Is tyrosine hydrophobic or hydrophilic?

Question

I’ve seen tyrosine classified as a hydrophobic amino acid due to its aromatic ring in some textbooks and as hydrophilic due to its hydroxyl group in other textbooks. How does tyrosine actually function in a peptide under physiological conditions (aqueous polar medium and pH~7)?

Answer 1

I have seen this in on-line content as well as in textbooks. The hydroxyl group attached to the aromatic gives it a polar characteristic while its aromatic ring gives it the hydrophobic characteristic. Because it has these two opposite characteristics, there continues to be confusion about where this is a hydrophobic or hydrophilic amino acid. For example, even phenylalanine only has an aromatic ring, tyrosine is more soluble than it in water. In contrast, tyrosine has been found through x-ray crystallography to reside in a hydrophobic environment regardless of it hydroxyl group.

Based on what the textbooks and online resources use as their criteria will determine if they characterize tyrosine as polar or non-polar. For academic purposes, I would go by what your textbook characterizes it as. For research purposes, know that tyrosine has this unique dual property that can be exploited in experiments.

To answer your question of how it functions in neutral pH and in aqueous medium, tyrosine will act as a hydrophilic amino acid.