Amino Acid Protonation States for Tyrosine

Question

a) At which pHs will Tyr’s amine be protonated? Deprotonated? What about its carboxylic acid and its OH? b) At which pHs will a solution with many tyrosine molecules contain a mixture of OH and O–? c) Based on your analysis in (a), which combinations of protonated and deprotonated functional groups can occur on Tyr, and at which pHs? d) How does all of this change if Tyr is part of a polypeptide?

I need help with just part d of this question. I know that tyrosine is hydrophilic when part of a polypeptide, I'm just a little unclear on how to answer the question.

Answer 1

Hi Meera! Actually Tyrosine is hydroPHOBIC due to the benezene hydrocarbon ring (hydrophobic) being in greater abundance compared to the single alcohol group of the side chain.

When part of a polypeptide, the pH at which Tyrosine's side chain is protonated or deprotonated remains the same. In terms of the charge of the polypeptide at the certain pH, you would have to figure out the charge of each individual amino acid in the polypeptide then add the charges up to get the overall charge.

Side note: A trick to determine is the side chain is protonated or deprotonated, think of pH and pKa values going at war with each other. Which ever value is greater will "win" the hydrogen atom.

Hope this helps! Message me if you have any more questions!

Kind regards,

Jonathan