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Separation of Amino Acids by Paper Chromatography

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Separation of Amino Acids by Paper Chromatography ( separation-amino-acids-by-paper-chromatography )

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To best understand why different amino acids have unique Rf values, it is important to understand the structural features of these molecules. As the name suggests, each amino acid contains an amino group, -NH2, and a carboxylic acid group, -COOH. The molecular structure of a generic amino acid is provided below: Amine group Hydrogen H H2N C COOH R Side chain Acid group The 20 different amino acids that make up our proteins, and those of most other living things, differ in the identity of the side chain R. In glycine, the simplest amino acid, R is a hydrogen atom. Eight amino acids have R groups that consist of carbon atoms with attached hydrogen atoms. Two examples are valine for which R is –CH(CH3)CH3, and phenylalanine, which contains a benzene ring with R equal to –CH2(C6H5). These nonpolar hydrocarbon side chains are hydrophobic or “water-hating.” Hence, they tend to lower the water solubility of the corresponding amino acids. Six amino acids have polar but neutral R groups that tend to promote water solubility. For example, for serine R is –CH2OH. In two amino acids, glutamic acid and aspartic acid, the side chains carry carboxylic acid groups. For example, in glutamic acid, R is –CH2CH2COOH. Finally, three amino acids have basic R groups. One of these is lysine, for which R is – CH2CH2CH2CH2NH2. Both acidic and basic R groups tend to promote water solubility, though the solubility will be pH dependent. In fact, the water solubility of all amino acids varies with the acidity of the solution, i.e. the H+ ion concentration that is commonly communicated via pH values. This is because all amino acids, even those with neutral side chains, contain an acidic –COOH group and a basic -NH2 group. The most prevalent ionic form of an amino acid in solution therefore depends on the pH of the solution. As the equation below suggests, in solutions of low pH (high H+ concentration), the amino and acid groups are both protonated and this contributes a net plus charge. Near the neutral pH of 7, an H+ has dissociated from the carboxylic acid group and the positive and negative charges balance each other. In solutions of still higher pH (low H+ concentration), the amino group is in the –NH2 form and the net charge is negative because of the –COO-. This means that the rate of migration of an amino acid will

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