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Introduces compounds consisting of two or more amino acids linked in a chain.

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Ball and stick structure of a small peptide

Credit: Image copyright molekuul.be, 2014
Source: http://www.shutterstock.com
License: CC BY-NC 3.0

Are your cells smarter than you?

Cells in our bodies have an intricate mechanism for the manufacture of proteins. Humans have to use other techniques in order to synthesize the same proteins in a lab. The chemistry of peptide synthesis is complicated. Both active groups on an amino acid can react and the amino acid sequence must be a specific one in order for the protein to function. Robert Merrifield developed the first synthetic approach for making proteins in the lab, a manual approach which was lengthy and tedious (and, he won the Nobel Prize in Chemistry in 1984 for his work). Today however, automated systems can crank out a peptide in a very short period of time.


A peptide is a combination of amino acids in which the amino group of one amino acid has undergone a reaction with the carboxyl group of another amino acid. The reaction is a condensation reaction, forming an amide group (CO−N), shown below.

Condensation reaction of two amino acids to form a peptide

Credit: User:YassineMrabet/Wikimedia Commons
Source: http://commons.wikimedia.org/wiki/File:Peptidformationball.svg
License: CC BY-NC 3.0

Amino acids join together to form a molecule called a dipeptide. The C−N bond is called a peptide bond. The order of amino acids is by convention shown with the free amino group on the left and the free carboxyl group on the right.[Figure2]

A peptide bond is the amide bond that occurs between the amino nitrogen of one amino acid and the carboxyl carbon of another amino acid. The resulting molecule is called a dipeptide. Notice that the particular side chains of each amino acid are irrelevant since the R groups are not involved in the peptide bond.

The dipeptide has a free amino group on one end of the molecule and a free carboxyl group on the other end. Each is capable of extending the chain through the formation of another peptide bond. The particular sequence of amino acids in a longer chain is called an amino acid sequence. By convention, the amino acid sequence is listed in the order such that the free amino group is on the left end of the molecule and the free carboxyl group is on the right end of the molecule. For example, suppose that a sequence of the amino acids glycine, tryptophan, and alanine is formed with the free amino group as part of the glycine and the free carboxyl group as part of the alanine. The amino acid sequence can be easily written using the abbreviations as Gly-Trp-Ala. This is a different sequence from Ala-Trp-Gly because the free amino and carboxyl groups would be on different amino acids in that case. 


  1. What is a peptide bond?
  2. How is this different from an ester?
  3. You mix two amino acids together: glu and ser. If you reacted them, how many different dipeptides would you expect to get? List them.

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Image Attributions

  1. [1]^ Credit: Image copyright molekuul.be, 2014; Source: http://www.shutterstock.com; License: CC BY-NC 3.0
  2. [2]^ Credit: User:YassineMrabet/Wikimedia Commons; Source: http://commons.wikimedia.org/wiki/File:Peptidformationball.svg; License: CC BY-NC 3.0

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