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]
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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
Substituents on π system
The electronic properties of the substituents also have an influence on the strength of the attraction. Electron withdrawing groups (e. g. Cyano -CN) weaken the interaction, while electron donating substituents (e.g. amino –NH2) strengthen the cation-π binding. This relationship is illustrated quantitatively in the margin for several substituents. The origin of this effect is often attributed to donation into or out of the π-system, although recent computational results indicate that it is primarily due to direct through-space interactions between the substituents and the cation.[4]
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.
Cation-π Interaction in Nature
Nature’s building blocks consist of aromatic moieties, too. Amino acid side chains of tryptophan and tyrosine or the DNA bases are capable of binding to cationic species (not only metal ions, but also charged amino acid side chains, ...).[5][6] Therefore, cation-π interactions can play an important role in stabilizing the three dimensional structure of a protein. An alternative role of the cation-π interaction is seen in the nicotinc acetylcholine receptor which binds its endogenous ligand, acetylcholine (a positively charged molecule), via a cation-π interaction to the quaternary ammonium.[7]
Anion-π interaction
In many respects, anion-π interaction is opposite to cation-π interaction, although the underlying principles are identical. Significantly fewer 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).[8] The anion-π effect is advantageously exploited in chemical sensors for specific anions.[9]
Notes
- ^ 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:.
- ^ 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:.
- ^ S. E. Wheeler and K. N. Houk (2009). "Substituent Effects in Cation/π Interactions and Electrostatic Potentials above the Centers of Substituted Benzenes Are Due Primarily to Through-Space Effects of the Substituents". J. Am. Chem. Soc. 131 (9): 3126. doi:.
- ^ 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:.
- ^ J. P. Gallivan and D. A. Dougherty (1999). "Cation-π interactions in structural biology". PNAS 96 (17): 9459. doi:.
- ^ 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:.
- ^ 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:.
- ^ 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:.
References
- J. C. Ma, and D. A. Dougherty (1997). "The Cation-π Interaction". Chem. Rev. 97 (5): 1303. doi:..
- Dougherty, Da; Stauffer, Da (Dec 1990). "Acetylcholine binding by a synthetic receptor: implications for biological recognition". Science (New York, N.Y.) 250 (4987): 1558–60. doi:. ISSN 0036-8075. PMID 2274786.
See also
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