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14-September-2008 18:38:44 - Cation-pi interaction Cation-π interaction between benzene and a sodium cation. Cation-π interaction between benzene and a sodium cation. Cation-π interaction is a noncovalent molecular interaction between the face of an electron-rich π system e.g. benzene, ethylene with an adjacent cation e.g. Li+, Na+. This unusual interaction is an example of noncovalent bonding between a monopole cation and a quadrupole π system. Cation-π interaction energies are of the same order of magnitude as hydrogen bonds or salt bridges and play an important role in molecular recognition.1 The π system above and below the benzene ring leads to a quadrupole charge distribution. The π system above and below the benzene ring leads to a quadrupole charge distribution. Contents 1 Origin of the Effect 2 Influences on the Strength of the Cation-π Interaction 2.1 Nature of the Cation 2.2 Substituents on π system 2.3 Influence of the solvent 3 Cation-π Interaction in Nature 4 Anion-π interaction 5 See also 6 References Origin of the Effect Benzene, the model π 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 π 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 electric quadrupole a pair of dipoles, which do not cancel each other results. The negatively charged π system can then interact favorably with positively charged ions. Influences on the Strength of the Cation-π Interaction The cation-π 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 subsitutents on the π system, as well as the solvent. Nature of the Cation From electrostatics Coulomb's law, smaller and more positively charged cations lead to larger electrostatic attraction. The following table shows a series of Gibbs free energy changes for the interaction of benzene with several alkaline metals in the gas phase.2 The influence of the ionic radius, rion, is evident. M+ Li+ Na+ K+ Rb+ -ΔG kcal/mol 38 27 19 16 rion pm 76 102 138 152 Binding energy in kcal/mol for Na+ to benzene with prototypical substituents. Binding energy in kcal/mol for Na+ to benzene with prototypical substituents.3 Substituents on π system The electronic properties of the substituents on the π system also have an influence on the strength of the attraction. Electron withdrawing groups e. g. Cyano -CN decrease the amount of negative charge in the π system and thus weaken the interaction. On the contrary, electron donating substituents e.g. amino -NH2 increase the charge separation of the quadrupole and strengthen the cation-π 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-π 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 solvent polarity. Cationic Acetylcholine binding to a tryptophane residue of the nicotinamide acetylcholine receptor via a cation-π effect. Cationic Acetylcholine binding to a tryptophane residue of the nicotinamide acetylcholine receptor via a cation-π effect. Cation-π Interaction in Nature Nature's building blocks consist of aromatic moieties, too. Amino acid side chains of tryptophane and tyrosine or the DNA bases are capable of binding to cationic species not only metal ions, but also charged amino acid side chains, ....45 Therefore, cation-π interactions can play an important role in stabilizing the three dimensional structure of a protein. A very impressive example is given by the nicotinamide acetylcholine receptor whose molecular recognition mechanism of its substrate acetylcholine a positively charged molecule nearly entirely bases on cation-π interaction.6 Anion-π interaction In many respects, anion-π interaction is opposite to cation-π 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 π system has to be reversed. This is achieved by placing several strong electron withdrawing substituents along the π system e. g. hexafluorobenzene.7 The anion-π effect is advantageously exploited in chemical sensors for specific anions.8 See also π stacking Salt bridges References ^ Eric V. Anslyn, Dennis A. Dougherty 2004. Modern Physical Organic Chemistry. University Science Books. ISBN 9-78-891389-31-3 ^ J. C. Amicangelo, and P. B. Armentrout 2000. Absolute Binding Energies of Alkali-Metal Cation Complexes with Benzene Determined by Threshold Collision-Induced Dissociation Experiments and ab Initio Theory. J. Phys. Chem. A 104 48: 11420. doi:10.1021/jp002652f. ^ S. Mecozzi, A. P. West, and D. A. Dougherty 1996. Cation-π Interactions in Simple Aromatics: Electrostatics Provide a Predictive Tool. JACS 118 9: 2307. doi:10.1021/ja9539608. ^ M. M. Gromiha, C. Santhosh, and S. Ahmad 2004. Structural analysis of cation-π interactions in DNA binding proteins. Int. J. Biol. Macromol. 34 3: 203. doi:10.1016/j.ijbiomac.2004.04.003. ^ J. P. Gallivan and D. A. Dougherty 1999. Cation-π interactions in structural biology. PNAS 96 17: 9459. doi:10.1073/pnas.96.17.9459. ^ D. L. Beene, G. S. Brandt, W. Zhong, N. M. Zacharias, H. A. Lester, and D. A. Dougherty 2002. Cation-π Interactions in Ligand Recognition by Serotonergic 5-HT3A and Nicotinic Acetylcholine Receptors: The Anomalous Binding Properties of Nicotine. Biochemistry 41 32: 10262. doi:10.1021/bi020266d. ^ D. Quiñonero, C. Garau, C. Rotger, A. Frontera, P. Ballester, A. Costa, and P. M. Deyà 2002. Anion-π Interactions: Do They Exist?. Angew. Chem. Int. Ed. 41 18: 3389. doi:10.1002/1521-37732002091641:183389::AID-ANIE33893.0.CO;2-S. ^ P. de Hoog, P. Gamez, I. Mutikainen, U. Turpeinen, and J. Reedijk 2004. An Aromatic Anion Receptor: Anion-π Interactions do Exist. Angew. Chem. 116 43: 5939. doi:10.1002/ange.200460486. J. C. Ma, and D. A. Dougherty 1997. The Cation-π Interaction. Chem. Rev. 97 5: 1303. doi:10.1021/cr9603744. . v d e Chemical bonds Strong Covalent bonds Antibonding Sigma bonds: 3c-2e bent bond · 3c-4e Hydrogen bond, Dihydrogen bond, Agostic interaction · 4c-2e Pi bonds: π backbonding · Conjugation · Hyperconjugation · Aromaticity · Metal aromaticity Delta bond: Quadruple bond · Quintuple bond · Sextuple bond Coordinate covalent bond · Hapticity Ionic bonds Cation-pi interaction · Salt bridge Metallic bonds Metal aromaticity Weak Hydrogen bond Dihydrogen bond · Dihydrogen complex · Low-barrier hydrogen bond · Symmetric hydrogen bond · Hydrophile Other noncovalent van der Waals force · Mechanical bond · Halogen bond · Aurophilicity · Intercalation · Stacking · Entropic force · Chemical polarity other Disulfide bond · Peptide bond · Phosphodiester bond Note: the weakest strong bonds are not necessarily stronger than the strongest weak bonds Retrieved from http://en..org/wiki/Cation-pi_interaction Categories: Chemical bonding Views Article Discussion this page History Personal tools Log in / create account Navigation Main page Contents Featured content Current events Random article Search Go Search Interaction Community portal Recent changes Contact Donate to Help Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Cite this page This page was last modified on 28 May 2008, at 20:57
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