Isomers/Enantiomers of Amino Acids

Pre-College Topics High School Life
1

<p>Not sure where to put this.</p>

<p>[The</a> Amino Acids](<a href=“http://chemed.chem.purdue.edu/genchem/topicreview/bp/1biochem/amino2.html#table]The”>http://chemed.chem.purdue.edu/genchem/topicreview/bp/1biochem/amino2.html#table)</p>

<p>Scroll down a little and you’ll see a big list of amino acids and their structures.</p>

<p>Which ones out of those amino acids have only one isomer (no enantiomers are possible)?</p>

<p>I know what an isomer is, but I’m a little confused as how to figure out which ones only have one. Any help is greatly appreciated. Thanks! :)</p>

2

<p>Glycine. Ok so a molecule has 2^n enantiomers if it has n chiral centers. A chiral center is a carbon atoms that has 4 different substituents. Basically if a carbon has 4 different substituents there are 2 possible relative orientations for its substituents. Look at this picture to get an image of it:
<a href=“http://www.lbl.gov/Science-Articles/Archive/assets/images/2002/Apr-22-2002/Enantiomers.jpg[/url]”>http://www.lbl.gov/Science-Articles/Archive/assets/images/2002/Apr-22-2002/Enantiomers.jpg&lt;/a&gt;
Remember that isomers have the same molecular formula. Enantiomers have the same molecular formula as well, however they are different molecules due to the fact that the relative position of the various substituents of all the chiral carbons are different.</p>

<p>So for a molecule to have no enantiomers it must have n = 0 chiral centers. Hence every carbon in the molecule must have less than 4 different substituents (if the carbon is double bonded to something, it has less than 4, if it has 2 hydrogens bonded to it that is less than 4 different substituents as well). In other words, if a carbon had a Chlorine, Fluorine, Bromine, and a Hydrogen bonded to it, it would be chiral. If it had a Chlorine, Fluorine and 2 Hydrogens it would not.</p>

<p>So now notice that all amino acids have a carbon bonded to an H3N+ (amino group), a COO- (carboxyl group), a Hydrogen, and what I’ll call an “R” group. In the case of Aspartate, R = CH2COO-. So in the case of Glycine R = H. But remember that carbon already has a Hydrogen bonded to it, hence it has less than 4 different substituents bonded to it. Hence the carbon is not chiral and Glycine does not have a pair of enantiomers. But if you look at every other amino acid, they all have 4 different substituents bonded to that central carbon, and hence that carbon is chiral and the molecule has 2 possible enantiomers.</p>