Science Buddies: "Ask an Expert"

Hi,

What is isoelectric point? And thanks for the advice, I didn't know that there was a point where proteins are least soluble

Hi Chiazuohui,

Good question. Proteins are composed of amino acids, which can have free carboxyl (COOH) groups or amino (NH2) groups, (all amino acids have at least one amino and one carboxyl group, but these are combined in a covalent peptide bond in proteins). Above a pH of 4.5, and carboxyl groups will have a negative charge (COO-), and below pH 7, the amino groups will have a positive charge (NH3+). The pH at which the negative and positive charges are balanced and give the protein a net neutral charge is called the isoelectric point.

Here is a website that explains the general principle of isoelectric point:

http://en.wikipedia.org/wiki/Isoelectric_point

Here is some additional information about isoelectric point and characteristics of proteins:

http://www.chemistryquestion.com/Englis ... ation.html

Casein has an isoelectric point of 4.6, so is least soluble at this pH. In milk at pH 6.7 the casein would have more carboxyl groups ionized compared to protonated amino groups, so would have an overall net negative charge.

Does this information give you any ideas for your experiment?


Donna Hardy