Dissociative recombination cross section and branching ratios of protonated dimethyl disulfide and N-methylacetamide.

Dimethyl disulfide (DMDS) and N-methylacetamide are two first choice model systems that represent the disulfide bridge bonding and the peptide bonding in proteins. These molecules are therefore suitable for investigation of the mechanisms involved when proteins fragment under electron capture dissociation (ECD). The dissociative recombination cross sections for both protonated DMDS and protonated N-methylacetamide were determined at electron energies ranging from 0.001 to 0.3 eV. Also, the branching ratios at 0 eV center-of-mass collision energy were determined. The present results give support for the indirect mechanism of ECD, where free hydrogen atoms produced in the initial fragmentation step induce further decomposition. We suggest that both indirect and direct dissociations play a role in ECD.

Towards the standard-module approach to disulfide-linked polypeptide nanostructures. I. Methodological prerequisites and mass spectrometric characterization of the test two-loop structure.

Potentially biologically-active nanostructures can be created from single chains of unmodified peptides by cross-linking different regions of the chain by disulfide bonds and cleaving the chain at specified sites to obtain the final configuration. The availability of techniques for assembly and characterization of such structures was tested on a two-loop structure created from a 21-residue linear peptide. Directed intra-molecular disulfide bond formation was performed by inserting partial sequences favoring intra-molecular SS bond formation ("loops") separated by partial sequences disfavoring such a process ("spacers") into the precursor sequence. Peptide bond cleavage by partial acid hydrolysis at specific sites (GG, NP/DP) inside the loops opened them; the same process in the spacer separated the loops. Synthesis, oxidation and bond cleavage were monitored by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI ToF MS). The hydrolysis fragments of the produced nanostructures were characterized by tandem electrospray ionization Fourier transform mass spectrometry (ESI FT-MS) with collisional and electron capture dissociations. The latter technique was especially useful as it cleaves SS bonds preferentially. The feasibility of the proposed synthesis approach and the adequacy of the analysis techniques for the test structure were demonstrated.

Improved low-energy electron injection systems for high rate electron capture dissociation in Fourier transform ion cyclotron resonance mass spectrometry.

New low-energy electron injection systems based on indirectly heated dispenser cathodes facilitate electron capture dissociation (ECD) in Fourier transform ion cyclotron resonance (FTICR) mass spectrometry. In this joint report, details are presented of the design and performance of these systems on two commercial FTICR instruments, 9.4 T Bruker BioAPEX in Uppsala and 4.7 T IonSpec Ultima in Odense. New results include obtaining meaningful one-scan MS/MS data for isolated precursor ions with millisecond irradiation times. The ECD rate improvement is not only due to the larger total electron current, but the larger emitting area as well.

Quantitative analysis of aroma compounds in carrot (Daucus carota L.) cultivars by capillary gas chromatography using large-volume injection technique.

Dynamic headspace sampling was used to collect aroma compounds from raw samples of four carrot (Daucus carota L.) cultivars (Brasilia, Duke, Fancy, and Cortez). The collected volatiles were analyzed by capillary GC-FID and GC-MS using large-volume cool on-column injection (LVI-COC). Of the 36 compounds identified, 6 had not been previously detected in carrots. Significant differences between the carrot cultivars were found for 31 of the identified volatiles as well as for total monoterpenes, sesquiterpenes, and total volatile content. Mono- and sesquiterpenes accounted for about 98% of the total volatile mass in all cultivars. LVI-COC injection was used to determine the loss of carrot volatiles during concentration of headspace samples under a stream of nitrogen. The loss among major monoterpenes in the concentrated samples varied from 16% for p-cymene to >40% for alpha-pinene as compared to nonconcentrated samples. The loss among high-boiling sesquiterpenes varied from not detectable (beta-caryophyllene, alpha-humulene, and caryophyllene oxide) to approximately 7% for (E)- and (Z)-gamma-bisabolene.