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2.4 Chemical properties of a-amino acids a. Acid/base nature a-amino acids contain both acidic (-COOH) and basic (-NH2) groups. Unfortunately, though, the picture is not as simple as this. In the solid crystalline state the a-amino acids exist as zwitterions, formed by the transfer of protons (H+) from the -COOH to the -NH2 groups. For a-amino acids without acidic or basic side chains these zwitterions have charged groups but are neutral overall. This is shown on the left. |
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Zwitterions remain when the a-amino acid is dissolved in water at pH7. Addition of an acid, supplying more protons, produces ions with a surplus positive charge: | ||||
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Addition of an alkali, providing hydroxide ions, produces ions with a surplus negative charge: | ||||
  Figure 6 Electrophoresis. An electric potential difference is applied across a plate of gel. Molecules separate on the gel since they move at speeds that depend on their size and charge. |
We can describe a-amino acids as amphoteric as they can react with both acid and alkali. They are effective buffers in biological systems.
The situation is more complicated in a-amino acids that have acidic or basic R groups , e.g. glu or lys (see Figure 3 ). The structures in Figure 3 show the main ionic form of each a-amino acid at pH7. At very low pH all a-amino acids exist as ions with an overall positive charge, while at high pH they exist as ions with an overall negative charge. For each a-amino acid there is a pH between these extremes at which its molecules are neutral overall. This value is called the isoelectric point for the a-amino acid. At its isoelectric point the a-amino acid molecules will not move when placed in an electric field. The separation technique called electrophoresis relies on molecules with different isoelectric points moving at different speeds when kept at a fixed pH and placed in an electric field (see Figure 6). |
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Unilever Education Advanced Series: Proteins | ||||||||||
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