-DEAE cellulose columns are rarely used at pH greater than 8.5. Why?

DEAE cellulose columns are weak anion exchangers. This means that they contain a positively charged resin that binds to negatively charged proteins. Raising the pH above 8.5 will induce a negative charge on the resin.

Explanation:

Diethylaminoethyl cellulose (DEAE-C) resin carries the dimethylamino group (CH2-CH2-NH+-CH2-CH3) which is positively charged at neutral pH. This positively charged resin interacts with negatively charged protein samples. Being a weak anion exchanger, DEAE-C only remains partially protonated at most pH values. For effective anion exchange, the resin should be kept at 2 pH values below its PKa value.Since DEAE C columns possess a PKa of 8.5, raising the pH further will deprotonate the resin and induce a negative charge on it, it will, therefore, not bind to positively charged proteins.

Mass, charge

Neutrons = 0 charge
Protons + Neutrons = mass number/atomic weight