Question

. A mixture of amino acids consisting of arginine, cysteine, glutamic acid, histidine, leucine, and serine is applied to a strip of paper and subjected to electrophoresis using a buffer at pH 7.5. What are the directions of migration of these amino acids and what are their relative mobilities?

Answer 1

Electrophoretic mobilities are determined by the average net charge on the amino acid of arginine, cysteine,glutamic acid, histidine, leucine and serine but one is more than 1 pH unit away from any

pKa, and thus will have a single predominant ionic species available by inspection of the titration curve with the exception is cysteine, for which one must use the Henderson-Hasselbalch equation and the pKr

of 8.2 to deduce that about 20% of the cysteine molecules carry a negative charge at pH 7.5