The strength of van der Waals interactions varies with the distance, R, between interacting species. The force of interaction between interacting species increases with
decreasing distance until they are separated by the van der Waals contact distance (see arrow marked A). Repulsion due to interaction between the electrons of each atom or
molecule then supervenes. While individual van der Waals interactions are extremely weak, the cumulative effect is nevertheless substantial for macromolecules such as DNA and
proteins with many atoms in close contact.
Multiple Forces Stabilize Biomolecules
The DNA double helix illustrates the contribution of multiple forces to the structure of biomolecules. While each individual DNA strand is held together by covalent bonds, the two strands
of the helix are held together exclusively by noncovalent interactions such as hydrogen bonds between nucleotide bases (Watson–Crick base pairing) and van der Waals interactions
between the stacked purine and pyrimidine bases. The double helix presents the charged phosphate groups and polar hydroxyl groups from the ribose sugars of the DNA backbone to
water while burying the relatively hydrophobic nucleotide bases inside. The extended backbone maximizes the distance between negatively charged phosphates, minimizing
unfavorable electrostatic interactions.