Stabilization of a tyrosine O-sulfate residue by a cationic functional group: formation of a conjugate acid-base pair.

Sulfated tyrosine [Tyr(SO3H)]-containing peptides showed characteristic peak patterns in their liquid secondary-ion mass spectrometry (LSIMS) spectra. Protonated molecules were desulfated more easily than their deprotonated counterparts. Therefore, the stabilities of the Tyr(SO3H) residues were well-reflected by peak patterns in their positive-ion spectra. These intrinsic peak patterns were investigated by comparing the behavior of each Tyr(SO3H) residue in acidic solution. As the peptide chain was lengthened and the number of cationic functional groups increased, the peak representing the [MH]+ of a Tyr(SO3H)-containing peptide became more prominent than that representing the desulfated [MH-SO3]+. These alterations in peptide structure also increased the stability of the Tyr(SO3H) residue in acidic solution. Based on the desulfation mechanism of an aryl monosulfate, we predicted that intramolecular cationic functional groups would stabilize Tyr(SO3H) residues by forming conjugate acid-base pairs (or salt bridges) both in the gaseous phase and in acidic solution. In accordance with this theory, Arg residues would take primary responsibility for this self-stabilization within Tyr(SO3H)-containing peptides. Moreover, a long peptide backbone was expected to have a weak protective effect against desulfation of the [MH]+ in the gaseous phase. Tyr(SO3H) residues were also stabilized by adding an external basic peptide containing multiple Arg residues. Formation of such intermolecular acid-base pairs was demonstrated by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) which detected conjugated peptide ions. The energetically favorable formation of conjugate acid-base pairs prompted by Tyr(SO3H) residues might be a driving force for protein folding and protein-protein interaction.