Amino acids

1.1 Basic structure of amino acids

Amino acids are compounds which have a carboxy group at one end and an amino group at the carbon atom next to the carboxy group, the so called a-carbon (see fig. 1.1). Several amino acids contain additional acidic or basic groups.

The carboxy group will donate a proton to the amino group, so that an amino acid (in the absence of other acids or bases) will carry both a negative and a positive charge, making the whole molecule appear uncharged (zwitter-ion).

The simplest amino acid is glycine, where R is a hydrogen atom. Since the a-carbon carries only 3 different ligands (carboxy group, amino group and hydrogen), it is not enantiomeric. Thus glycine is not chiral, unlike all other amino acids which carry 4 different ligands on the a-carbon. We thus distinguish D- and L-amino acids, only L-amino acids occur in proteins and in mammalian metabolism. D-amino acids do occur in bacterial cell walls (murein) however.

1.2 The isoelectric point

From your chemistry lessons you know how to determine the pKaof an acid or base, the pH at which half of the molecules are charged. A compound which can act as both acid and base (like an amino acid) has another important property: The isoelectric point pI, which is the pH at which the number of positive charges on the molecule is the same as the number of negative charges. At the pI the molecule would therefore appear uncharged. At this pH the molecules ability to interact with water is lowest, and therefore its solubility.

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