In chemistry , π-EFFECTS OR π-INTERACTIONS are a type of non-covalent interaction that involves π systems . Just like in an electrostatic interaction where a region of negative charge interacts with a positive charge, the electron-rich π system can interact with a metal (cationic or neutral), an anion, another molecule and even another π system. Non-covalent interactions involving π systems are pivotal to biological events such as protein-ligand recognition.
* 1 Types
* 2 Metal-π interactions
* 2.1 Linear systems
* 2.2 Cyclic systems
* 3 Anion π-interactions * 4 π-effects in biological systems * 5 References
The most common types of π-interactions involve:
* Metal-π Interactions: involves interaction of a metal and the face of a π system, the metal can be a cation (known as cation-π interactions ) or neutral * Polar-π interactions: involves interaction of a polar molecule and quadrupole moment a π system.
Polar π interaction between water molecule and benzene
* Aromatic-aromatic interactions (π stacking): involves interactions of aromatic molecules with each other.
* Arene-perfluoroarene interaction: electron-rich benzene ring interacts with electron poor hexafluorobenzene .
Arene perfluoroarene stacking
* π donor-acceptor interactions: interaction between low energy empty orbital (acceptor) and a high energy filled orbital (donor).
Donor-acceptor interaction between hexamethylbenzene (donor) and tetracyanoethylene (acceptor)
* Anion-π interactions: interaction of anion with π system * Cation-π interactions: interaction of a cation with a π system * C-H-π interactions: interaction of C-H with π system: These interactions are well studied using experimental as well as computational techniques.
ETHYLENE – π In the most simple linear π systems, bonding to metals takes place by two interactions. Electron density is donated directly to the metal like a sigma bond would be formed. Also, the metal can donate electron density back to the linear π system (ethylene ) from the metal’s d orbital to the empty π* orbital of ethylene . Electron density donated to the alkene π* orbital Electron density donated to the metal like a Sigma bond
ALLYL – π
The specifics for binding of π cyclic systems are much more complex and depend on the electrons, the HOMO , and the LUMO in each individual case of molecules. Cyclic π systems can bind monohapto or polyhapto depending on the individual situation. This means that π bonds can bind individually to the metal or there can be a single bond from the center of a benzene or cyclopentadienyl complex . Of course the bonding modes (η1, η3, η5, etc.) determine the number of donated electrons (1, 3, 5, etc.). The diversity of these cyclic complexes allows for a seemingly endless number of metallic structures.
The use of organometallic structures led by π – metal bonding
plays an enormous role in the catalysis of organic reactions . The
π metal interactions can also be involved directly with the function
of ligands on the catalyst .
Another π metal interaction directly involved with catalysis
involves π stacking .
Due to reasons explained earlier in the article, the bonding between
a nucleophilic olefin and an electrophilic
Interaction between benzene and an anion, "X−"
Anion and π-aromatic systems (typically electron deficient) create an interaction that is associated with the repulsive forces of the structures. These repulsive forces involve electrostatic and anion-induced polarized interactions. This force allows for the systems to be used as receptors and channels in supramolecular chemistry for applications in the medical (synthetic membranes, ion channels) and environmental fields (e.g. sensing, removal of ions from water).
The first X-ray crystal structure that depicted anion-π interactions was reported in 2004. In addition to this being depicted in the solid state, there is also evidence that the interaction is present in solution.
π-EFFECTS IN BIOLOGICAL SYSTEMS
Reaction of SAM with nucleophile
π-effects have an important contribution to biological systems since they provide a significant amount of binding enthalpy. Neurotransmitters produce most of their biological effect by binding to the active site of a protein receptor. Pioneering work of Dennis A. Dougherty is a proof that such kind of binding stabilization is the effect of cation-π interactions of the acetylcholine (Ach) neurotransmitter. The structure of acetylcholine esterase includes 14 highly conserved aromatic residues. The trimethyl ammonium group of Ach binds to the aromatic residue of tryptophan (Trp). The indole site provides a much more intense region of negative electrostatic potential than benzene and phenol residue of Phe and Tyr. S-Adenosyl methionine (SAM) can act as a catalyst for the transfer of methyl group from the sulfonium compound to nucleophile. The nucleophile can be any of a broad range structures including nucleic acids, proteins, sugars or C=C bond of lipids or steroids. The van der Waals contact between S-CH3 unit of SAM and the aromatic face of a Trp residue, in favorable alignment for catalysis assisted by cation-π interaction.
A great deal of circumstantial evidence places aromatic residues in the active site of a number of proteins that interact with cations but the presence of cation-π interaction in biological system does not rule out the conventional ion-pair interaction. In fact there is a good evidence for the existence of both type of interaction in model system.
* ^ Anslyn, E.V.; Dougherty, D.A. Modern Physical Organic
Chemistry; University Science Books; Sausalito, CA, 2005 ISBN
* ^ Meyer, EA; Castellano, RK; Diederich, F (2003). "Interactions
with aromatic rings in chemical and biological recognition".
Angewandte Chemie International Edition in English. 42 (11):
1210–50. PMID 12645054 . doi :10.1002/anie.200390319 .
* ^ K. Sundararajan; K. Sankaran; K.S. Viswanathan; A.D. Kulkarni;
S.R. Gadre (2002). "H-π Complexes of acetylene-ethylene: A matrix
isolation and computational study".
J. Phys. Chem. A . 106 (8): 1504.
Bibcode :2002JPCA..106.1504S. doi :10.1021/jp012457g .
* ^ K. Sundararajan; K.S. Viswanathan; A.D. Kulkarni; S.R. Gadre
(2002). "H-π Complexes of acetylene-benzene: A matrix isolation and
computational study". J. Mol. Str. (Theochem) . 613: 209. Bibcode
:2002JMoSt.613..209S. doi :10.1016/S0022-2860(02)00180-1 .
* ^ J. Rebek (2005). "Simultane Verkapselung: Moleküle unter
Angewandte Chemie . 117 (14): 2104. doi :10.1002/ange.200462839
* ^ J. Rebek (2005). "Simultaneous Encapsulation: Molecules Held at
Angewandte Chemie International Edition . 44 (14): 2068.
doi :10.1002/anie.200462839 .
* ^ S. Grimme (2004). "Accurate description of van der Waals
complexes by density functional theory including empirical
corrections". Journal of Computational