I'm trying to label a protein with fluorescent dye (TMR succinimidyl ester), but having trouble getting the protocol to work.
The protein seems to be stable in distilled water at a the recommended concentration of about 2mg/ml (determined by OD280 with an absorption coefficient calculated from its sequence).
However, the protocol calls for the addition of 1:10 1M sodium bicarbonate to raise the pH to ~8.5. When I perform this step the solution immediately turns cloudy. I have also encountered this precipitation problem when dialyzing against PBS.
What steps can I take to get around this? I assume a lower concentration of protein might prevent precipitation, but I have read that the labeling reaction becomes very inefficient below 1mg/ml.