My protein have an Tm of 65℃ after elution with 200mM imidazole ,concentration is 0.2mg/ml,Then I concentrated protein by Millipore 30K ultrafiltration tube to an concentration to 2mg/ml, then I aspirate some for thermal shift assay ,then I didn't get an S shape signal curve and signal is decrease with temperature which means my protein loss it 3D structure , interesting ,the protein have an PH about 12,but the filtered buffer is still have Ph 8.0,As I detect by PH test stripes, My buffer is 50mM NaPoi PH 8.0,300mM NaCl,0.1mM TCEP ,5% glycerol. Did anyone met the same problem or have some suggestions to explain this, thanks all