Histidyl-glycyl Containing Peptides. Characterization and Complexation Properties of H(L-His-Gly)2-R With Hydrogen and Alkali Metal Ions in the Gas-Phase

The two histidyl-glycyl containing peptides, H(L-His-Gly)(2)-OH and its methyl ester (H(L-His-Gly)(2)-OCH3, have been structurally characterized by liquid secondary-ion mass spectrometry, Both high-internal-energy ion fragmentations produced in the source and metastable decompositions occurring in the first field-free region have been studied. The mass spectra show the presence of y-, a- and b-type ions, a, being the most abundant fragment ion. The metastable decompositions are dominated by the loss of a water molecule and by y-type ions. The interactions of the two peptides with alkali metal ions (Li, Na, K) have been evaluated both by normal mass spectrometry and by tandem mass spectrometry to obtain selected daughter-ion spectra. The occurrence of mono-, bi- and trimetallated species has been detected in the gas phase. While, in the ease of the protonated species, y-type ions are predominant, in the presence of an alkali metal ion (Cat), they show lower abundances (Cat=Li) or are absent (Cat=Na, K) both in the mass spectra produced in the source and in metastable decompositions. In most of these cases, a very intense low-internal-energy ion, which is represented by [a(3),+Cat-H](+) and which can be produced by interaction of the metal with a deprotonated amide nitrogen, is recorded. This mechanism should be favored by the ''anchoring'' effect exerted by the Imidazole ring of the histidine which promotes interaction with metals. Other metastable decompositions yield abundant [b(3)+Cat-H](+) ions or involve the loss of the side-chain of the histidine. The formation of [b(3)+Cat+OH](+) ions, observed only in the case of the free acid peptide, is due to the interaction of the metal ion with the C-terminus carboxyl group, Bi- and trimetallated species have also been detected in the gas phase and characterized.