Problem 25 - Basic Principles of Organic Chemistry

Question

Indicate the steps that would be necessary to attach each of the amino acids listed to the N-terminus of a peptide chain. Assume that any side-chain functions in the peptide are suitably protected, but do not assume that the amino acids will couple with the peptide without suitable protection of their functional groups. a. lysine b. aspartic acid c. cystine d. serine

Step-by-Step Solution

Answer

Protect functional groups, activate the carboxyl group, attach to the peptide, then deprotect.

Step 1: Protect the Amino Acid Functional Groups

Before attaching each amino acid to the N-terminus of a peptide chain, protect the functional groups that might react during the coupling process. For lysine, protect the amine group on the side chain with a protecting group like Boc (tert-butyloxycarbonyl). For aspartic acid, protect the carboxyl group with a group like tert-butyl. For cystine, ensure the thiol groups are protected, often using acetamidomethyl or trityl groups. For serine, typically, the hydroxyl group is protected with a benzyl group.

Step 2: Activate the Carboxyl Group for Coupling

Activate the carboxyl group of each amino acid so it can be attached to the N-terminus of the peptide chain. This is often done using a coupling reagent like DCC (dicyclohexylcarbodiimide) or HBTU (O-(benzotriazol-1-yl)-N,N,N′,N′-tetramethyluronium hexafluorophosphate).

Step 3: Attach to the Peptide Chain

Once the functional groups are protected and the carboxyl group activated, attach the amino acid to the N-terminus of the peptide chain under mild conditions that prevent side reactions. Make sure the amino terminus of the peptide is deprotected prior to this step to allow coupling.

Step 4: Deprotect the Newly Attached Amino Acid

After successful attachment, remove the protecting groups without disrupting the newly formed peptide bonds. This might involve using specific deprotection conditions like acidic treatment for Boc groups or oxidative conditions for disulfide-linked cystine.

Key Concepts

Amino Acid Protection

Amino acids are the building blocks of peptides, but their reactivity can cause unintended side reactions during synthesis. To control this, we protect certain parts of the amino acid by blocking reactive groups temporarily. In peptide synthesis, protecting amino acids is crucial to preventing unwanted reactions. For example, lysine has an amine side group that is highly reactive, so we use a Boc (tert-butyloxycarbonyl) group to protect it.
Similarly, cystine has reactive thiol groups that we might protect using acetamidomethyl groups. For aspartic acid, the carboxyl group may be capped with a tert-butyl group, while serine often has its hydroxyl group protected by a benzyl group. This prevents them from reacting during the synthesis until we need them to.
These protections ensure that each amino acid only reacts where and when we want it to, like attaching to the N-terminus of a peptide chain.

Peptide Bond Formation

Forming peptide bonds is the process of linking amino acids together into chains. This process simulates how proteins are made naturally in cells. In chemical synthesis, the goal is to repeat this reaction carefully to build peptides with specific sequences.
When an amino acid is added to a growing peptide chain, the carboxyl group of the new amino acid reacts with the amino group of the peptide. To ensure this reaction occurs smoothly, we must first "activate" the carboxyl group.
This carefully controlled reaction forms a peptide bond, linking the new amino acid to the chain. Once all desired amino acids are attached, a complete peptide structure is formed.

Functional Group Activation

Before amino acids can be attached to a peptide chain, their carboxyl groups need to be activated. Activation makes it easier for the amino acid to form a bond with the peptide's N-terminus.
Common activation methods involve reagents like DCC (dicyclohexylcarbodiimide) or HBTU (benzotriazol-1-yl) that help form reactive intermediates. These intermediates facilitate the formation of peptide bonds by making the carboxyl group more reactive.
With an activated carboxyl group, the amino acid is primed to join the peptide chain without causing other reactions to occur.

Protecting Groups

Protecting groups are temporary modifications to amino acids, safeguarding reactive parts during peptide synthesis. These groups are chosen based on the functional group they protect.
For example, an amine group may be protected with a Boc group, while a thiol may be protected using trityl. The choice of protecting group depends on both the target aminodelivery and the specific conditions of the synthesis.
Importantly, these groups must be stable enough to withstand peptide synthesis, yet easily removable afterward to restore the amino acid’s original functionality.

Solid-Phase Synthesis

Solid-phase synthesis is a method that simplifies peptide synthesis. Invented by Bruce Merrifield, this technique involves attaching the starting peptide to an insoluble resin. This keeps the peptide immobile, while allowing reagents to wash over it during the synthesis process.
Advantages of solid-phase synthesis include the ease of removing excess reagents and impurities by simple washing. Further, it allows for automation of the peptide synthesis process. After all amino acids have been added in sequence, the complete peptide is cleaved from the resin.
Solid-phase synthesis makes it much easier to build long or complex peptides by repeating cycles of protection, activation, coupling, and deprotection.