A comparison of the fragmentation pathways of [Cu(II)(Ma)(Mb)]•2+ complexes where Ma and Mb are peptides containing either a tryptophan or a tyrosine residue.

[Cu(II)(M(a))(M(b))](•2+) complexes, where M(a) and M(b) are dipeptides or tripeptides each containing either a tryptophan (W) or tyrosine (Y) residue, have been examined by means of electrospray tandem mass spectrometry. Collision-induced dissociations (CIDs) of complexes containing identical peptides having a tryptophan residue generated abundant radical cations of the peptides; by contrast, for complexes containing peptides having a tyrosine residue, the main fragmentation channel is dissociative proton transfer to give [M(a) + H](+) and [Cu(II)(M(b)-H)](•+). When there are two different peptides in the complex, each containing a tryptophan residue, radical cations are again the major products, with their relative abundances depending on the locations of the tryptophan residue in the peptides. In the CIDs of mixed complexes, where one peptide contains a tryptophan residue and the other a tyrosine residue, the main fragmentation channel is formation of the radical cation of the tryptophan-containing peptide and not proton transfer from the tyrosine-containing peptide to give a protonated peptide.

Binding energies of the silver ion to alcohols and amides: a theoretical and experimental study.

The silver ion binding energies to alcohols (methanol, ethanol, n-propanol, i-propanol, and n-butanol) and to amides (acetamide, N-methylacetamide, N, N-dimethylacetamide, formamide, N-methylformamide, and N, N-dimethylformamide) have been calculated using density functional theory (DFT) and measured using the threshold collision-induced dissociation (TCID) method. For DFT, the combined basis sets of ECP28MWB for silver and 6-311++G(2df,2pd) for the other atoms were found to be optimal using a series of test calculations on Ag (+) binding to methanol and to formamide. In addition, the Ag (+) binding energies of all ligands were evaluated with nine functionals after full geometric optimizations. TCID binding energies were measured using a triple quadrupole mass spectrometer. Reasonable to good agreements were obtained between the calculated and experimental silver(I) binding energies. Ligation of Ag (+) to the alcohols was primarily via the oxygen, although n-propanol and n-butanol exhibited additional, bidentate coordination via the CH hydrogens. By contrast, silver(I) binding to the amides was all monodentate via the carbonyl oxygen. There appears to be strong correlations between the binding energies and the polarizabilities of the ligands.

Analysis of perchlorate in foods and beverages by ion chromatography coupled with tandem mass spectrometry (IC-ESI-MS/MS).

A new IC-ESI-MS/MS method, with simple sample preparation procedure, has been developed for quantification and confirmation of perchlorate (ClO4-) anions in water, fresh and canned food, wine and beer samples at low part-per-trillion (ng l(-1)) levels. To the best of our knowledge, this is the first time an analytical method is used for determination of perchlorate in wine and beer samples. The IC-ESI-MS/MS instrumentation consisted of an ICS-2500 ion chromatography (IC) system coupled to either an API 2000 or an API 3200 mass spectrometer. The IC-ESI-MS/MS system was optimized to monitor two pairs of precursor and fragment ion transitions, i.e., multiple reaction monitoring (MRM). All samples had oxygen-18 isotope labeled perchlorate internal standard (ISTD) added prior to extraction. Chlorine isotope ratio (35Cl/37Cl) was used as a confirmation tool. The transition of 35Cl16O4- (m/z 98.9) into 35Cl16O3- (m/z 82.9) was monitored for quantifying the main analyte; the transition of 37Cl16O4- (m/z 100.9) into 37Cl16O3- (m/z 84.9) was monitored for examining a proper isotopic abundance ratio of 35Cl/37Cl; and the transition of 35Cl18O4- (m/z 107.0) into 35Cl18O3- (m/z 89.0) was monitored for quantifying the internal standard. The minimum detection limit (MDL) for this method in de-ionized water is 5 ng l(-1) (ppt) using the API 2000 mass spectrometer and 0.5 ng l(-1) using the API 3200 mass spectrometer. Over 350 food and beverage samples were analyzed mostly in triplicate. Except for four, all samples were found to contain measurable amounts of perchlorate. The levels found ranged from 5 ng l(-1) to 463.5+/-6.36 microg kg(-1) using MRM 98.9-->82.9 and 100 microl injection.

Amide bond cleavage in deprotonated tripeptides: a newly discovered pathway to "b2 ions.

The fragmentation reactions of the [M-H](-) ions of the tripeptides H-Gly-Leu-Sar-OH, H-Leu-Gly-Pro-OH and H-Gly-Leu-Gly-OH have been investigated in detail using energy-resolved mass spectrometry, isotopic labelling and MS(3) experiments. It is shown that the major route to the "b(2) ions involves loss of a neutral amine from the a(3) ([M-H-CO(2)](-)) ion rather than being formed directly by fragmentation of the [M-H](-) ion. When there is no C-terminal amidic hydrogen (Sar, Pro), loss of a neutral amine is the dominant primary fragmentation reaction of the a(3) ion. However, when there is a C-terminal amidic hydrogen (Gly), elimination of the N-terminal amino acid residue is the major fragmentation reaction of the a(3) ion and formation of the "b(2) ion is greatly reduced in importance. It is proposed that the "b(2) ions are deprotonated oxazolones.

Elucidation of fragmentation mechanisms of protonated Peptide ions and their products: a case study on glycylglycylglycine using density functional theory and threshold collision-induced dissociation.

The fragmentation mechanisms of protonated triglycine and its first-generation dissociation products have been investigated using a combination of density functional theory calculations and threshold collision-induced dissociation experiments. The activation barrier measured for the fragmentation of protonated triglycine to the b(2) ion and glycine is in good agreement with a calculated barrier at the B3LYP/6-31++G(d,p) level of theory reported earlier [Rodriquez, C. F. et al. J. Am. Chem. Soc. 2001, 123, 3006-3012]. The b(2) ion fragments to the a(2) ion via a transition state structure that is best described as acylium-like. Contrary to what is commonly assumed, the lowest energy structure of the a(2) ion is not an iminium ion, but a cyclic, protonated 4-imidazolidone. Furthermore, fragmentation of the b(2) to the a(1) ion proceeds not via a mechanism that results in HNCO and H(2)C=C=O as byproducts, as have been postulated, but via a transition state that contains an incipient a(1) ion and an incipient carbene. The fragmentation of a(2) to a(1) proceeds via a transition state structure that contains the a(1) ion, CO and an imine as incipient components.