Aromatic Amino Acids: Definition, Types, Absorbance, and Function

Aromatic amino acids (AAA) are a class of α-amino acids containing aromatic rings, including phenylalanine (Phe), tyrosine (Tyr) and tryptophan (Trp). Among them, phenylalanine and tryptophan are essential amino acids. Tyrosine is a semi-essential amino acid. In the body, aromatic amino acids are precursors for the synthesis of monoamine neurotransmitters (including catecholamines and 5-hydroxytryptamine). In addition, many plants and microorganisms can synthesize aromatic amino acids. Many herbicides can inhibit the synthesis of aromatic amino acids in weeds, making them harmless to humans and animals.

What are Aromatic Amino Acids?

Aromatic amino acids refer to amino acids with a benzene ring structure in their molecular structure. They mainly include tyrosine, phenylalanine, and tryptophan. These compounds have no or little charge, and they vary between hydrophobic and non-hydrophobic, such as phenylalanine and tyrosine. The aromatic ring of aromatic amino acids is highly stable and does not react easily with other compounds or chemical elements. Aromatic compounds exist in the human body except for aryl compounds. For example, every nucleotide in our DNA and RNA contains aromatic molecules. Sometimes, histidine amino acids are mistakenly grouped in the aromatic amino acid group. The amino group of this compound can be aromatic-like, but with a weak positive charge and hydrophilic properties.

Which Amino Acids are Aromatic?

Aromatic amino acids play a vital role in biological processes and are the building blocks of proteins, contributing to the structural and functional diversity of these macromolecules. In the field of biochemistry, aromaticity refers to the unique property exhibited by certain organic compounds that have a ring structure with alternating double bonds, which results in enhanced stability and unique chemical reactivity. In terms of amino acids, three specific residues are classified as aromatic due to the presence of aromatic rings in their side chains: phenylalanine, tyrosine, and tryptophan.

• Phenylalanine (Phe)

Phenylalanine is an essential amino acid, which means that it must be obtained from the diet as the body cannot synthesize it on its own. Structurally, phenylalanine has a benzene ring on its side chain, which gives this amino acid its characteristic aromaticity. The phenyl group consists of a six-carbon ring with alternating single and double bonds, which results in increased resonance stabilization and electron delocalization. This unique structure gives phenylalanine special chemical properties that affect its role in protein structure and enzyme function.

In proteins, phenylalanine residues often participate in hydrophobic interactions as the nonpolar nature of the phenyl group promotes its aggregation with other hydrophobic amino acids inside proteins. This arrangement helps improve the stability of protein structure by reducing the exposure of nonpolar residues to the surrounding water environment. In addition, phenylalanine can play a key role in signal transduction pathways and molecular recognition events by acting as a recognition site for protein-protein interactions or ligand binding.

• Tyrosine (Tyr)

Tyrosine is a non-essential amino acid, which means that it can be synthesized in vivo from another amino acid, phenylalanine. Structurally, tyrosine contains a phenolic group (-OH) attached to a benzene ring, providing an additional functional group that distinguishes it from phenylalanine. The presence of the hydroxyl group confers unique properties to tyrosine, allowing it to participate in various biochemical reactions and signaling pathways.

The aromaticity of the benzene ring in tyrosine allows it to play a role in protein structure and function, similar to phenylalanine. However, the hydroxyl group in tyrosine provides additional possibilities for hydrogen bonding and enzyme catalysis. Tyrosine residues are often located in the active site of enzymes, and the hydroxyl group can participate in substrate binding or catalytic reactions by interacting with other amino acids or cofactors. In addition, tyrosine plays a crucial role in cellular signaling pathways because it can be phosphorylated to form phosphotyrosine, a key modification involved in signal transduction and regulation of cellular processes.

• Tryptophan (Trp)

Tryptophan is an essential amino acid and a precursor for the synthesis of important biomolecules such as serotonin and niacin (vitamin B3). Structurally, tryptophan contains an indole ring on its side chain, which gives this amino acid aromaticity and unique properties. The indole ring is composed of a benzene ring fused to a five-membered nitrogen-containing ring, forming a complex structure with enhanced stability and diverse chemical reactivity.

The aromatic nature of the indole ring of tryptophan plays a key role in protein structure and function, as tryptophan residues are often located in hydrophobic regions of proteins, where they contribute to structural stability and ligand binding. In addition, tryptophan is also known for its role in fluorescence spectroscopy, as the indole ring can fluoresce when excited by ultraviolet light. This property has been widely used to study protein folding, ligand binding, and protein-protein interactions using fluorescence techniques.

Are Aromatic Amino Acids Polar?

When it comes to the question of whether aromatic amino acids are polar, the chemical properties of these amino acids must be considered. Polar molecules are those that have partial positive and negative charge areas, which lead to interactions with water molecules. Nonpolar molecules, on the other hand, have no distinct charge areas and do not interact easily with water. Among the aromatic amino acids, phenylalanine is a nonpolar amino acid due to its hydrophobic nature. The benzene ring in the phenylalanine side chain consists of only carbon and hydrogen atoms, which do not interact easily with water molecules. Therefore, phenylalanine is considered nonpolar and is usually found in the hydrophobic core of proteins, away from water molecules. Unlike phenylalanine, tyrosine and tryptophan are both aromatic amino acids with some polarity. Tyrosine contains a hydroxyl group (-OH) in its side chain, while tryptophan has a nitrogen-containing indole ring. These functional groups can participate in hydrogen bonding interactions with water molecules, making tyrosine and tryptophan more polar than phenylalanine.

Aliphatic and Aromatic Amino Acids

Aliphatic amino acids are amino acids that consist of aliphatic side chain functional groups. These compounds are non-polar and hydrophobic amino acids. Generally, aliphatic amino acids can be found inside protein molecules, but alanine and glycine are two exceptions that can be found inside or outside protein molecules. Some examples of aliphatic amino acids include alanine, isoleucine, leucine, proline, and valine. Sometimes, methionine is also considered an aliphatic amino acid, but it contains a sulfur atom in the side chain, which makes it as unreactive as true aliphatic amino acids. In aliphatic amino acids, the hydrophobicity increases as the number of carbon atoms in the side chain increases. Since aliphatic amino acid molecules have an equal distribution of charge throughout the molecule, these compounds do not react strongly in the presence of other molecules as there is no distinct positive or negative charge.

Fig. 1. Amino acids with aromatic side chains.

Aliphatic and aromatic amino acids are biochemical compounds that have basic amino acid functional groups and some important side chains. The key difference between aliphatic and aromatic amino acids is that aliphatic amino acids do not have a ring structure with alternating double bonds characteristic whereas aromatic amino acids have a ring structure with alternating double bonds characteristic. Alanine, isoleucine, leucine, proline, and valine are some examples of aliphatic amino acids while tyrosine, phenylalanine, and tryptophan are some examples of aromatic amino acids.

Absorbance of Aromatic Amino Acids

Aromatic amino acids absorb ultraviolet (UV) light. Most proteins contain aromatic amino acids, so UV spectrophotometry can be used to quantify proteins. Using ultraviolet (UV) absorbance to measure protein concentration is a relatively simple method for quantifying proteins. For example, phenylalanine absorbs light at approximately 257 nm, tyrosine absorbs light at approximately 274 nm, and tryptophan absorbs light at approximately 280 nm. By analyzing the absorbance peaks of these amino acids, researchers can quantitatively determine their concentration in a sample. The absorbance of aromatic amino acids is currently an important factor in various fields of research, including biochemistry, pharmaceuticals, and food science. Researchers often use spectrophotometric techniques to measure the absorbance of these amino acids for a variety of purposes.

Aromatic Amino Acids Function

• Phenylalanine plays a vital role in the production of neurotransmitters such as dopamine, norepinephrine, and epinephrine.
• Phenylalanine is also a precursor to tyrosine, which is synthesized through an enzymatic hydroxylation process.
• Tyrosine is involved in the synthesis of important molecules such as thyroid hormones, melanin, and catecholamines.
• Tyrosine is a precursor for the synthesis of important signaling molecules and hormones, including dopamine, norepinephrine, epinephrine, and thyroid hormones.
• Tyrosine residues in proteins can undergo post-translational modifications such as phosphorylation, which plays a vital role in cell signaling pathways.
• Tryptophan is a precursor for the synthesis of important molecules such as serotonin, melatonin, and niacin (vitamin B3).
• The delocalization of π electrons in the aromatic rings gives these amino acids UV absorbance, which can be used for protein structure studies and determination.
• The hydrophobicity of the aromatic rings also plays a role in protein folding and stability.
• Aromatic amino acids also participate in protein-ligand interactions, where the aromatic rings can stack with other aromatic rings or interact with ligands through π-π interactions.
• Aromatic amino acids can participate in hydrogen bonding and electrostatic interactions, contributing to the specificity and affinity of protein-protein and protein-ligand interactions.

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