Checked the pH after the precipitate forms and it is between pH 8-8.5, which I guess it should be as the coupling buffer is pH 8.3.
The coupling buffer is 0.1M NaHCO3, 0.5M NaCl pH 8.3
The protein has been dialysed and stored in this buffer and when I add the protein to the beads, I top up with some buffer to ensure the beads are incubated in a decent volume. Its during this step the precipitation occurs so I don't understand how adjusting the pH would help? (sorry)