pKa of amino acid residue in enzyme

[Chris]

Hi

I have a histidine which I believe is acting as a base to abstract a proton
from  a cysteine at the active site of an enzyme I am working on and was
wondering if anyone knew of any ways to measure the pKa of either the
histidine and/or the cysteine (preferably not by NMR) . I do have alanine
mutants of both the cysteine and histidine if those are at all useful in this.

Thanks in advance for your help

Chris


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