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The production of neutrons carrying at least 20% of the proton beam energy (x L > 0.2) in e + p collisions has been studied with the ZEUS detector at HERA for a wide range of Q 2 , the photon virtuality, from photoproduction to deep inelastic scattering. The neutron-tagged cross section, ep → e ′ Xn, is measured relative to the inclusive cross section, ep → e ′ X, thereby reducing the systematic uncertainties. For x L > 0.3, the rate of neutrons in photoproduction is about half of that measured in hadroproduction, which constitutes a clear breaking of factorisation. There is about a 20% rise in the neutron rate between photoproduction and deep inelastic scattering, which may be attributed to absorptive rescattering in the γp system. or 0.64 < x L < 0.82, the rate of neutrons is almost independent of the Bjorken scaling variable x and Q 2 . However, at lower and higher x L values, there is a clear but weak dependence on these variables, thus demonstrating the breaking of limiting fragmentation. The neutron-tagged structure function, F

Charge states of noncovalent protein complexes in the gas phase are known to affect their propensity for unfolding and dissociation. In this work, C-reactive protein (CRP) pentamer and Concanavalin A (ConA) tetramer at different charge states were subjected to collision induced dissociation (CID) and surface induced dissociation (SID) in a modified quadrupole/ion mobility/time-of-flight mass spectrometer. Charge manipulation was achieved through solution addition of charge reducing (triethylammonium acetate) or supercharging (3-nitrobenzylalcohol) reagents. The results show that charge reduction increases the stability of the proteins to dissociation and suppresses unfolding of the precursors. While CID becomes less effective at dissociation of charge reduced CRP and ConA, SID showed better preserved subunit contacts that are useful for quaternary structure elucidation. In contrast, supercharging of CRP and ConA leads to facile dissociation into subunits even for CID. The extent of precursor unfolding also increases with greater charge. Another interesting finding is that low-charge multimer products (dimers, trimers, etc.) seem to be collapsed after being released from the complexes. Further investigation is necessary to fully understand this behavior. The data presented here suggest that charge manipulation can be used to "tune" the dissociation behavior of noncovalent protein complexes in order to obtain the most useful information desired for structural analysis.

Silicon nanopowder (5-50 nm) was applied as a matrix for the analysis of small molecules in laser desorption/ionization mass spectrometry. In contrast with conventional matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry, the matrix background interference in the low mass range was significantly reduced. Effects of the particle size and sample preparation procedures on the background mass spectra and the analyte signal intensity have been investigated, and an optimized powder and sample preparation protocol was established. Several surface characterization tools have been applied as well. Both positive mode and negative mode laser desorption/ionization have been applied to different analytes including drugs, peptides, pesticides, acids, and others. Detection limits down to the low femtomole per microliter levels were achieved for propafenone and verapamil drugs. The method developed was found relatively tolerant to salt contamination, which allowed the direct analysis of morphine and propaphenone in untreated urine and triazine herbicides in a soil extract. The new silicon-nanoparticle-assisted laser desorption ionization method was found to be highly selective, which may be due to analyte-dependent precharging in solution, prior to vacuum laser desorption. Some aspects of the charge-transfer mechanism have been studied and discussed. In comparison with standard MALDI matrixes, the silicon nanopowder requires much lower laser fluence (contributing to a reduced background) has much better surface homogeneity, and is more tolerant to salt interference, which makes it an easily applicable practical tool at a potentially low cost.

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