the [[Ionic_radius|ionic radius]], <math>r_{\mathrm{ion}}</math>, is evident.
<blockquote>
{| class="wikitable"
|-
! M<sup>+</sup>
| Li<sup>+</sup>
| Na<sup>+</sup>
| K<sup>+</sup>
| Rb<sup>+</sup>
|-
! -<math>\Delta</math>G [kcal/mol]
| 38
| 27
| 19
| 16
|-
! <math>r_{\mathrm{ion}}</math> [pm]
| 76
| 102
| 138
| 152
|}
</blockquote>
[[Image:Cationpi_Substituenteffect.png|thumbnail|380px|Binding energy (in kcal/mol) for Na<sup>+</sup> to benzene with prototypical substituents.<ref>{{cite journal|author=S. Mecozzi, A. P. West, and D. A. Dougherty |journal= [[JACS]] |title=Cation-<math>\pi</math> Interactions in Simple Aromatics: Electrostatics Provide a Predictive Tool |year= 1996 |volume=118 |issue=9 |pages=2307 |doi=10.1021/ja9539608}}</ref>]]
===Substituents on <math>\pi</math> system===
The electronic properties of the [[substituent|substituents]] on the <math>\pi</math> system also have an influence on the strength of the attraction. [[Inductive_effect|Electron withdrawing groups]] (e. g. [[Nitrile|Cyano]] -CN) decrease the amount of negative charge in the <math>\pi</math> system and thus weaken the interaction. On the contrary, [[Inductive_effect|electron donating substituents]] (e.g. amino –NH<sub>2</sub>) increase the charge separation of the quadrupole and strengthen the cation-<math>\pi</math> binding. This relationship is illustrated quantitatively in the margin for several substituents.
===Influence of the solvent===
Additionally, the nature of the solvent also determines the relative strength of the bonding. Most data on cation-<math>\pi</math> interaction is acquired in the gas phase, as the attraction is most pronounced in that case. Any intermediating solvent molecule will attenuate the effect, which is why it becomes less pronounced with increasing [[Chemical_polarity|solvent polarity]].
[[Image:Cationpi_Acetylcholine.png|thumbnail|200px|Cationic [[Acetylcholine]] binding to a [[tryptophane]] residue of the nicotinamide acetylcholine receptor via a cation-<math>\pi</math> effect.]]
==Cation-<math>\pi</math> Interaction in Nature==
Nature’s building blocks consist of aromatic moieties, too. [[Amino_acid|Amino acid]] side chains of [[tryptophane]] and [[tyrosine]] or the [[DNA_base|DNA bases]] are capable of binding to cationic species (not only metal ions, but also charged amino acid side chains, ...).<ref>{{cite journal|author=M. M. Gromiha, C. Santhosh, and S. Ahmad |journal= Int. J. Biol. Macromol. |title=Structural analysis of cation-<math>\pi</math> interactions in DNA binding proteins |year= 2004 |volume=34 |issue=3 |pages=203 |doi=10.1016/j.ijbiomac.2004.04.003}}</ref><ref>{{cite journal|author=J. P. Gallivan and D. A. Dougherty |journal= [[PNAS]] |title=Cation-<math>\pi</math> interactions in structural biology |year= 1999 |volume=96 |issue=17 |pages=9459 |doi=10.1073/pnas.96.17.9459}}</ref> Therefore, cation-<math>\pi</math> interactions can play an important role in stabilizing the three dimensional structure of a [[Protein_structure|protein]]. A very impressive example is given by the [[Nicotinic_acetylcholine_receptor|nicotinamide acetylcholine receptor]] whose molecular recognition mechanism of its [[Substrate_%28biochemistry%29|substrate]] [[acetylcholine]] (a positively charged molecule) nearly entirely bases on cation-<math>\pi</math> interaction.<ref>{{cite journal|author=D. L. Beene, G. S. Brandt, W. Zhong, N. M. Zacharias, H. A. Lester, and D. A. Dougherty |journal= Biochemistry |title=Cation-<math>\pi</math> Interactions in Ligand Recognition by Serotonergic (5-HT<sub>3A</sub>) and Nicotinic Acetylcholine Receptors: The Anomalous Binding Properties of Nicotine |year= 2002 |volume=41 |issue=32 |pages=10262 |doi=10.1021/bi020266d }}</ref>
==Anion-<math>\pi</math> interaction==
In many respects, [[anion]]-<math>\pi</math> interaction is opposite to cation-<math>\pi</math> interaction, although the underlying principles are identical. Significantly less examples are known to date. In order to attract a negative charge, the charge distribution of the <math>\pi</math> system has to be reversed. This is achieved by placing several strong [[Inductive_effect|electron withdrawing substituents]] along the <math>\pi</math> system (''e. g.'' hexafluorobenzene).<ref>{{cite journal|author=D. Quiñonero, C. Garau, C. Rotger, A. Frontera, P. Ballester, A. Costa, and P. M. Deyà |journal= [[Angew. Chem. Int. Ed.]] |title=Anion-<math>\pi</math> Interactions: Do They Exist? |year= 2002 |volume=41 |issue=18 |pages=3389 |doi=10.1002/1521-3773(20020916)41:18<3389::AID-ANIE3389>3.0.CO;2-S |doilabel=10.1002/1521-3773(20020916)41:18<3389::AID-ANIE3389>3.0.CO;2-S}}</ref> The anion-<math>\pi</math> effect is advantageously exploited in chemical sensors for specific anions.<ref>{{cite journal|author=P. de Hoog, P. Gamez, I. Mutikainen, U. Turpeinen, and J. Reedijk |journal= [[Angew. Chem.]] |title=An Aromatic Anion Receptor: Anion-<math>\pi</math> Interactions do Exist |year= 2004 |volume=116 |issue=43 |pages=5939 |doi=10.1002/ange.200460486}}</ref>
==See also==
* [[Stacking_%28chemistry%29|<math>\pi</math> stacking]]
* [[Salt_bridge_(protein)|Salt bridges]]
== References ==
<references/>
* {{cite journal |author=J. C. Ma, and D. A. Dougherty |journal=Chem. Rev. |title=The Cation-π Interaction |year=1997 |volume=97 |issue=5 |pages=1303 |doi=10.1021/cr9603744}}.
{{Chemical bonds}}
[[Category:Chemical bonding]]