Question

why does the charge on a protein vary with change in PH

Answer 1

protein are made up od amino acid which are v.much sensitive to pH. and if u see the str of protein it consist COO- as well as H3O+ means it contain chage a well and act a a buffer but if pH change it can alter the 3d str of protein in turn charges also vary..

Answer 2

I only learned about this recently, so I hope someone will correct me if I've misunderstood something.

Some amino acid residues have an amine group or carboxylic acid group in the side chain. These, as well as the amino-terminus and carboxy-terminus of the whole protein, will lose or gain protons at different pH's. At a very low pH, everything is protonated (giving you COOH and NH3+), but as the pH increases, the different aforementioned groups will lose their protons at specific pH's. First the carboxylic acid groups (becoming COO-), then, eventually, the amine groups (becoming NH2) as well. When a proton is lost, that means you suddenly have one more electron than protons, and that will give you a more negative charge. If you then reduce the pH again, the extra protons in solution will protonate first the amine and then the carboxylic acid groups once again, giving you a more positive charge.

Answer 3

Here's a hint - it's all about pKa