Some general aspects of the chemistry of Organo-alkali metal ions. An overview of recent work

Dissociation pathways of alkali-cationized peptides: opportunities for C-terminal peptide sequencing

Dissociation pathways of alkali-cationized peptides have been studied using multiple stages of mass spectrometry (MSx) with a quadrupole ion trap mass spectrometer. Over 100 peptide ions ranging from 2 to 10 residues in length and containing each of the 20 common amino acids have been examined. The formation of the [b(n-1) + Na + OH]+ product ion is the predominant dissociation pathway for the majority of the common amino acids. This product corresponds to a sodium-cationized peptide one residue shorter in length than the original peptide. In a few cases, product ions such as [b(n-1) + Na - H]+ and those formed by loss, or partial loss, of a sidechain are observed. Both [b(n-1) + Na + OH]+ and [b(n-1) + Na - H]+ product ions can be selected as parent ions for a subsequent stage of tandem mass spectrometry (MS/MS). It is shown that these dissociation patterns provide opportunities for peptide sequencing by successive dissociation from the C-terminus of alkali-cationized peptides. Up to seven stages of MS/MS have been performed on a series of [b + Na + OH]+ ions to provide sequence information from the C-terminus. This method is analogous to Edman degradation except that the cleavage occurs from the C-terminus instead of the N-terminus, making it more attractive for N-terminal blocked peptides. The isomers leucine and isoleucine cannot be differentiated by this method but the isobars lysine and glutamine can.

A study of metal chelation of dinucleotide analogs in the gas phase by fast-atom bombardment mass spectrometry

Fast-atom bombardment (FAB) mass spectrometry was used to investigate the interaction of proton and alkali metal ions with dinucleotide analogs such as T-n-T (T = thymine moiety, n = polyether chain, e.g., triethylene, tetraethylene, pentaethylene, and hexaethylene ether 1-4), A-n-T (A = adenine unit 5-8), and T-n-OMe (9-12) in 3-nitrobenzyl alcohol matrix. The [M + H](+) ion is the most abundant ion for the A-n-T series, whereas in 1-4 and 9-12 the (TC2H4)(+) ion is the most abundant. Formation of [M + H -C2H4O](+) ions, a characteristic fragmentation of crown ethers under electron ionization, is observed for compounds 1-12 and is more pronounced in 6 and 7. An abundant [M - H](-) ion is observed for all the compounds studied under negative ion FAB due to the presence of the (-CO-NH-CO-) group of thymine, an indication of existence of intramolecular H bonding. The FAB mass spectra of 1-12 with alkali metal ions (Li(+), Na(+), K(+), Rb(+), and Cs(+)) showed formation of abundant metal-coordinated ions ([M + Met](+) and [TC2H4 + Met](+)). Compounds 3, 4, 6, 7, and 10-12 showed ions due to the substitution of the thymine moiety by a hydroxyl group ([M + Met - 108](+), Met = metal ion). For compound 3 alone, substitution of two thymine groups ([M + Met - 216](+)) was observed. Metastable ion studies were used to elucidate the structures of these potentially significant ions, and the ion formule were confirmed with high resolution measurements. Selectivity toward metal complexation with ligand size was seen in the T-n-T and A-n-T series and was even more pronounced in A-n-T series. These dinucleotide analogs fall in the following order of chelation of alkali metal ions, acyclic glymes < dinucleotide analogs (acyclic glymes substituted with nitrogen bases) < crown ethers, which places them in perspective as receptor models.

Injection of reagent ions into the selvedge region in fast-atom bombardment mass spectrometry

A divided probe that incorporates a potassium aluminosilicate glass target and an analyte/glycerol matrix target, spatially separated, was used to inject potassium ions (K(+)) into the high-pressure "selvedge" region formed above the analyte/glycerol matrix target during fast-atom bombardment (FAB); [M+K](+) adduct ions that represent the types of gas-phase neutral molecules present in the selvedge region are observed. Computer modeling assisted in designing the divided target and an additional ion optical element for the FAB ion source to optimize interactions between K(+) ions and the desorbed neutral molecules. The capability of injecting K(+) ions into the FAB experiment has utility in both mechanistic studies and analyses. Experimental results here are consistent with a model for the desorption/ionization processes in FAB in which some types of neutral analyte molecules are desorbed intact and are subsequently protonated by glycerol chemical ionization. Unstable protonated molecules undergo unimolecular decomposition to yield observed fragment ions. The use of K(+) cationization of analytes for molecular weight confirmation is demonstrated, as well as its utility in FAB experiments in which mixtures are encountered.