For example, the titration curve for glycine looks like this: At "pH 2. A mixture of a weak acid and its conjugate base is a buffer In both regions, we can add small amounts of acid or base, and the pH will not change much. Thus, an amino acid has two regions in which it can act as a buffer. The buffer regions for glycine are pH 1. Related questions How do I determine the molecular shape of a molecule? What is the lewis structure for co2?
What is the lewis structure for hcn? How is vsepr used to classify molecules? The basic amino acids which have positively charged side chains at neutral pH have relatively high examples.
Amino acids undergo reactions characteristic of carboxylic acids and amines. The reactivity of these functional groups is particularly important in linking amino acids together to form peptides and proteins, as you will see later in this chapter.
Simple chemical tests that are used to detect amino acids take advantage of the reactivity of these functional groups. Ninhydrin is used to detect fingerprints because it reacts with amino acids from the proteins in skin cells transferred to the surface by the individual leaving the fingerprint. Amino acids can act as both an acid and a base due to the presence of the amino and carboxyl functional groups. For a solution containing a mixture of amino acids, the different amino acids can be separated on the basis of the direction and relative rate of their migration when placed in an electric field at a known pH.
This group of amino acids is characterized by having very similar, although not identical, values for pKl the pK of the -COOH group in the range of 1. Amino acids with an ionizable R group Table have more complex titration curves with three stages corresponding to the three possible ionization steps; thus they have three pK a values. The third stage for the titration of the ionizable R group merges to some extent with the others. The titration curves of two representatives of this group, glutamate and histidine, are shown in Figure The isoelectric points of amino acids in this class reflect the type of ionizing R groups present.
For example, glutamate has a pI of 3. This is a result of the presence of two carboxyl groups which, at the average of their pK a values 3. Similarly, the pI of histidine, with two groups that are positively charged when protonated, is 7.
Figure The titration curves of a glutamate and b histidine. The pK a of the R group is designated pK R. Another important generalization can be made about the acidbase behavior of the 20 standard amino acids.
All the other amino acids have pK a values too far away from pH 7 to be effective physiological buffers Table , although in the interior of proteins the pK a values of amino acid side chains are often altered. Ion-exchange chromatography is the most widely used method for separating, identifying, and quantifying the amounts of each amino acid in a mixture. This technique primarily exploits differences in the sign and magnitude of the net electric charges of amino acids at a given pH, which are predictable from their pK a values or their titration curves.
The chromatographic column consists of a long tube filled with particles of a synthetic resin containing fixed charged groups; those with fixed anionic groups are called cation-exchange resins and those with fixed cationic groups, anion-exchange resins.
A simple form of ion-exchange chromatography on a cation-exchange resin is described in Figure The affinity of each amino acid for the resin is affected by pH which determines the ionization state of the molecule and the concentration of other salt ions that may compete with the resin by associating with the amino acid.
A modern enhancement of this and other chromatographic techniques is called high-performance liquid chromatography HPLC. This takes advantage of stronger resin material and improved apparatus designed to permit chromatography at high pressures, allowing better separations in a much shorter time. For amino acids, the entire procedure has been automated, so that elution, collection of fractions, analysis of each fraction, and recording of data are performed automatically in an amino-acid analyzer.
Figure shows a chromatogram of an amino acid mixture analyzed in this way. Figure Ion-exchange chromatography. An example of a cation-exchange resin is presented. At pH 3. As a result, they move through the column at different rates. As for all organic compounds, the chemical reactions of amino acids are those characteristic of their functional groups.
Because all amino acids contain amino and carboxyl groups, all will undergo chemical reactions characteristic for these groups.
For example, their amino groups can be acetylated or formylated, and their carboxyl groups can be esterified. We will not examine all such organic reactions of amino acids, but several widely used reactions are noteworthy because they greatly simplify the detection, measurement, and identification of amino acids.
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