Charge of your protein depends on the pH of the buffer in the first place. I would use Expasy Protparam tool to calculate approximate pKa of your protein for overall idea of its charge. It´s just a guess value, so it might be wrong, but most likely it will be close to your proteins real pKa. I don´t know any reliable method to calculate the charge precisely when the structure is unknown.

You can’t

Dear Sandeep,

Do you have an idea on the sequence of your protein?

Try using the software (http://compbio.clemson.edu/sapp/delphi_webserver/) to estimate the surface charge of your protein.

Hope this helps,

Ana

You can use Protein Calculator V.3 web server to know the charge of your protein at particular pH , you have to input your query protein sequence and tick the check box according to your requirement before submission. Here I am giving the link - http://protcalc.sourceforge.net/ .