Thursday, September 10, 2015

Difference between heterocyclic aminoacids and aromatic aminoacids

"Are not heterocyclic aminoacids coming under aromatic amino acids? Then why aren't they coming under aromatic amino acids? Why a separate category?"

This question was asked by Anonymous on my "Amino acids with electrically charged side chains mnemonic" blog.

What's an aromatic compound?
An aromatic (or aryl) compound contains a set of covalently bound atoms with specific characteristics:
- An arrangement of alternating single and double bonds. (A delocalized conjugated pi system)
- All the contributing atoms in the same plane. (Coplanar structure)
- Stable.

Types of aromatic compounds:
Homocyclic: CnHn where n is an even number.
Heterocyclic: One or more of the atoms in the aromatic ring is of an element other than carbon.

What's an aromatic amino acid?
Aromatic amino acids are amino acids that include an aromatic ring.
Tyrosine, phenylalanine, histidine and tryptophan have aromatic ring side chains.

What's a heterocyclic  amino acid?
A heterocyclic amino acid is one that has a ring structure in which at least one atom is not carbon.
Proline is the only cyclic amino acid, which is heterocyclic as it has an N atom in the ring.

So the answer to your question would be.. No, heterocyclic aminoacids should not be then considered a subset of aromatic aminoacids. 
Proline is most definitely not an aromatic amino acid since aromaticity requires a conjugated π system with 4n + 2 π electrons, but proline has no π bonds to begin with. A heterocyclic molecule is not necessarily aromatic.

Cool fact: The nucleotides (Guanine, cytosine, thymine, uracil, and adenine) are also aromatic compounds.

That's all!

Thanks molecularbiologistproblems and cyclopentadiene, for correcting the mistakes made in this post (I made too many T_T)
You guys are geniuses! =)



  1. Your definition of aromatic amino acid is misinformed. A homocyclic compound doesn't necessarily have to be aromatic. Aromatic compounds are not always just rings of carbon. You also forgot about histidine, which is an aromatic heterocyclic amino acid. The nucleotides (guanine, cytosine, threonine, uracil, and adenine) are also all aromatic compounds. Aromaticity has to do with hyperconjugation of the pi bounds between members of the ring structure that produce a unique type of stability.

    1. Thank you for the correction! :)

    2. Definitely better, but I made the mistake of calling threonine a nucleotide. I meant thymine. Threonine is an amino acid. Oops! Cyclopentadiene ( pointed out (which I'm noticing now) that because of how this blog post is structured, it seems like you're calling proline an aromatic amino acid. It's not aromatic because it has no pi bonds. It is, however, heterocyclic. See his post here:
      Sorry, I'm not trying to be nitpicky here or anything even though it totally seems that way.

    3. Oh. I think I get it now. I'll correct it again!


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