Wednesday, August 19, 2015

Amino acids with nonpolar aliphatic side chains



As I mentioned here on the blog, in an earlier post (this post), the standard amino acids differ in the chemical composition of their side chains. The 20 standard amino acids can be divided in 5 groups, according to the physico-chemical properties of the side chain, in particular, according to their polarity. Before starting to talk about this division, I want to mention that it is a division that involves some ambiguities, which I will highlight as they appear in the posts that I will devote to this matter.
I'll start by talking about the amino acids with nonpolar aliphatic side chain. First of all, it should be explained what does it means nonpolar and aliphatic. Nonpolar means that there are no significant asymmetries in the distribution of electrons on atoms. Stated more simply, if a molecule (or a side chain) it is non-polar, it contains atoms with similar electronegativities. As mentioned in a previous post (this post), if a molecule is composed only of carbon and hydrogen, it is considered non-polar. Similarly, if a side chain of an amino acid is composed only of carbon and hydrogen atoms, it is considered to be nonpolar. Regarding the term "aliphatic", this relates to the absence of aromatic rings, which are benzene ring derivatives; therefore they are cyclic structures with six vertices, all of them corresponding to carbon atoms and 3 double bonds therein (in fact they are not three double bonds, instead they are six bonds with the connection order of 1.5, but this would complicate things and may be considered three double bonds). Therefore, all amino acids having in its side chain only carbon and hydrogen atoms and that show no aromatic rings, belong to the class of amino acids with nonpolar aliphatic side chains.
They are:

Glycine – it is the simplest amino acid with a side chain consisting only of hydrogen. As the hydrogen is too small to have a major role in the interaction with other amino acid side chains, and do not have by itself a polar (or nonpolar) significant behavior, this amino acid appears in this category by deleting parts, namely because in the other categories did not make sense to include it. Glycine has the distinction of being the only standard amino acid that does not have stereoisomers because its a carbon is not chiral because it is not connected to four different substituents.
 
Alanine – its side chain is a methyl group (-CH3), which fits perfectly in the definition of nonpolar aliphatic side chain.


 Valine, leucine and isoleucine - their side chains are more complex than that of alanine, but they are composed exclusively of carbon and hydrogen atoms.
 

Methionine – another amino acid that appears in this group somewhat by a process of elimination. The sulfur atom is an inner position of the chain (is a thioether group), and does not significantly affect the polarity thereof.
 
All amino acids in this group will tend to establish London dispersion forces (so-called "hydrophobic interactions") with neighboring amino acids and, therefore, in a 3D structure of a protein, they tend to appear in proximity to each other.
I just wanted to finish with an idea that is often said in the wrong way. The amino acids shown in this post are not nonpolar amino acids, they are amino acids with nonpolar side chains. No amino acid is nonpolar because they have two very polar groups (amino and carboxylic) connected to a carbon.

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