Merrifield Solid-Phase Peptide Synthesis: Videos & Practice Problems

The Merrifield solid phase peptide synthesis (SPPS) is an automated technique used to construct peptide chains on a solid support, specifically utilizing polystyrene beads. Polystyrene is a polymer formed through the polymerization of styrene, which consists of a benzene ring bonded to two double-bonded carbons. In the context of Merrifield synthesis, chloromethylated polystyrene is employed, where a chloromethyl group (–CH₂Cl) is introduced to the polystyrene structure.

To initiate the synthesis, a protected amino acid is used. Typically, amino acids have a free amino group, but in this process, the amino group is protected by an acetyl or acyl group. This protection is crucial as it prevents unwanted reactions during the synthesis. The reaction occurs through an SN2 mechanism, where the oxygen from the protected amino acid attacks the methylene carbon of the chloromethyl group, displacing the chlorine atom and forming a covalent bond between the amino acid and the polystyrene bead.

The process allows for the sequential addition of amino acids, effectively growing the peptide chain on the solid support. This method provides the flexibility to synthesize peptides of varying lengths. Once the desired peptide chain is formed, it can be cleaved from the polystyrene bead, allowing for the isolation of the synthesized peptide. Overall, the Merrifield solid phase peptide synthesis method is a powerful tool in peptide chemistry, enabling efficient and controlled peptide synthesis.

The Merrifield peptide synthesis method is a widely used technique for constructing peptides through a series of well-defined steps. The process begins with the preparation of protected amino acids, which is essential for preventing unwanted reactions during synthesis. This involves a condensation reaction between a carboxylic acid and the N-terminus of an amino acid, resulting in the formation of a protected amino acid through the loss of water. The acyl group added to the amino group serves as a protective measure.

In the first step of the synthesis, the protected amino acid undergoes an SN2 reaction with chloromethylated polystyrene beads. During this reaction, the oxygen from the amino acid attacks the carbon attached to the chlorine, displacing the chloride ion and attaching the protected amino acid to the polystyrene support. This sets the stage for the next phase of the synthesis.

Step two involves hydrolysis, where the acyl protecting group is removed from the N-terminus of the amino acid, exposing the free amino group. This unprotected amino acid can then participate in further reactions. The synthesis can be repeated multiple times, allowing for the sequential addition of amino acids. Each cycle consists of peptide bond formation, where another protected amino acid is added to the N-terminus of the growing peptide chain. This reaction is facilitated by the coupling agent DCC (dicyclohexylcarbodiimide), which helps to absorb the water produced during the reaction, leading to the formation of a new peptide bond.

Following the peptide bond formation, another hydrolysis step occurs to remove the protecting group from the newly added amino acid, again exposing the N-terminus for further elongation. This cycle of adding protected amino acids and deprotecting them can be repeated as needed to achieve the desired peptide length.

Finally, in step four, the completed peptide chain is released from the polystyrene support. This is accomplished by treating the structure with hydrofluoric acid (HF), which cleaves the bond between the peptide and the polystyrene bead. The result is the formation of the desired peptide, along with some byproducts from the reaction.

Overall, the Merrifield peptide synthesis method is a powerful and efficient approach for synthesizing peptides, allowing for precise control over the sequence and length of the peptide chains produced.

To synthesize the dipeptide composed of valine and methionine using a Merrifield synthesis, the process begins with a polystyrene bead, which serves as a solid support for the reaction. The first step involves reacting the polystyrene bead with chloromethoxyethane, a compound characterized by a chlorine atom attached to a methylene group (–CH₂–) linked to an ether (–O–) and an ethyl group. This reaction is facilitated by a catalyst, tin(IV) chloride (SnCl₄), which helps to attach the chloromethoxyethane group to the bead.

Next, the amino acids valine and methionine are introduced to the polystyrene bead in their protected forms. For valine, the structure includes a carboxyl group (–COOH), an amino group (–NH₂), and a protecting group, which is typically an isopropyl group. The reaction occurs as the protected valine reacts with the activated bead, displacing the chlorine atom and forming a bond with the bead. This step is followed by the addition of dicyclohexylcarbodiimide (DCC), which activates the carboxyl group for peptide bond formation, and then the use of aqueous acid (H₃O⁺) to cleave the protecting group, yielding the valine residue attached to the bead.

In the subsequent step, methionine, which contains a sulfur atom in its structure, is also introduced in its protected form. The methionine structure includes a carboxyl group, an amino group, and a protecting group. Similar to the previous step, the protected methionine reacts with the bead, facilitated by DCC, followed by the removal of the protecting group using H₃O⁺. This results in the attachment of methionine to the growing peptide chain.

Finally, to cleave the dipeptide from the polystyrene bead, hydrofluoric acid (HF) is used. This step effectively removes the polystyrene support, leaving behind the desired dipeptide, which consists of the amino acid sequence of valine and methionine. The final structure of the dipeptide includes the amino groups and the respective side chains of both amino acids, completing the synthesis process.