Question

Draw the different structures of Ala, Arg, and Cys at pH = 1, 6, 10, 14. Calculate the pI of each amino acid.

Answer 1

At pH = 1, alanine and cysteine will have protonated their amine groups while arginine, apart from its amine will also have its guanidyl group protonated and exist in their cationic form.

At pH = 6, alanine will exist in its zwitterionic form (pI of Ala is 6) while cysteine will exist in its anionic form, with the carboxylic acid group protonated in equilibrium with its deprotonated form and arginine, with its basic guanidyl side-chain will exist in an equilibrium with its amine group on the alpha carbon protonated and unprotonated.

At pH = 10, alanine and cysteine will exist in their anionic form, with the acid group deprotonated while arginine will exist in an equilibrium with its zwitterionic form and with its guanidyl group unprotonated.

At pH = 14, all the three amino acids will exist in their anionic form, with their carboxylic acid groups deprotonated.