STUDY OF IONIZATION OF TYROSINE RESIDUES IN PROTEINS BY 2ND-DERIVATIVE UV SPECTROSCOPY

Four spectrally different forms of tyrosine residues were shown to be present in proteins, namely, nonionized residues, either buried or exp osed to solvent, and ionized residues buried or exposed to solvent. A method for determining the pK(a), values of the tyrosine residues in p roteins was proposed. It is based on the decrease in the absorption in tensity in the second derivative of the UV spectrum at 284.2 mn, which is the wavelength of the isobestic point corresponding to the transit ion of the nonionized tyrosine residues from the buried to the exposed state. Several proteins were studied by this method; the results obta ined were found to be close to the corresponding published data. This method is simpler than the conventional UV titration.