Question 27

[yfangl097543]

Since the isoelectric point is the pH when the net charge on the molecule is 0, does that mean it is the pH where the carboxylic acid on most of the lysines are deprotonated and half of the amine groups are protonated (since there are 2 amine groups and only one carboxylic acid group per lysine)? Also, in this case, is the cathode positive or negative?

[admin]

It is a bit different than that, it is well explained here: http://en.wikipedia.org/wiki/Isoelectric_point

[admin]

bump

[hua8986059]

so Amino acids with acidic side chains would have isoelectric point of lower than 7 right?

[mohsinalam5969]

@hua8986059

The pI will be at a lower pH because the acidic side chain introduces an "extra" negative charge. So the neutral form exists under more acidic conditions when the extra -ve has been neutralised. For example, for aspartic acid the neutral form is dominant between pH 1.88 and 3.65, pI is halfway between these two values, i.e. pI = 1/2 (pKa1 + pKa3), so pI = 2.77.
Hope you understamd.Thanks