Phenolic hydroxyl ionization in calotropins from Calotropis gigantea latex

The ionization of the phenolic hydroxyl groups in calotropins DI and DII isolated from the latex of Calotropis gigantea has been studied by spectrophotometric titration at 295 nm in the pH range 6–13.2. Of the 12 tyrosine residues of calotropin DI and 13 tyrosine residues of calotropin DII, only four residues were ionized reversibly in the pH range 8.9–10.7 with the apparent pK of 9.7. The remaining tyrosine residues were ionized irreversibly in the pH range 11.2–13.2 with the apparent pK of 11.5. Both calotropins showed time-dependent ionization of phenolic groups at 295 nm in the pH range 11.5–12.0. Chemical modification with tetranitromethane suggested the presence of three tyrosine residues on the surface of each calotropin molecule.