CD spectra of different cation forms of spike protein. The scanning fluorimetry data of the two batches shows the protein is both stable and functional, with heparin binding activity. A and C : CD spectra were recorded on J-1100 spectrometer between 180 and 260 nm of spike protein (0.5 mg/ml). B and D : Secondary structure were analysed by program CDSSTR of A and C, respectively. A and B : the first batch of spike protein (PBS buffer). C and D : the second batch of spike protein (PBS, 20% glycerol).