PEPTIDE SYNTHESIS

PEPTIDE SYNTHESISTo form a peptide bond, the N-terminus of one molecule must be joined to the C-terminus of another molecule.

DCC is a commonly used coupling agent.25.6 Peptide SynthesisCopyright 2012 John Wiley & Sons, Inc.Klein, Organic Chemistry 1e 25-2

DCC coupling mechanism:25.6 Peptide SynthesisCopyright 2012 John Wiley & Sons, Inc.Klein, Organic Chemistry 1e 25-4

DCC coupling mechanism:25.6 Peptide SynthesisCopyright 2012 John Wiley & Sons, Inc.Klein, Organic Chemistry 1e 25-5

DCC coupling mechanism:25.6 Peptide SynthesisCopyright 2012 John Wiley & Sons, Inc.Klein, Organic Chemistry 1e 25-6

DCC coupling cannot give the selectivity necessary to connect amino acids in a specific sequence.25.6 Peptide SynthesisCopyright 2012 John Wiley & Sons, Inc.Klein, Organic Chemistry 1e 25-7

The N-terminus of one molecule can be protected, and the C-terminus of the other can be protected.

WHAT is a protecting group?25.6 Peptide SynthesisCopyright 2012 John Wiley & Sons, Inc.Klein, Organic Chemistry 1e 25-8

Once they are protected, the groups can be coupled using DCC, only giving one possible result.

Deprotection yields the desired dipeptide.25.6 Peptide SynthesisCopyright 2012 John Wiley & Sons, Inc.Klein, Organic Chemistry 1e 25-9

Removing the protecting group

Another popular amine protecting group is Fmoc.

The mechanism for Fmoc installation is similar to that for the Boc protecting group.Protecting groups are often sterically hindered. WHY?Copyright 2012 John Wiley & Sons, Inc.Klein, Organic Chemistry 1e 25-14

Fmoc removal is generally performed with a base.

Draw the resulting products.Again, the evolution of gas helps push the reaction forward.Copyright 2012 John Wiley & Sons, Inc.Klein, Organic Chemistry 1e 25-15

The carboxylic acid group of the amino acid is often protected with an ester.

Methyl and benzyl esters are popular. WHAT do they look like?Hydrolysis is generally used to cleave the ester. WHAT conditions are necessary?25.6 Peptide SynthesisCopyright 2012 John Wiley & Sons, Inc.Klein, Organic Chemistry 1e 25-16

Benzyl esters can also be removed with H2 or with HBr.

HOW many total reaction steps will be necessary to couple two amino acids in a specific order using protecting groups?25.6 Peptide SynthesisCopyright 2012 John Wiley & Sons, Inc.Klein, Organic Chemistry 1e 25-17

Below are the steps required to couple alanine and glycine.

Practice with SKILLBUILDER 25.5.25.6 Peptide SynthesisCopyright 2012 John Wiley & Sons, Inc.Klein, Organic Chemistry 1e 25-18

The methods of protection, DCC coupling, and deprotection are not practical for forming larger peptides.Imagine an average of 95% yield for each reaction, what would the overall yield be for the formation of a tetrapeptide?The Merrifield synthesis is a superior method reducing the number of synthetic steps significantly.First, amino acid groups are attached to the surface of polymer beads. WHY are beads used?Copyright 2012 John Wiley & Sons, Inc.Klein, Organic Chemistry 1e 25-19The Merrifield synthesis is a superior method reducing the number of synthetic steps significantly.

WHY is the Boc protecting group necessary in this step?The Merrifield peptide synthesisCopyright 2012 John Wiley & Sons, Inc.Klein, Organic Chemistry 1e 25-20

The protecting group is removed, and a different amino acid is coupled to the chain.

WHY is the Boc protecting group necessary in this step?This process eliminates the need to protect the carboxylic acid group.Copyright 2012 John Wiley & Sons, Inc.Klein, Organic Chemistry 1e 25-21

The Merrifield synthesis allows for long chains to be synthesized through sequential addition of each amino acid one at a time.Purification is simple. WHY?When the synthesis is complete, the chain is cleaved from the polymer bead using HF.In 1969, Merrifield used the technique to synthesize ribonuclease (128 residues) with an overall yield of 17%.That is an average of > 99% yield on each reaction step.Merrifield won the Nobel prize in 1985Copyright 2012 John Wiley & Sons, Inc.Klein, Organic Chemistry 1e 25-22Merrifield synthesis of Ile-Gly-Leu-Ala-Phe