Merrifield Solid-Phase Peptide Synthesis - Online Tutor, Practice Problems & Exam Prep

The Merrifield solid phase peptide synthesis method involves four key steps: amino acid protection, attachment to a chloromethylated polystyrene bead via an SN2 reaction, hydrolysis to deprotect the amino group, and peptide chain release using hydrofluoric acid. This automated process allows for the sequential addition of protected amino acids, forming peptide bonds through condensation reactions, while facilitating the isolation of synthesized peptides. Understanding this method is crucial for applications in organic synthesis and peptide chemistry.

Merrifield Solid-Phase Peptide Synthesis Concept 1

Everyone, in this video, we're going to take a look at the Merrifield solid phase peptide synthesis process. Now, it itself is an automated method that uses a polystyrene bead in synthesizing peptide chains. Remember, polystyrene itself is a polymer created from ethylene, benzene, or styrene. If we take a look here, styrene is a benzene ring connected to two double-bonded carbons.

And we can say, we can link them together through polymerization to create our polystyrene. Our polystyrene, we're going to highlight as an orange structure. Now, the Merrifield solid phase peptide overview is that we use a chloromethylated polystyrene. So we've added, basically, to one of these rings here. We've added a CH2Cl, just to one of the two rings.

And we're going to use what's called a protected amino acid. Now typically, we'd have this free amino group here on this end, but we've protected it by using an acyl group here in order to protect it. And in that way, through an SN2 process, this oxygen can come here, hit this methylene carbon, and kick out the chlorine. And in this way, we've linked this protected amino acid to my polystyrene. Now, the way it works is the polystyrene, again, remember it's a bead, a physical bead, and what we're doing is we're growing a peptide chain on that bead and we can grow the chain as long as we need. And at the end, we can cleave off that chain that we've grown on the bead, and in that way, isolate a peptide chain.

So, this is basically the whole process of the Merrifield solid phase peptide synthesis method.

Merrifield Solid-Phase Peptide Synthesis Concept 2

Hey everyone. So, when we're talking about the Merrifield peptide synthesis method, we're gonna see that it has 4 required steps. Now, step 0 is not part of these 4 required steps, that's just prep work that we do outside of this process. In it, we're gonna have amino acid protection. So a condensation reaction occurs between a carboxylic acid and the N-terminus end of an amino acid.

So here's our carboxylic acid and the amino acid. It's a condensation reaction, so we have the loss of water. And in this process, we created our protected amino acid. Here, we have added this silyl group to our amino group to protect it. And what we're gonna say next is step 1 finally, we have an SN2 process, where the protected amino acid reacts with a chloromethylated polystyrene bead.

To show the polystyrene chain in a simple five version, we just have this "P" here. And what's gonna happen is this oxygen comes in, hits here, and kicks out the CH₂Cl from the CH₂, and we've attached this protected amino acid to this polystyrene bead. So here it goes. This leads us to step 2, which is a hydrolysis reaction where we have the deprotection of the N-terminus end of the amino acid.

So, basically, we're gonna remove the acyl protecting group here, exposing that amino acid. So now we have that unprotected amino acid. And at this point, you're gonna go into the cyclic part of the Merrifield synthesis process. We're gonna attach as many amino acids as we want to exposed amino acid ends.

And this can go several times. We could do 3a to 3b, back to 3a to 3b over and over again as many times as we want. So here we're going to say that 3a is our peptide bond formation, which is another protected amino acid attaches to the N-terminus end of the amino acid. This is done with the help of DCC, which will help to adsorb the water that's lost. And we're gonna say here, we lose water.

So remember, this is just a simplified way of explaining what's going on. DCC there has just helped to facilitate this process to happen. And we're gonna say here, losing that water gives us now this carbonyl connected to the amino group. It still has its alpha carbon and R group. And then we're gonna have this still protected and here.

So we have this structure here. Now, we can go into hydrolysis for part 3b, which again remember is the deprotection of the N-terminus end of the amino acid. So all that is is us removing this acyl group. So when we do that, we get this unprotected amino end. Now, again, we can go if we wanted this way and react with yet another protected amino acid, increasing the chain length, or instead, I can just book it this way and just use HF and end the process.

Here, we're gonna say, step 4 is the peptide chain release. The peptide chain is released upon treatment with hydrofluoric acid. This would sever this connection of the growing amino acid chain from the polystyrene bead. When we do that, we've exposed now the peptide chain. We'll just add the rest of what's attached.

So there goes my dipeptide that I've just formed. We'd also have the rest of the structure as waste. So that benzene is still there. So I have the CH₂ group, and it would combine with the Fluorine from the HF. But here, we don't really care too much about the structure; we care about the peptide that I just made.

Alright. So this is the Merrifield peptide synthesis process.

Merrifield Solid-Phase Peptide Synthesis Example 1

Hey, everyone. So here it says identify the steps needed to prepare the following dipeptide. So we have valine and methionine based on a Merrifield synthesis. Alright. So what we're gonna start with is we're gonna start with our polystyrene bead.

And what we're gonna do here is we're gonna actually react this with a chloromethoxyethane group. So chloromethoxyethane is Cl connected to CH₂, connected to oxygen, connected to an ethyl. And that's gonna go over our catalyst to help this happen, which is Tin 4 Chloride. So what this process does is it gives us, so remember we talked about this CH₂Cl group? That's how we get it on with this first additional step.

And now what we have to do is we have to add these 2 amino acids to this polystyrene bead and then cleave it off at the end. So we're gonna say here, when we introduce these amino acids, remember we introduced them as protected groups. So with valine, here's our carboxyl group. Here's the amino group. Here's the protected group, and remember, it's valine.

Its group would be this isopropyl group. Right? We know that this would come in, hit here, and kick this out, and we get this whole group being added. Remember, this would be step 1.

Step 2 would be DCC, and then step 3 would be H₃O⁺ to cleave off the protected end. Doing these steps would give us this. Let's write this CH₂. And again, we've attached the amino acid to it. Don't forget the R group.

Alright. So now we have this. Now we gotta do it again. Now we wanna add the methionine. So yet this is another amino acid that has to be in its protected form.

So here goes the carboxyl. Alright. And then we're gonna say here to protect it and add it. And here, methionine, remember it has a sulfur involved. So we have an ethylene group, then sulfur, and then a methyl.

We do DCC again, followed by H₃O⁺, which would cleave off the protecting end. So we get, Okay. I'm gonna attach this amino acid, and we're gonna attach to it this group here. So we have that. We have NH.

Oops. We did that. Now we'd have this. Then we're gonna have NH₃⁺. There we go.

And then we do finally we're gonna do our HF step. Okay. Because remember, when we use HF here, it's gonna cut this part off completely. And what we care about is just the peptide. So what we have now is, and then NH₃⁺.

So this would be our final answer, and these would be the steps needed to grow this dipeptide from a polystyrene bead.

Provide the peptide chain structure produced from the following sequence of reagents. (Note: Show the ionic forms for the N and C terminus ends).

Everyone. So here, it says, provide the peptide chain structure produced from the following sequence of reagents. Note, show the ionic forms for the n and c terminus ends. So, remember we have our polystyrene here. We use our chloromethoxyethane over this catalyst in order to create this, our chloromethylated polystyrene. And what we're doing here is we're attaching to this methylene and amino acids, making a peptide chain. And what are the amino acids that we're attaching? Well, it's these ones here in spheres. We're adding threonine (T), phenylalanine (F), leucine (L), and glycine (G). Remember, these are the one-letter codes for amino acids. So these are the amino acids that we're going to link together. And based on the way I'm linking them here, I'm just gonna draw the peptide chain. Typically, we draw it from the N terminus to the carboxyl end on the right. So, we're gonna show that. So, here goes my protonated end here. And I'm gonna add the R groups afterwards. We're just showing the chain itself. Again, if you don't remember how to do this, make sure you go back and take a look at our chapter on amino acids to help you remember the process of doing this. Okay. And then here we have our carboxyl end. Again, the amino acids we have here are threonine (T), to phenylalanine (F), to leucine (L), to glycine (G). So here, we're gonna add the R groups here. So if we're talking about threonine, this would be its R group. For phenylalanine, it's this CH2 connected to a benzene ring. For leucine, here, we'd have it as this. And finally, glycine, the simplest amino acid, has as its R group just an H. You don't have to show it if you don't want to. But here, in essence, this is what we're doing. We don't even need to go through too much of this process. We know we're linking these four amino acids. We know at the end we're going to expose the amino end. And we know HF is just gonna cleave off this growing peptide chain from the polystyrene bead. Right. So, again, this would be the, in this case, our tetrapeptide chain that we've created from the Merrifield synthesis process.

Starting from polystyrene, provide the synthetic pathway in forming the tripeptide Gly-Leu-Ser using the Merrifield peptide synthesis method. 

Everyone, so it says, starting from polystyrene, provide the synthetic pathway in forming the tripeptide. Here we have Glycine, Leucine, and Serine using the Merrifield peptide synthesis method. Alright. So the first thing we're going to do is we have to chloromethylate the polystyrene. So here goes our polystyrene. It's chloromethylated; we're going to use our chloromethoxyethane. That is ClCH2OC2H5 over our catalyst of Tin(IV) chloride. That would add CH2Cl to the structure. And now we just have to go through the process of adding our amino acids. So here we're dealing with Glycine first. Remember with glycine, it's the simplest amino acid. Its R group is just an H. Here is the acyl group that's added to the amino end. We facilitate this through DCC, and then we use H3O+ to expose and remove the acyl end and expose that amino group. So, here we have CH2. And again, we're connecting here. Now it would be an NH3+. Now we do it again to add the next amino acid, which is Leucine. So here we'll do it in a different color. Here is our group. It's this structure. We've protected this. DCC facilitates the condensation reaction of what happened between the exposed amino and the NH3+ with this protected amino acid, and then H3O+ would remove this acyl group from this protected amino acid end. So, here, this gets longer and longer. So we have this. And then finally, we add the last amino acid here, which we're going to say here is this carboxyl end. Ok. And then our R group here is this. And here, we're going to use DCC again to help facilitate it, H3O to remove the acyl end, and then finally, HF to just cut it off from the whole thing. So what we're going to get here at the end is our, and here, I'm just going to draw it as a carboxyl end to the amine to the amino end. Usually, it's the opposite, but just for simplicity, we'll keep it this way. So we have this here, which is then connected here to this, and we're going to have here our R group, and then our exposed amino end here. So this would represent our final tripeptide that we created through the Merrifield process. So, this would be our final answer.