This page looks briefly at the hydrolysis of proteins into their constituent amino acids using hydrochloric acid.
The chemistry of the reaction
If you have already studied the hydrolysis of amides under acidic conditions, you will find that this is basically the same reaction. That's not surprising because what biologists and biochemists call a peptide link (in proteins, for example) is what chemists call an amide link. With an amide like ethanamide, the carbon-nitrogen bond in the amide group is broken and you get a carboxylic acid formed:
\[CH_3CONH_2 + H_2O + H^+ \rightarrow CH_3COOH + NH_4^+\]
Now imagine doing the same thing with a simple dipeptide made of any two amino acids.
Instead of ammonium ions, you get positive ions made from the -NH2 groups reacting with hydrogen ions.
You need the extra hydrogen ion in the equation (compared with the amide equation) to react with the -NH2 group on the left-hand end of the dipeptide - the one not involved in the peptide link.
If you scale this up to a polypeptide (a protein chain), each of the peptide links will be broken in exactly the same way. That means that you will end up with a mixture of the amino acids that made up the protein - although in the form of their positive ions because of the presence of the hydrogen ions from the hydrochloric acid.
Doing the reaction
There are two ways of carrying out this reaction - an old, slow method, and a new, fast one.
The old slow way
The protein is heated with 6 M hydrochloric acid for about 24 hours at 110°C. (6M hydrochloric acid is slightly more than semi-concentrated.)
The new fast way
Protein samples are placed in tubes in a sealed container containing 6 M hydrochloric acid in an atmosphere of nitrogen. The whole container is then placed in a microwave oven for about 5 - 30 minutes (depending on the protein) with temperatures up to 200°C. The hydrochloric acid vaporizes, comes into contact with the protein samples and hydrolyses them. This method is used to hydrolyse small samples of protein during protein analysis.