Elucidation study of thermal decomposition products of human serum albumin using liquid ionization mass spectrometry

The origin of the intense but unknown peaks at m/z 235 observed in liquid ionization (LI) mass spectra of middle ear effusion and serum was investigated by using related standard compounds and the collision induced dissociation techniques. The ions were observed as the base peaks in mass spectra of the aqueous fractions of middle ear effusion and serum after chloroform extraction and in those of authentic human serum albumin (HSA) too. The ions commonly observed in serous fluids could be estimated as tyrosil–valil interchain immonium ions arising from thermal decomposition of HSA during the measurement. Such thermally stable interchain immonium ions, also observed in some oligopeptides having Val–Tyr sequence as their fragment ions, are likely to be characteristic ions for large protein molecules.