Liquid secondary‐ion mass spectrometry of peptides containing multiple tyrosine‐ O ‐sulfates

The behavior of peptides containing multiple tyrosine‐ O ‐sulfates in liquid secondary‐ion mass spectrometry (LSIMS) has been investigated. In the positive‐ion spectra of the peptides containing two tyrosine‐ O ‐sulfates, Cionin and CCK‐associated C‐terminal nonapeptide (CAP‐9), the completely desulfated [M+H−2SO 3 ] + ions formed the base peaks, accompanying the significantly less‐intense [M+H] + and [M+H−SO 3 ] + ions. In the negative‐ion spectra of these peptides, the [M−H] − and [M−H−SO 3 ] − ions gave prominent peaks with significantly weaker [M−H−2SO 3 ] − ions. In the case of a peptide containing three tyrosine‐ O ‐sulfates, [Tyr(SO 3 H) 1 ]CAP‐9, the completely desulfated [M+H−3SO 3 ] + ion again formed the base peak in the positive‐ion spectrum. On the other hand, the sulfated tyrosine‐containing [M+H] + , [M+H−SO 3 ] + , and [M+H−2SO 3 ] + ions were of negligible abundance compared to the spectra of peptides containing two tyrosine‐ O ‐sulfates. We observed an intriguing ‘ladder fragmentation pattern’ in the negative‐ion spectrum of this triply‐sulfated peptide. The ladder consisted of the [M−H] − , [M−H−SO 3 ] − , and [M−H−2SO 3 ] − ions, but without the completely desulfated [M−H−3SO 3 ] − ion. These characteristic fragmentation patterns of sulfated tyrosine‐containing peptides were considered to bear a close correlation with the inherent acid‐lability of a tyrosine‐ O ‐sulfate in solution. A possible mechanism has been proposed to explain the fragmentation patterns in the gaseous phase, in which a proton plays a decisive role.