The use of metal salts in electrospray ionization (ESI) of peptides increases the charge state of peptide ions, facilitating electron transfer dissociation (ETD) in tandem mass spectrometry. In the present study, K(+) and Ca(2+) were used as charge carriers to form multiply-charged metal-peptide complexes. ETD of the potassium- or calcium-peptide complex was initiated by transfer of an electron to a proton remote from the metal cation, and a c'-z• fragment complex, in which the c' and z• fragments were linked together via a metal cation coordinating with several amino acid residues, was formed. The presence of a metal cation in the precursor for ETD increased the lifetime of the c'-z• fragment complex, eventually generating c• and z' fragments through inter-fragment hydrogen migration. The degree of hydrogen migration was dependent on the location of the metal cation in the metal-peptide complex, but was not reconciled with conformation of the precursor ion obtained by molecular mechanics simulation. In contrast, the location of the metal cation in the intermediate suggested by the ETD spectrum was in agreement with the conformation of "proton-removed" precursors, indicating that the charge reduction of precursor ions by ETD induces conformational rearrangement during the fragmentation process.

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Site-specific fragmentation following Si:2p photoexcitation of (trifluorosilyl) (trimethylsilyl)methane [F3SiCH2Si(CH3)3 (FSMSM)] has been studied by means of photoelectron-photoion and photoion–photoion coincidence techniques. The total photoionization efficiency curve of FSMSM has only one broad peak near the Si:2p core-ionization threshold and no evidence is obtained for the occurrence of selective excitation of each of the two Si atoms. From the results of ab initio calculations of FSMSM, it is found that the magnitude of the Coulomb interaction between the valence electrons and the Si:2p core electrons is very similar at the two Si sites. As a result, the difference in the chemical shift of the 2p core level between the two Si sites is very small. However, the evidence for site-specific fragmentation of the molecule following the Si:2p core-level photoexcitation is obtained by analyzing the photoion–photoion coincidence spectra; the relative yields of H+–SiF+ and CH+n–SiF+3 (n=1–3) ion pairs are enhanced around 104.5 eV, and those of CH+3–F3SiCH2Si(CH3)+2, CH+3–SiCH3F+2, CH+3–Si(CH3)2F+, and H+–SiCH+3 are enhanced around 107 eV. These two thresholds are interpreted to correspond to the 2p core-level photoexcitation at the Si atom bonded to three F atoms and that at the Si atom bonded to three CH3 groups. Ionic fragmentation following excitation of these core electrons occurs selectively around the Si atom where the photoexcitation has taken place. The site-specific fragmentation scheme can be explained on the basis of the valence electron density around the excitation site.

Starlike molecules with arms consisting of a tert-butyldimethylsilyloligothienylenedimethylsilyl unit, MeSi[SiMe 2 (T) n SiMe 2 (t-Bu)] 3 (7-11; n ) 2-6, T ) thienylene) were prepared by the reactions of tris(chlorodimethylsilyl)methylsilane, which was chosen as a core, with the tert-butyldimethylsilyl-substituted lithiooligothienylenes used for construction of the arms. UV-visible absorption and fluorescence properties of 7-11 have been studied in a dioxane solution. The molecules 7-11 showed high fluorescence quantum yields with respect to the corresponding oligothiophenes. Compounds 7 and 8 revealed longer lifetimes than those of bi-and terthiophene. The results of X-ray crystallographic analysis of the bromo derivative 12 are described. The density functional theoretical calculations were also carried out to compare the geometrical structures and the excitation energies with the available experimental data.