2.4: Noncovalent Attractions in Biomolecules
Noncovalent attractions are associations within and between molecules that influence the shape and structural stability of complexes. These interactions differ from covalent bonding in that they do not involve sharing of electrons.
Four types of noncovalent interactions are hydrogen bonds, van der Waals forces, ionic bonds, and hydrophobic interactions.
Hydrogen bonding results from the electrostatic attraction of a hydrogen atom covalently bonded to a strong-electronegative atom like oxygen, nitrogen, or fluorine by another electronegative atom. The hydrogen atom develops a partial positive charge as the electronegative atom to which it is bonded draws the electron cloud near it. As a result, a weak interaction occurs between the δ+ charge of the hydrogen and the δ– charge on the neighboring electronegative atom. This type of interaction forms regularly between water molecules. Independent hydrogen bonds easily break; however, they occur in large numbers in water and organic polymers, creating a significant force in combination.
A second type of interaction called van der Waals is driven by temporary attractions between electron-rich and electron-poor regions of two or more atoms (or molecules) that are near each other. These interactions can contribute to the three-dimensional structures of proteins essential for their function.
Another type of interaction is ionic bonding that occurs between oppositely charged ions. In biological systems, ionic interactions arising from oppositely charged ions can also help stabilize biomolecules’ structure. Metal ions such as magnesium interact with negatively charged biomolecules such as DNA. The magnesium ion binds to the negative phosphate groups, thereby neutralizing the charge and helping to pack the long DNA polymer into solenoid or toroid structures.
Lastly, the hydrophobic effect is a noncovalent interaction in which hydrophobic molecules aggregate to minimize contact with water in an aqueous environment. As a consequence, the hydrophobic regions of a polypeptide become buried within the structure during protein folding.