Isoelectric pointThe isoelectric pointof an amino acid or peptide is the pH at which it carriesno net charge.It is often abbreviated as pIHH2NH2NNHpKa1= 2.2pKa2= 9.0pKa3= 12.5Net charge:+2+10-1pIof arginine = pKa2+ pKa32= 10.75
Ion-exchange chromatographyCation-exchange using a resin with sulfonic acid groups attached.SO3-SO3-CH2CHCHCH2nDowex-50SO3-This resin would be a cationic – exchange resin.The resin would bind to positively charged molecules.The positive charges of the amino acids will bind to the resin by electrostatic forces. An amino acid with a higher [AA+]/[AA-] ratio willelute before one with a lower ratio; in general, an amino acid with a lower pIwill elute before one with a higher pI.Hydrophobic R groups of some of the amino acids may also interact with theNonpolar benzene ring, causing them to bind a little better to the resin.
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