Shigeo Hayakawa

Experimental Evidence for an Inverse Hydrogen Migration in Arginine Radicals

Radicals formed by electron transfer to protonated arginine have been predicted by theory to undergo an inverse migration of the hydrogen atom from the C(alpha) position to the guanidine carbon atom. Experiments are reported here that confirm that a fraction of arginine and arginine amide radicals undergo such an inverse hydrogen migration. The rearranged arginine and arginine amide C(alpha) radicals are detected as stable anions after charge inversion by collisions with Cs atoms of precursor cations at 3 and 50 keV kinetic energies. RRKM calculations on the B3-PMP2/aug-cc-pVTZ potential energy surface indicate that arginine radicals undergo rapid rotations of the side chain to reach conformations suitable for C(alpha)-H transfer, which is calculated to be fast (k > 10(9) s(-1)) in radicals formed by electron transfer. By contrast, H-atom transfer from the guanidine group onto the carboxyl or amide C=O groups is >50 times slower than the C(alpha)-H atom migration. The guanidine group in arginine radicals is predicted to be a poor hydrogen-atom donor but a good H-atom acceptor and thus can be viewed as a radical trap. This property can explain the frequent observation of nondissociating cation radicals in electron capture and electron transfer mass spectra of arginine-containing peptides.

Internal energy distribution in charge inversion mass spectrometry using alkali metal targets

Abstract A charge inversion mass spectrometer (PMS/NMS) by using alkali metal targets has been developed which produces clearer differentiation of hydrocarbon isomers than collision induced dissociation (CID). In PMS/NMS, mass-selected positive ions are made to collide with an alkali metal target, and the resulting negative ions formed upon two-electron transfer are mass analyzed. On the basis of the observed target-density dependence of product ion intensity and thermochemical considerations, we propose that the process of dissociative negative ion formation in PMS/NMS is by way of near-resonant neutralization, followed by spontaneous dissociation of the neutrals and then endothermic negative ion formation. CID spectra and PMS/NMS spectra have been measured for two types of so-called thermometer molecules, namely partially deuterated methanol and W(CO)6. The differences between the CID spectra and the PMS/NMS spectra of partially deuterated methanol demonstrate that the major process in PMS/NMS involves dissociation of the excited neutral species. The internal energy deposition of the charge inversion measured for W(CO)n+ (n = 4–6) ions indicates that dissociation occurs in the energy-selected neutrals formed by way of near-resonant neutralization. The relative peak intensities in the PMS/NMS spectra for some hydrocarbons depend strongly on the alkali metal target used, indicating the importance of internal energy in the dissociation of the excited neutral intermediates. These results demonstrate the utility of PMS/NMS as a technique for the investigation of the dissociation of energy-selected neutral intermediates and for isomeric differentiation.

Histidine-containing radicals in the gas phase.

Radicals containing the histidine residue have been generated in the gas phase by femtosecond electron transfer to protonated histidine-N-methylamide (1H+), Nalpha-acetylhistidine-N-methylamide (2H+), Nalpha-glycylhistidine (3H+), and Nalpha-histidylglycine (4H+). Radicals generated by collisional electron transfer from dimethyldisulfide to ions 1H+ and 2H+ at 7 keV collision energies were found to dissociate completely on the microsecond time scale, as probed by reionization to cations. The main dissociations produced fragments from the imidazole side chain and the cleavage of the C(alpha)CO bond, whereas products of NCalpha bond cleavage were not observed. Electron transfer from gaseous potassium atoms to ions 3H+ and 4H+ at 2.97 keV collision energies not only caused backbone NCalpha bond dissociations but also furnished fractions of stable radicals that were detected after conversion to anions. Ion structures, ion-electron recombination energies, radical structures, electron affinities, and dissociation and transition-state energies were obtained by combined density functional theory and Møller-Plesset perturbational calculations (B3-PMP2) and basis sets ranging from 6-311+G(2d,p) to aug-cc-pVTZ. The Rice-Ramsperger-Kassel-Marcus theory was used to calculate rate constants on the B3-PMP2 potential energy surfaces to aid interpretation of the mass spectrometric data. The stability of Nalpha-histidylglycine-derived radicals is attributed to an exothermic isomerization in the imidazole ring, which is internally catalyzed by reversible proton transfer from the carboxyl group. The isomerization depends on the steric accessibility of the histidine side chain and the carboxyl group and involves a novel cation radical-COO salt-bridge intermediate.

Study of the dissociation of neutral intermediates using charge inversion mass spectrometry

In charge inversion experiments using a tandem mass spectrometer, mass-selected positive ions are made to collide with an alkali metal target, and the resulting negative ions formed upon two-electron transfer are mass-analyzed. The internal energy depositions are measured for the so-called thermometer molecule W(CO)6. The difference between the centered value of the internal energy of W(CO)6 and the energy level of the precursor ion is in good agreement with the ionization energy of the Cs target. This correlation indicates that dissociation occurs from energy-selected neutrals formed via near-resonant neutralization.

Discrimination of C3H4+ isomeric ions by charge inversion mass spectrometry using an alkali metal target

Abstract Charge inversion mass spectrometry was performed using an MS/MS instrument in which mass-selected positive ions were made to collide with alkali metal targets, and the resulting negative ions formed upon two-electron transfer were mass analyzed. Charge inversion spectra using Cs targets were measured for C3H4+ ions produced from allene (CH2CCH2) and propyne (CH3CCH) by electron impact. The peak associated with C3H4- in the charge inversion spectra is twice as intense for propyne as for allene, whereas the profile of the peaks associated with C3Hn− (n = 0–2) is similar for both isomeric precursors. The kinetic energy release values at FWHM of the peak associated with C3H3− formed from allene ions and propyne ions are 0.28 ± 0.04 eV and 0.64 ± 0.06 eV, respectively. A clear differentiation between the isomeric precursors can be made on the basis of these differences in the charge inversion spectra. The differences observed are attributed to the formation of excited C3H4 states by near-resonant neutralizations of allene and propyne ions. Collisionally activated dissociation (CAD) spectra using He targets were measured for the same C3H4+ isomeric precursor ions. The CAD spectra are similar for both isomeric precursor ions, although slight differences were detected in the weak peaks associated with the species formed upon CC bond cleavage. Charge inversion mass spectrometry using alkali metal targets was found to provide a much clearer differentiation between the isomeric ions of unsaturated hydrocarbons than CAD.

Kinetic-energy-sensitive mass spectrometry for separation of different ions with the same m/z value

A double-focusing mass spectrometer (MS) equipped with a superconducting-tunnel-junction (STJ) detector has been applied to measure relative ionization cross-sections for the production of ions that are accompanied by different ion species with the same mass-to-charge (m/z) value. The STJ detector fabricated for this study enables kinetic energy (E) measurement of incoming individual ions at a counting rate of up to approximately 100 k ions/s and an energy resolution (DeltaE/E) of 15%. Both high counting rate and high-energy resolution are necessary to independently determine both m and z and not the m/z value only in ion-counting MS experiments. Ions such as (14)N(2) (2+) and (14)N(+) with the same m/z value can be clearly discriminated using a kinetic-energy-sensitive MS. This fine discrimination capability allows direct determination of relative ionization cross-sections of the homonuclear diatomic ions (14)N(2) (2+)/(14)N(2) (+) and (16)O(2) (2+)/(16)O(2) (+), which are difficult to measure due to the strong interference by the signals of their dissociated atomic ions with noticeably large ionization cross-sections. The new instrument requires no low-abundance heteronuclear diatomic molecules of the forms (14)N(15)N or (16)O(17)O to carry out ionization studies and thus, is expected to be useful in fields such as atmospheric science, interstellar science, or plasma physics.

Definitive evidence for the existence of a long-lived vinylidene radical cation, H2C=C+.

Charge inversion mass spectra of C2H2+ ions produced by electron impact from HC≡CH and CH2=CCl2 were measured using K and Cs targets. Clear differences in the charge inversion spectra between HC≡CH and CH2=CCl2 indicate that the C2H2+ ion formed from CH2CCl2 is H2C=C+⋅. The lifetime of H2C=C+⋅ is found to be longer than 8.5 μs, and the state is proposed to be the 2B1 ground-state rather than the 4A2 state.

A new technique to study the dissociation of energy-selected neutral intermediates

Abstract The dissociation of excited species is one of the most important types of chemical reactions, and it has been investigated by using either molecular collision or photon excitation. Mass spectrometry is suited to generate and probe unstable intermediates in the gas phase and has enabled the investigation of the dissociation of various stable and unstable ions. However, investigation of neutral species using this technique has been difficult because of their lack of electronic charge. In this work we have used mass spectrometry to measure collision-induced dissociation (CID) spectra and charge-inversion spectra of CD 3 OH + and CH 3 OD + . The major dissociation process in CID was found to involve elimination of a hydrogen atom from the methyl group, whereas dissociation in the charge-inversion mass spectrometer was found to be via elimination of a hydrogen atom from the hydroxyl group. Hydrogen atom elimination from the hydroxyl group has also been reported as the major process in the photo-induced dissociation of neutral methanol. This demonstrates the usefulness of charge-inversion mass spectrometry as a technique for the investigation of the dissociation of neutral intermediates.

Dissociation mechanism of electronically excited C3H4 isomers by charge inversion mass spectrometry

Abstract The mechanism for dissociation of C3H4 isomers to form neutral radicals was investigated by charge inversion mass spectrometry using alkali metal targets. Electronically excited allene and propyne were formed from the corresponding positive ions by neutralization with alkali metal, and the neutral fragments resulting from dissociation of the excited molecules were converted into negative ions which were mass-analyzed and detected. From the comparison of peak intensities and peak widths among allene, propyne and 3,3,3-trideutero-propyne (CD3CCH), the dissociation mechanism of electronically excited C3H4 was elucidated. The principal feature of the dissociation process involves the loss of two H atoms from the electronically excited hydrocarbons, which is attributed not to two independent CH bond cleavages, but rather to loss of a H2 molecule. The loss of a H2 molecule has a higher activation energy than loss of a H atom, despite the final state of the former having a lower heat of formation than that of the latter. In a CH bond cleavage, the weaker the bond, the more easily it is cleaved. Cleavage of a CC bond is substantially less favorable than the loss of hydrogen atoms, even though the former has a lower energy than the later. This may be attributed to the lower frequency of the CC bond vibration compared with that of the CH bond.

Study of the dissociation of a charge‐reduced phosphopeptide formed by electron transfer from an alkali metal target

Doubly protonated phosphopeptide (YGGMHRQET(p)VDC) ions obtained by electrospray ionization were collided with Xe and Cs targets to give singly and doubly charged positive ions via collision-induced dissociation (CID). The resulting ions were analyzed and detected by using an electrostatic analyzer (ESA). Whereas doubly charged fragment ions resulting from collisionally activated dissociation (CAD) were dominant in the CID spectrum with the Xe target, singly charged fragment ions resulting from electron transfer dissociation (ETD) were dominant in the CID spectrum with the Cs target. The most intense peak resulting from ETD was estimated to be associated with the charge-reduced ion with H2 lost from the precursor. Five c-type fragment ions with amino acid residues detached consecutively from the C-terminal were clearly observed without a loss of the phosphate group. These ions must be formed by N--Calpha bond cleavage, in a manner similar to the cases of electron capture dissociation (ECD) and ETD from negative ions. Although the accuracy in m/z of the CID spectra was about +/-1 Th because of the mass analysis using the ESA, it is supposed from the m/z values of the c-type ions that these ions were accompanied by the loss of a hydrogen atom. Four z-type (or y--NH3, or y--H2O) ions analogously detached consecutively from the N-terminal were also observed. The fragmentation processes took place within the time scale of 4.5 micros in the high-energy collision. The present results demonstrated that high-energy ETD with the alkali metal target allowed determination of the position of phosphorylation and the amino acid sequence of post-translational peptides.