• π-Interactions
Cation-π interactions
Alkaline earth metal cations with
π-bonds through a non-covalent
weak interaction, namely cation-π
interaction.
Cation-π interactions play a very
important role in biological
systems.
π−π Interaction or π–π stacking (0 – 10 kcal/mol)
Weak electrostatic interaction between aromatic rings.
There are two general types: (a) face-to-face and (b) edge-to-face:
Distribution of electron
density in benzene molecule
Face-to-face π-stacking interactions are responsible for the slippery
feel of graphite. Similar π-stacking interactions help stabilize DNA
double helix.
(a) Top and (b) side views of the layered structure of graphite, held together
by face-to-face -interactions.
Herringbone packing of anthracene molecules
Binding of anti-Alzheimer’s drug Aricept to active site of
acetylcholinesterase from Torpedo californica
Polar-π interactions
1.9 kcal/mol
1.4 kcal/mol
1.6 kcal/mol
pKa = 6.39 (X=Y=H)
π−Donor-Acceptor interactions
• van der Waals (dispersion) forces
•
•
•
•
Bond energy is very weak (0.1-1 kcal/mol)
Exists between almost all atoms and molecules
Arise from atomic or molecular dipole
Interactions between the fluctuating induced dipoles (due to
instantaneous and short-lived vibrational distortions)
• Hydrophobic Effect
Hydrophobic binding of organic guests in aqueous solution.
The displacement of water molecules from a hydrophobic cavity is
responsible for the enthalpic hydrophobic effect.
Entropic Hydrophobic Effect
Two organic molecules creating a hole within an aqueous phase,
giving rise to the entropic hydrophobic effect – one hole is more
stable than two.
Enthalpic Hydrophobic Effect
Binding of p-xylene in a cyclophane
K = 9.3 × 10-3 M-1 (H2O)
ΔG = -22 kJ mol-1 (at 293 K)
ΔH = -31 kJ mol-1 (favored)
TΔS = -9 kJ mol-1 (disfavored)
N
+
Solvent effect on binding
N
H
Solvation considerations during the host-guest complexation of a metal cation