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-SO3](+) ions. In the negative-ion spectra of these peptides, the [M-H](-) and [M-H-SO3](-) ions gave prominent peaks with significantly weaker [M-H-2SO(3)](-) ions. In the case of a peptide containing three tyrosine-O-sulfates, [Tyr(SO3H)(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-SO3](+), and [M+H-2S(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-SO3](-), 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.