Question

Describe the process of solid-phase peptide synthesis.

Short answer

Answer: In the Merrifield method of solid-phase peptide synthesis, the key steps involved are: 1) Attachment of the first amino acid to the solid support, 2) Deprotection of the α-amino group, 3) Coupling of the subsequent amino acids, 4) Washing and repetition of deprotection and coupling steps, and 5) Cleavage of the peptide from the resin and final purification.

Step-by-step solution

1. Introduction to Solid-phase Peptide Synthesis

Solid-phase peptide synthesis (SPPS) is a method used for the chemical synthesis of peptides and proteins. This technique involves the sequential addition of amino acids to a growing peptide chain while anchored to a solid support. The process simplifies the synthesis of peptides by allowing for faster reaction times and easier purification steps compared to solution-phase synthesis. The most common approach for SPPS is the Merrifield method, which uses a resin as the solid support.

2. Role of the Solid Support

In the SPPS process, the solid support (usually a resin) is responsible for anchoring and immobilizing the growing peptide chain. This feature allows for the efficient removal of excess reagents and side products during the synthesis, simplifying purification and yielding the desired peptide sequence. The resin consists of small beads with a large surface area, providing multiple attachment sites for the growing peptide chain.

3. Attachment of the First Amino Acid

The first amino acid is attached to the solid support by forming a covalent bond between the carboxyl group of the amino acid and the free available functional groups on the resin. The α-amino group of the amino acid is protected by a temporary protecting group, such as Fmoc (9-fluorenylmethoxycarbonyl), to prevent unwanted side reactions.

4. Deprotection

Next, the Fmoc protecting group is removed by treating the resin-bound amino acid with a deprotection solution, such as a solution of piperidine in dimethylformamide (DMF). This process reveals the α-amino group, making it available for the coupling reaction with the following amino acid.

5. Coupling

The next amino acid (also protected with an Fmoc group) is added to the reaction along with an activating agent, which facilitates the amide bond formation between the α-amino group of the incoming amino acid and the carboxyl group of the resin-bound amino acid. This process is called coupling, and it results in the elongation of the peptide chain by one amino acid.

6. Washing and Repetition

After coupling, the resin is washed to remove any excess reagents and side products. These washing and deprotection steps are repeated for each amino acid until the desired peptide sequence is obtained.

7. Cleavage and Final Purification

Once the desired peptide sequence is assembled, the peptide is cleaved from the resin and fully deprotected using a cleavage cocktail, usually containing trifluoroacetic acid (TFA). The peptide is then precipitated with cold ether, collected by centrifugation, and purified using high-performance liquid chromatography (HPLC) to obtain the final product. In summary, solid-phase peptide synthesis is an efficient and widely used method for peptide synthesis that involves the sequential addition of amino acids to a growing peptide chain while attached to a solid support. The process includes the steps of attachment, deprotection, coupling, repetition, and cleavage, followed by purification to obtain the desired peptide sequence.