Study of ionization of tyrosine residues in proteins by second-derivative UV spectroscopy

Four spectrally different forms of tyrosine residues were shown to be present in proteins, namely, nonionized residues, either buried or exposed to solvent, and ionized residues buried or exposed to solvent. A method for determining the pK _a values of the tyrosine residues in proteins was proposed. It is based on the decrease in the absorption intensity in the second derivative of the UV spectrum at 284.2 nm, which is the wavelength of the isobestic point corresponding to the transition of the nonionized tyrosine residues from the buried to the exposed state. Several proteins were studied by this method; the results obtained were found to be close to the corresponding published data. This method is simpler than the conventional UV titration. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1394–1398, July, 1997.