Non-covalent interactions and water

Covalent bond

Strong and sharing electrons in a valence shell

Non covalent bonds

Weaker and based on unequal sharing of electrons between nuclei, polar molecules

Intramolecular bonds

Interaction between biomolecules and domains of biomolecules

Non-polar covalent bond

C - H

Polar covalent bonds

Moderately polar bond -

S - H

Polar bond

N - H

O - H

N - C

O - C

Dipole

Partial separation of charge

Electronegativity

H < C = S << N < O

Hydrogen bonds in water

Central water molecule acts as a hydrogen donor to other molecules

Central water molecule acts as a hydrogen acceptor from other molecules

Hydrogen donor examples

N attached to H and O attached to H

Hydrogen acceptors

N not bonded to anything

O not bonded to anything

O double bonded to C

Hydrogen bonds

A type of dipole-dipole interaction and the most common one in biology

Among the strongest of non-covalent interactions: around 30kJ/mole

Only strong if all the 3 atoms aligned

H atom bonded to electronegative atom —— Another electronegative atom with lone pair electrons

Charge-Charge interactions (ion pairs)

• Salt bridges

• Electrostatic interactions between opposite charge

• Stronger than H bond

• Extend greater distances

Charge-Charge interactions

Ion pairs buried within the hydrophobic interior of the protein are more stable

Van der Waals forces

• Attraction between stable or inducible dipoles

• Only works at very short, optimal distance

• Weaker than other dipole interactions - large amount of interactions —> stabilising

Hydrophobic interactions

Association of non-polar groups with most energy attributed to exclusion of water (increased entropy)

Strength of hydrophobic interactions

3-10 kJ/mol

Strength of Van der Waals Forces

0.4-4kJ/mol

Strength of Electrostatic interactions

40-200kJ/mol

Strength of H bonds

2-30kJ/mol

Strength of non covalent bonds

Van der Waals Forces

Hydrophobic interactions

H bonds

Electrostatic interactions