KEY2CHEM

Intermolecular Forces in Large Molecules

Noncovalent and intermolecular interactions play important roles in many biological and polymer systems. For example, hydrogen bonding between DNA base pairs holds the two strands of DNA together. Additionally, the strong hydrogen bonds between adjacent polymer strands in Kevlar provide rigidity and structural strength; Kevlar is used in bulletproof vests and helmets.


Example 1.

 

What is the strongest type of intermolecular force found between lipids (fats)?

A. hydrogen bonding

B. ion-dipole

C. dispersion

 

 

Solution

 

C. dispersion

Lipids tend to be nonpolar molecules, which interact via dispersion forces.

 


Example 2.

A micelle can form from molecules that have a polar (hydrophilic) portion and a nonpolar (hydrophobic) portion. If the following molecule were to form a micelle (with other molecules of itself) in water, what would the micelle structure look like?

A. 

B. 

C. 

 

 

 

 

 

Solution

A. 

The polar part of the micelle will interact with polar water molecules on the outside of the micelle (water is the solvent). The nonpolar tails will interact with one another.


Example 3.

 

An enzyme has an active site pocket with several different polar amino acids. What type of substrate is LEAST likely to interact with the enzyme’s active site?

A. polar

B. ionic

C. nonpolar

 

 

 

 

Solution

 

C. nonpolar

The polar active site will have stronger intermolecular interactions with polar (dipole-dipole or hydrogen bonding) and ionic (ion-dipole) substrates and weaker interactions with nonpolar (dipole-induced dipole) substrates.