Reversing Polarity to Conserve Chirality
By: Carol A. Rouzer, VICB Communications
Published: June 24, 2010
The Johnston lab develops an innovative approach to chiral peptide synthesis.
The concept of chirality - nonsuperimposable mirror images - is a recurring theme in nature. It is immediately evident when comparing the right and left extremities of a bilaterally symmetrical organism, such as the hands of a human being. The two hands may be placed palms together, and they will line up as if each hand were “seeing” itself in a mirror. However, if the hands are placed one on top of the other, both palms down, the fingers and thumbs do not line up - they are not superimposable (Figure 1). In the case of the two halves of a human body, both the right and left sides are present. But chirality also occurs at the molecular level, and in that case, usually only one “side” is found.
Figure 1. Hands are an example of chiral objects. Held with palms together they are mirror images of one another, but they cannot be superimposed if both palms are down. In contrast, the bookends are also mirror images of one another, but they are superimposable when facing in the same direction. These objects are not chiral.
Molecular chirality in biochemistry results from the fact that carbon atoms, which form the framework of all bio-molecules,
often bind four other atoms or groups of atoms in a tetrahedral shape (Figure 2). If all four binding partners are different, the resulting molecule will exist as two non-superimposable mirror images, or enantiomers. When such molecules are synthesized in a test tube, both enantiomers will usually be formed in equal quantities. However, when chiral molecules occur in nature, usually only one enantiomer is found. The reason for this is that biomolecules are most often synthesized by enzymes which are themselves an enantiomer existing in only one form (Figure 3). Just as a right glove can only fit onto the right hand, the structure of a molecule synthesized by a “right handed” enzyme will be determined by the geometry of that enzyme.
Figure 2. Example of a chiral molecule with carbon Figure 3. Diagram of the structure of an enzyme. The protein
at the center and four different atoms attached at is formed from two long chains of amino acids coiled into a
each point of a tetrahedron. The three views on the specific shape that is a single enantiomer. Its mirror image is
left are the same molecule, rotated on an axis drawn not found in nature. Image courtesy of Wikimedia Commons
through the iodine and carbon atoms. The same is true under the GNU Free Documentation License.
for the three views on the right. The molecules on the
left and right are mirror images of each other, but
cannot be superimposed.
Molecular chirality poses a particular challenge for the chemist who wishes to synthesize biologically relevant molecules in the laboratory. He/she may choose to accept the fact that both enantiomers will form in the test tube environment and develop a way to isolate the biologically relevant one, which is often quite difficult. The alternative approach is to develop a chiral laboratory environment that will produce only the desired enantiomer. This is the approach taken by Jeff Johnston and his group, who have just reported their exciting new approach to the chiral synthesis of peptides. [Shen and Johnston Nature 465, 1027-1032 (24 June 2010) doi:10.1038/nature09125 Article]. Proteins are comprised of chains of twenty different chiral amino acid building blocks which are present in only one enantiomeric form (Figure 4).
Figure 4. Generalized structure of an amino acid consisting of a central carbon atom, a carboxyl group (COOH), an amino group (NH2), a hydrogen atom (H) and a fourth group (R) that can be one of 20 different possibilities. The enantiomer on the left is used in protein synthesis. The one on the right is rarely found in living organisms. Image courtesy of Wikimedia Commons under the GNU Free Documentation License.
In the cell, proteins are synthesized on complex structures, the ribosomes, which link the carboxylic acid functional group of one amino acid to the amino group of the next amino acid forming an amide (peptide) bond one at a time (Figure 5). The laboratory synthesis of shorter amino acid chains (peptides) first devised by Bruce Merrifield in 1963, takes a similar approach, starting with the pre-formed amino acids, and forming each peptide bond in turn. This approach revolutionized the field of peptide biochemistry and won Merrifield the 1984 Nobel Prize in Chemistry. Now refined and automated, Merrifield’s method is widely used, but it has its disadvantages. These include the requirement for large excesses of reagents, and the possibility that the chirality of an amino acid may be inverted (switched to the opposite enantiomer) under the conditions of the process. The Johnston lab’s approach to chiral peptide synthesis avoids these pitfalls.
Figure 5. Structure of a peptide made from just two amino acids. The peptide
bond is formed from the carboxyl group of the first amino acid and the amino
group of the second, indicated by the arrows.
The Johnston lab’s innovative method does not start with amino acids. Instead, they use alpha-bromo-nitroalkanes and amines as building blocks (Figure 6). This choice has a particular advantage for the Johnston group because they have already developed methods to synthesize chiral versions of each kind of molecule. A second advantage to their approach is that the reaction can be run under very mild conditions, helping to prevent reversal of chirality in either of their starting materials or the final product.
Figure 6. Examples of a chiral alphabromo-nitroalkane (top left) and a chiral amine (top right) used for peptide synthesis by the Johnston group. The bromo, nitro, and amino groups are indicated by arrows. The bottom structure is the peptide resulting from the reaction of the top two compounds. The arrows indicate the atoms that form the peptide.
The reaction between the alpha-bromonitroalkane and the amine that forms the new peptide bond will tweak the interest of any organic chemist due to its interesting mechanism. In peptide synthesis using amino acids as starting materials, the amino group of one amino acid donates electrons to the carboxyl carbon atom of the second amino acid in order to form the peptide bond. In the Johnston group’s approach, the amino group of the starting amine serves as the acceptor for electrons donated by the alpha-bromo-nitroalkane, which eventually forms the carbon atom of the new peptide bond. This reversal of bond-forming polarity, referred to as umpolong in organic chemistry, provides an interesting mechanistic puzzle for future research. It also ensures that syntheses of particularly difficult peptides can be carried out successfully without loss of desired chirality. The Johnston group is now actively applying their newfound technology to the synthesis of difficult peptide natural products.