The chemical formula of taurinated detyrosinated a1CT has an expected mass of 2803.1276. This differs from the measured value by 8.8 ppm, well within the measurement accuracy of the instrument and analysis mode in which the data were collected. Based on the highresolution measurements and the chemical database search, we concluded that the most likely explanation for the a1CT-Y+X is that "X" is a taurine residue added to a detyrosinated C-terminus of a1-tubulin. This possibility was explored further with fragmentation of the putatively assigned signal. The expected mass of a y1 fragment of taurine in negative ion mode is 124 Da. The fragmentation spectrum in Figure shows a y-ion peptide fragmentation series beginning at 124.2 Da, and demonstrates clear sequence coverage of the a1CT-Y sequence for 13 additional residues. The sequence information obtained via this method confirms the assignment based on Figure . Fragmentation spectra show that the unidentified peptide is a1CT-Y with a Cterminal adduct, "X". Peaks that are observed in the fragmentation spectra are noted with the corresponding amino acid and denote y-type ions. Interestingly, the y1 ion expected for a c-terminal taurine residue is 124Da and this signal is observed in the fragmentation spectrum highly suggestive of taurine inclusion. accurate mass measurement. Further, this fragmentation data strongly suggests that taurine is bonded to the C-terminus of a1CT-Y.