PLASMA DESORPTION MASS-SPECTROMETRY OF PHOSPHOPEPTIDES - AN INVESTIGATION TO DETERMINE THE FEASIBILITY OF QUANTIFYING THE DEGREE OF PHOSPHORYLATION

The intact ion yields of phosphotyrosine, phosphothreonine and mono- and diphosphoserine residue-containing peptides have been compared with the non-phosphorylated sequences using plasma desorption mass spectrometry. Equimolar mixtures of the phosphorylated (M(p)) and non-phosphorylated peptides (M) were also analysed. The positive mass spectra of these mixtures show a higher intensity of the [M + H]+ compared with the [M(p) + H]+. In the negative mass spectrum, the bias towards the [M - H]- compared with the [M(p) - H]- was reduced, but the spectra generally did not accurately reflect the stoichiometry.