Peptide Sequencing: Partial Hydrolysis with Cyanogen Bromide - Online Tutor, Practice Problems & Exam Prep

Cyanogen Bromide (BRCN) is a highly specific reagent used for peptide sequencing, cleaving peptide bonds at the carboxyl group of methionine. The mechanism involves an SN2 reaction where nucleophilic sulfur attacks carbon, forming a sulfonium ion. This is followed by an intramolecular SN2 reaction leading to a five-membered methionine derivative ring. Acid-catalyzed hydrolysis then cleaves the methionine derivative from the peptide chain, regenerating the carbonyl and protonating the nitrogen. Understanding this process is crucial for analyzing peptide structures and sequences.

Peptide Sequencing: Partial Hydrolysis with Cyanogen Bromide Concept 1

Hey, everyone. So let's take a look at peptide sequencing, and here we're going to talk about partial hydrolysis by Cyanogen Bromide. Now, Cyanogen Bromide, which is BrCN, only cleaves peptide bonds on the carboxyl group of methionine. And we're going to say, yes. Enzymes can be very specific in where they cleave in terms of amino acids, but when it comes to cyanogen bromide, it's even more specific.

And, it has no exceptions. So this would be a great thing to use when it comes to partial hydrolysis of our peptide chain, which represents the first step in peptide sequencing. So now that we know that cyanogen bromide is so highly specific, let's go to the next video, and let's take a look at an example deploying it.

Peptide Sequencing: Partial Hydrolysis with Cyanogen Bromide Example 1

Hey everyone. So in this example question, it says, cyanogen bromide was used to cleave an unknown peptide. Propose a possible peptide sequence given the following fragments. And here's a big hint: there are no methionine to methionine peptide bonds.

Right. So since there are no methionine to methionine peptide bonds, that means that these two are not connected. So we can bring down this fragment by itself. And remember that cyanogen bromide cleaves at the carboxyl side, right, the carboxyl group. So here, it was cleaved, so this fragment was separated from something.

What was it separated from? It had to have been this fragment here. Now, at this point, we've connected these two fragments together, but where does this other methionine fragment go? Does it go to the beginning of this peptide sequence or at the end? Remember, we can only cleave on the carboxyl side, so this side here.

If I put it here, it wouldn't work because again I'm cutting the carboxyl side. This fragment would not be able to be by itself. So methionine wasn't on this side. Methionine had to be over here on this side. We cut or cleave the carboxyl side and that's how we want it to be by itself as one of our three fragments.

Right. So here, this represents the sequence, a possible sequence of this peptide chain. So we have methionine followed by this four amino acid fragment, and then followed by this three amino acid fragment. And this will be our final answer.