The emergence of proteomics has placed great interest in the understanding of the mechanisms of MS/MS fragmentation of peptides under low-energy collision-induced dissociation. In this work, we describe the presence of anomalous fragments, which correspond to neutral loss elimination of internal amino acids from ions of the b series in quadrupole ion trap MS/MS spectra from naturally occurring peptides. Internal amino acid elimination occurred preferentially with aliphatic amino acids. The phenomenon was more apparent when doubly charged precursors were fragmented and was inhibited when peptides were N-acetylated at the N-terminus. Fragmentation of isomeric peptides where some internal amino acids were relocated in N-terminal position produced MSn spectra indistinguishable from those of the original peptides, indicating that some b ions underwent a structural rearrangement process. Formation of anomalous fragments required a minimum activation time. Our data are consistent with a nucleophile attack of the N-terminal nitrogen over the electrophilic carbonyl carbon at one peptide bond, forming a cyclic b ion intermediate that, by reopening at preferential sites, exposes internal amino acids to the C-terminal side.
The need to acquire restaurant services may be motivated by various reasons but, whatever the reason, the potential customer has to search for information on the desired service in order to compare the alternatives and make the best purchase decision. Knowing how potential clients make this information search is fundamental for restaurant company managers so that they can carry out marketing communications strategies that facilitate the selection of their restaurant by potential customers. This paper examines the external information search process carried out by potential customers. It aims to determine what information search activities are carried out and what the motivating factors are for these activities. A survey was used to analyse the services offered by restaurants in the city of Zaragoza in the north‐east of Spain. The results of the survey and implications for restaurant managers are discussed.
This paper describes experience with the commercially available LCQ quadrupole ion trap mass spectrometer applied to the off-line analysis of peptides and proteins. The standard front end of the electrospray probe was replaced with a micromanipulator which, with the aid of a magnifying device, allowed the use of a variety of miniaturized spraying interfaces. The low sample consumption and extended analysis times of these devices were ideally suitable to obtain improved results in terms of sensitivity and mass accuracy. This needed a careful optimization of the number of ions stored inside the trap (ion target parameter) and required spectrum averaging of many scans. A method is presented for the mathematical fitting of ZoomScan spectra to theoretical isotopic distributions, which allowed the mass determination of large peptides with more accuracy than that achieved by conventional deconvolution algorithms. A very simple on-line desalting configuration is also described which needed no external micro-high-performance liquid chromatographic pumps, and can be easily mounted using the built-in syringe delivery system of the LCQ. This set-up allowed extended analysis times of 'in-gel' protein digests in subpicomole amounts. Finally, the multiple fragmentation capabilities of the ion trap were found to be extremely useful for the analysis of peptide modifications such as phosphorylation and for sequencing individual peptides from highly complex MHC-bound peptide pools.
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