Purpose of aromatic rings in biochemistry?

[jackmell]

May I ask what are some of the reasons we see so much aromatic, hetrocyclic chemistry in biology? I realize that several amino acids are aromatic and so to nucleic acids, other biomolecules. What exactly do the aromatic rings bring to biology in order for us to see it so profusely utilized?

 

[Yanick]

In DNA the pi electrons of the nitrogenous bases stabilize the helical structure by pi-stacking interactions.

Many proteins which work through radical Chemistry form transient tyrosine and/or tryptophan radicals.

Also there are cofactors for redox Chemistry known as flavins (see FAD/FADH₂) which are integral parts of electron transport processes owing to their ability to do single electron Chemistry and due to their aromaticity in the fully oxidized form.

These are all I could come up with off the top of my head.

 

[Ygggdrasil]

1. Because these compounds gain so much stabilization through their conjugated pi systems (i.e. they are resonance stabilized), it may be easier for the body to synthesize aromatic compounds over other molecules of similar complexity. This stability may also increase their halflives within the organism, making them less prone to degradation (important, for example, for DNA).

2. In binding reactions between two biomolecules, there are two important factors influencing the strength of binding: the enthalpy change (ΔH) of the interaction and the entropy change (ΔS) of the interaction. While many think of the enthalpy of the interaction (how strong are the intermolecular bonds between two interacting molecules), the entropy of the interaction is just as important. Molecules that are conformationally flexible in the free state lose a considerable amount of energy upon binding to their target as this binding usually locks the molecule into a single conformation. Aromatic rings are nice in this regard because they are already conformationally locked and will not lose as much entropy upon binding.

3. As Yanick mentioned, aromatic rings can aid intermolecular interactions on the enthalpy side as well. A number of fairly strong intermolecular interactions involve aromatic rings, including pi-pi stacking interactions and cation-pi interactions.

4. The conjugated pi systems in aromatic rings are great for helping to delocalize charges and radicals. As Yanick mentioned, this property allows the aromatic rings to facilitate a number of chemical reactions.

 

[jackmell]

Hi guys, thanks!

I like the entropy argument: small change in entropy with rings easier than one which creates lots of order as in locking conformations in a flexible molecule, and I like the association with hydrogen bonding used profusely in the reversible-reactions of biochemistry.