Spectrophotometric titration of phenoxyl groups of sheep brain tubulin

Spectrophotometric titration of phenoxyl group of sheep brain tubulin carried out in 6M guanidine hydrochloride indicated 36 tyrosine residues per dimer of tubulin. A plot of log alpha/(1-alpha) versus pH showed that 26 residues titrated with apparent pK of 10.4 and 10 residues titrated with apparent pK of 11.0. The high pK value of tyrosine could be attributed to the possibility that the molecules containing tyrosine residues were not completely utilised as these residues could have been partially shielded from the solvent. Alternatively, they could have proximal negative charges.