Question

A mixture of valine, glycine, isoleucine, aspartic acid, phenylalanine, arginine, lysine, asparagine, and methionine are put into an electrophoresis apparatus, with the buffer pH=6.5. Sort each amino acid according to its charge in the buffer with a pH of 6.5.

Answer 1

The pH at which the amino acid behave as neutral, called the isoionic point or isoelectric point of amino acid( pI). If the pH of buffer is higher than the pI, then the amino acid will be negatively charged. If the pH is lower than the pI, then the amino acid will be positively charged.

pI of valine = 5.96, negatively charged in pH 6.5
pI of glycine = 5.97, negatively charged
pI of isoleucine = 6.02, negatively charged
pI of aspartic acid =2.77, negatively charged
pI of phenylalanine = 5.48, negatively charged
pI of arginine=10.76, positively charged
pI of lysine = 9.74, positively charged
pI of asparagine= 5.41, negatively charged
pI of methionine= 5.74, negatively charged

Otherwise we can determine the charge from their structures. For the Group I amino acids in which have non polar aliphatic or aromatic R group substitutions like glycine, alanine, valine, leucine, isoleucine, proline, phenylalanine, methionine, and tryptophan. These all amino acids have pI in range of 5-6 pH.

Only two acidic amino acids Asparatic acid and Glutamic acid have pI around 2-3 pH.

Only three basic amino acids Arginine, Lysine and histidine have pI around basic pH around 9-11.