|
Chemguide: Support for CIE A level Chemistry Learning outcome 34: Nitrogen compounds 34.4: Amino acids Learning outcome 34.4.1 This statement is about the essential chemistry of amino acids. Before you go on, you should find and read the statement in your copy of the syllabus. Important background Read the page introducing amino acids. You should read down as far as the optical activity of the amino acids (as a useful revision), but you do not need the last bit about D and L configuration. The acid-base behaviour of amino acids Simple amino acids I am taking these to be amino acids where the R group doesn't contain either an amino group or a carboxylic acid group. Read the page about the acid-base behaviour of amino acids. You can ignore the last section titled "Why isn't the isoelectric point of an amino acid at pH 7?" Amino acids with other amino or carboxylic acid groups present These include amino acids like lysine or aspartic acid.
Let's assume that only the groups at the right-hand end of these molecules become involved in forming zwitterions. The zwitterions would be:
In solution in water, the other groups would react with the water. For example, the basic amino group at the left-hand end of the lysine molecule would accept a hydrogen ion from the water like this:
Notice that the new ion has now got 2 positive and 1 negative charge, giving a net charge of +1. A solution of lysine in water would be alkaline because of the hydroxide ions formed in this equilibrium. In fact it didn't matter which of the two amino groups we assumed was a part of the original zwitterion - you would end up with the same final ion. In aspartic acid, the acidic -COOH group at the other end also reacts with water, losing a hydrogen ion to a water molecule.
Notice that this time, the final ion has a net 1- charge, and a solution of aspartic acid in water would be acidic because of the hydroxonium ions formed. We will have some more to say about this in statement 34.4.3.
© Jim Clark 2020 |
