Fragmentations of [M − H] − anions of peptides containing tyrosine sulfate. Does the sulfate group rearrange? A joint experimental and theoretical study

RATIONALE To investigate the fragmentations in the negative‐ion electrospray mass spectra of peptides containing tyrosine sulfate. METHODS Possible fragmentation mechanisms were explored using a Waters QTOF2 tandem mass spectrometer in concert with calculations at the CAM‐B3LYP/6‐311++g(d,p) level of theory. RESULTS The major negative ion formed in the ESI‐MS of peptides containing tyrosine sulfate is [(M − H) − SO 3 ] − and this process normally yields the base peak of the spectrum. The basic backbone cleavages of [(M − H) − SO 3 ] − allowed the sequence of the peptide to be determined. Rearrangement reactions involving the formation of HOSO 3 − and [(M − H) − H 2 SO 4 ] − yielded minor peaks with relative abundances ≤ 10% and ≤ 2%, respectively. CONCLUSIONS The mass spectra of the [M − H] − and [(M − H) − SO 3 ] − anions of peptides containing tyrosine sulfate allowed the position of the tyrosine sulfate group to be determined, together with the amino acid sequence of the peptide. Copyright © 2013 John Wiley & Sons, Ltd.