Functional Groups in Organic Chemistry
Acids and Bases
Organic Acids and Bases
|One aspect of acid-base chemistry you need to pay a bit of attention to is the titration of amino acids. The MCAT plays something of a game with the amino acids. Although you don't need to memorize all the structures, you do need to go into the exam pretty well familiar with amino acid chemistry.|
Although the full biochemistry course is not a formal prerequisite for the MCAT, your knowledge of the amino acids (and most basic biochemistry) does need to be a bit past the typical Bio 101 course if you want to score a superior score on the exam. Just push the envelope a little bit, and you will be fine.
Standard α-amino acids possess two ionizable groups, a carboxyl group and an amine group bonded to a central carbon. After peptide bond formation these two groups will no longer be ionizable, having been taken up as part of the amide linkage (except at the C- and N- termini).
Seven of the amino acid constituents of proteins also possess an ionizable side chain in addition to the two groups on the central carbon. Four amino acids contain acidic side chains. Aspartic acid and glutamic acid contain a carboxyl group, cysteine a sulfhydral group, and tyrosine a phenolic hydroxyl group. Three amino acids contain basic side chains. Lysine, arginine and histidine all contain an amine group.
The curves describing the titration of a simple amino acid (one carboxyl and one amine group) with a strong base (beginning in acidic pH with the amine in ammonium form) shows the loss of each proton in step-wise fashion. The pH at which the molecule carries no net electric charge is the isoelectric point, which is the average of the two pKas for the successive ionization steps. The titration curves of amino acids with acidic or basic groups in their side chains have three steps with the isoelectric point midway between the pKa values of the monocation and monoanion forms. The titration curves of proteins are complex, having many ionizable groups.