Noncovalent Attractions in Biomolecules

Covalent bonds form when pairs of electrons are shared between interacting atoms. These strong bonds require a large amount of energy to break.

While covalent bonds are intramolecular forces that connect atoms into molecules, intermolecular and noncovalent attractions stabilize groups of atoms between molecules or within different parts of a larger molecule.

Though individually weak, many noncovalent interactions together can keep molecules associated for an extended time. 

Four major types of noncovalent attractions occur in biological systems: ionic interactions, hydrogen bonds, Van der Waals forces, and hydrophobic interactions.

Ionic interactions occur between oppositely charged ions. 

The DNA backbone has many negatively charged phosphate groups close to each other. To stabilize the molecule, cations such as magnesium interact with the phosphates groups, neutralizing the net charge on the DNA. 

A hydrogen bond forms when a hydrogen atom that is covalently bonded to a highly electronegative atom, such as oxygen or nitrogen, interacts with the lone pair of electrons on another electronegative atom.

In DNA, the complementary strands are paired together by hydrogen bonds. A base from one strand shares its covalently bonded hydrogens with nitrogen or oxygen atoms on another base on the opposite strand.

Van der Waals interactions occur when two molecules approach each other closely. 

These nonspecific attractive forces result from temporary dipoles generated by the rapid movement of electrons throughout the molecule. However, when the molecules get too close, electrostatic repulsion overturns Van der Waals interactions.

Lastly, hydrophobic interactions are a forced association of hydrophobic groups due to repulsion from the water molecules. 

In an aqueous environment, the hydrophobic parts of lipid molecules cannot easily disrupt the hydrogen bonding between water molecules. 

The interaction between the water molecules is stronger than the interaction between water and the lipid molecules. So, the hydrophobic parts of the lipids associate, causing the lipids to aggregate.