Amino acids are a group of 21 different molecules which, when bonded in sequence, make up proteins. They consist of an organic side chain bonded to an amine and a carboxylic acid (-COOH) group (hence the name amino acid). These two different functional groups make amino acids bifunctional.
Table of Properties
A table of amino acid name, abbreviation and properties is as follows:
|Side chain polarity
|Side chain acidity or basicity
α-amino acids (α is the Greek letter alpha) have the general formula RCH(NH2)COOH. There are around 20 naturally occurring α-amino acids which polymerise by condensation polymerisation to form all known polypeptides (which form the primary structure of proteins) in biological organisms during gene translation. Some organisms contain atypical amino acids in their proteins, such as the D-amino acids, selenocystine or pyrolysine. These atypical amino acids are particularly common in organelles such as mitochondria and are encoded in variants of the standard genetic code, such as amber codons.
Due to the acidic carboxylic acid group and the alkaline amine group on amino acids they can form ions with themselves called zwitterions. The word zwitterion is derived from the German for hermaphrodite, and in this case refers to the fact that amino acid zwitterions have both positive and negative charges.
Formation of zwitterions
Zwitterions form from amino acids when the hydrogen from the carboxylic acid group (-COOH) dissociates from the molecule and forms a dative covalent bond with the nitrogen in the amine group (-NH2). This forms an ion with two charges: RCH(NH3+)COO-.
Physical properties of α-amino acids due to zwitterions
The formation of zwitterions dramatically affects the physical properties of amino acids. The zwitterion is the predominant form in the solid phase and aqueous solution. Due to the two charges on zwitterions, there is a much stronger intermolecular attraction between the ions, raising the melting point of an amino acid far above the melting point of organic compounds with similar molecular mass and numbers of electrons. For example, the amino acid glycine decomposes at 262 °C (without melting) where propanoic acid melts at -21 °C.
Optical isomerism of amino acids
All α-amino acids except glycine CH2(NH2)COOH and proline are complex enough to have the two optical isomers that are mirror images of each other (L and R). This is because they have four different groups around the alpha carbon, forming a chiral centre.
All living organisms use exclusively L optical isomers of amino acids for their proteins. The reasons of this are unknown and difficult to explain from the evolutionary point of view, as a naturally occurring chemical reaction normally produces L and R isomers in equal proportions, and most of the physical properties of the L and R forms are identical.
- Chemistry 2, Ratcliff B. & Eccles H., 2001, Cambridge University Press, ISBN 0-521-79882-5