Efficiency of collisionally-activated dissociation and 193-nm photodissociation of peptide ions in fourier transform mass spectrometry

For tandem mass spectrometry, the Fourier transform instrument exhibits advantages for the use of collisionally-activated dissociation (CAD). The CAD energy deposited in larger ions can be greatly increased by extending the collision time to as much as 120 s, and the efficiency of trapping and measuring CAD product ions is many times greater than that found for triple-quadrupole or magnetic sector instruments, although the increased pressure from the collision gas is an offsetting disadvantage. A novel system that uses the same laser for photodesorption of ions and their subsequent photodissociation can produce complete dissociation of larger oligopeptide ions and unusually abundant fragment ions. In comparison to CAD, much more internal energy can be deposited in the primary ions using 193-nm photons, sufficient to dissociate peptide ions of m/z > 2000. Mass spectra closely resembling ion photodissociation spectra can also be obtained by' neutral photodissociation (193-nm laser irradiation of the sample) followed by ion photodesorption.     

Bond-selective photodissociation of aliphatic disulfides

The photochemistry of aliphatic disulfides is presented. The photolysis products are photoionized with coherent vacuum ultraviolet radiation and analyzed by time-of-flight mass spectrometry. With 248-nm excitation, the predominant dissociation pathway is S—S bond cleavage. With 193-nm excitation, S—S bond cleavage, C—S bond cleavage, and molecular rearrangements are all observed as primary processes. The branching ratio for S—S bond cleavage relative to C—S bond cleavage is typically 1–2 orders of magnitude greater at 248 run than 193 run. This wavelength dependence cannot be explained readily by photodissociation from the ground electronic state. The ground state S—S bond energy, ～ 280 kJ/mol, is much larger than the C—S bond energy, ～ 235 kJ/mol. If dissociation occurred from the ground state, higher wavelength radiation would be expected to favor the lower energy process, but the opposite effect is observed. Thus, excited state photochemistry is indicated. These results are discussed with respect to the differences between low and high energy collision-induced dissociation of peptides that contain disulfide linkages and to the possibility of achieving bond-selective photodissociation of such ions.     

Sequence and side-chain specific photofragment (193 nm) ions from protonated substance P by matrix-assisted laser desorption ionization time-of-flight mass spectrometry

Photodissociation at 193 nm (6. 43 eV) of the protonated substance-P, [M + H]⁺ ions, in a delayed extraction matrix-assisted laser desorption ionization (MALDI) time-of-flight (TOF) mass spectrometer, is reported. The photofragment ion spectrum of substance P contains a complete series of a-type fragment ions and abundant side-chain cleavage ions. This article focuses on the utility of MALDI-TOF photodissociation for peptide sequencing.     

A collinear tandem time-of-flight mass spectrometer for infrared photodissociation spectroscopy of mass-selected ions

An apparatus based on collinear tandem time-of-flight mass spectrometer has been designed for the measurement of infrared photodissociation spectroscopy of mass-selected ions in the gas phase.The ions from a pulsed laser vaporization supersonic ion source are skimmed and mass separated by a Wiley-McLaren time-of-flight mass spectrometer.The ion of interest is mass selected,decelerated and dissociated by a tunable IR laser.The fragment and parent ions are reaccelerated and mass analyzed by the second time-of-flight mass spectrometer.A simple new assembly integrated with mass gate,deceleration and reacceleration ion optics was designed,which allows us to measure the infrared spectra of mass selected ions with high sensitivity and easy timing synchronization.     

Laser photo-induced dissociation using tandem time-of-flight mass spectrometry

A novel tandem time-of-flight (TOF) mass spectrometer has been developed for studying the photo-induced dissociation of large molecules and elemental clusters. It consists of a linear first stage TOF analyser for primary mass separation and precursor ion selection, and a second orthogonal reflecting field TOF analyser for product ion analysis. The instrument is equipped with a large volume throughput molecular beam source chamber allowing the production of jet-cooled molecules and molecular clusters, as well as elemental clusters, using either a pulsed laser vaporisation source (LVS) or a pulsed are cluster ion source (PACIS). A second differentially pumped chamber can be used with effusive sources, or for infrared laser desorption of large molecules, followed by laser ionisation. These primary ions can then be irradiated with a second, high energy laser to induce photodissociation. Detailed information about the fragmentation mechanisms can be deduced from the product ion mass spectra. Preliminary results on the photo-induced dissociation (PID) of the molecule ion of aniline at 266 nm are presented. In this case the molecule ions were generated via two-photon laser ionisation at 266 nm using an effusive source. Results for the collision-induced dissociation (CID) of the aniline molecule ion, using a commercial mass spectrometer equipped with an atmospheric pressure electrospray ionisation interface, are also presented. Copyright 2000 John Wiley & Sons, Ltd.     

Elucidating the various multi-phosphorylation statuses of protein functional regions by 193-nm ultraviolet photodissociation

The 193-nm ultraviolet photodissociation strategy could significantly improve the MS ability in elucidating the confused phosphorylation sites with multiple possible positions due to its ability in generating more phosphorylation site-determining ions and providing higher sequence coverage.     

Photodissociation of singly protonated peptides at 193 nm investigated with tandem time-of-flight mass spectrometry

Photodissociation at 193 nm of some singly protonated peptides generated by matrix-assisted laser desorption/ionization was investigated using tandem time-of-flight mass spectrometry. For peptides with arginine at the C-terminus, x, upsilon, and w fragment ions were generated preferentially while a and d fragment ions dominated for peptides with arginine at the N-terminus. These are the same characteristics as photodissociation at 157 nm reported previously. Overall, the photodissociation spectra obtained at 157 and 193 nm were strikingly similar.     

Peptide photodissociation at 157 nm in a linear ion trap mass spectrometer

The photodissociation by 157 nm light of singly- and doubly-charged peptide ions containing C- or N-terminal arginine residues was studied in a linear ion trap mass spectrometer. Singly-charged peptides yielded primarily x- and a-type ions, depending on the location of the arginine residue, along with some related side-chain fragments. These results are consistent with our previous work using a tandem time-of-flight (TOF) instrument with a vacuum matrix-assisted laser desorption/ionization (MALDI) source. Thus, the different internal energies of precursor ions in the two experiments seem to have little effect on their photofragmentation. For doubly-charged peptides, the dominant fragments observed in both photodissociation and collisionally induced dissociation (CID) experiments are b- and y-type ions. Preliminary experiments demonstrating fragmentation of multiply-charged ubiquitin ions by 157 nm photodissociation are also presented.     

Photon-induced dissociation with a four-sector tandem mass spectrometer

The feasibility of using photodissociation of protonated peptide molecules to sequence specific fragment ions with a 193-nm pulsed laser beam in a magnetic deflection tandem mass spectrometer of EBEB configuration was demonstrated. Although the short pulse (15 ns) and low repetition rate (100 Hz) of the excimer laser permitted the irradiation of only ～ 0.02% of the (M + H)⁺ ions exiting MS-1, a photon-induced decomposition spectrum of the heptapeptide angiotensio III (M _r 930.5) was produced that was practically the same (but with better signal-to-noise ratio) as that generated by collision-activated dissociation at the same low duty cycle. Because of the low and pulsed fragment ion currents, an array detector was used to record the spectra. A dependence between laser power and abundance of fragment ions was observed (increased power increases the relative abundance of ions of low mass). Laser power was varied from 6 to 80 mJ. Formation of fragment ions from a large peptide (melittin, M, 2844.75) was also observed. The results permit the design of modifications that may increase the fragment ion yield to 10% or higher, which would make photon-induced decomposition a useful method for magnetic deflection mass spectrometers.     

Fragmentation of oligosaccharide ions with 157 nm vacuum ultraviolet light

The 157 nm photofragmentation of native and derivatized oligosaccharides was studied in a linear ion trap and in a home-built matrix-assisted laser desorption/ionization (MALDI) tandem time-of-flight (TOF/TOF) mass spectrometer, and the results were compared with collision-induced dissociation (CID) experiments. Photodissociation produces product ions corresponding to high-energy fragmentation pathways; for cation-derivatized oligosaccharides, it yields strong cross-ring fragment ions and provides better sequence coverage than low- and high-energy CID experiments. On the other hand, for native oligosaccharides, CID yielded somewhat better sequence coverage than photodissociation. The ion trap enables CID hybrid MS3 experiments on the high-energy fragment ions obtained from photodissociation.     

Laser photodissociation of organometallic compounds on a cryosubstrate

The photodissociation dynamics of organometallic compounds (tetramethyltin, trimethylgallium, trimethylindium and dimethylzinc) adsorbed on a quartz substrate at 100 K have been studied by time-of-flight mass spectrometry, detecting mainly CH₃ photofragments. Shapes of the time-of-flight spectra depend on the flux of the dissociation laser at 193 nm and the thickness of multilayers of organometallic compounds. In thick layers, not only a low energy component but also a high energy component are observed in the time-of-flight spectrum of the CH₃ photofragments. In thin layers, relaxation processes take place so quickly that the product time-of-flight profiles are characterized by a Maxwell–Boltzmann temperature profile.     

Tandem time-of-flight mass spectrometer for photodissociation of biopolymer ions generated by matrix-assisted laser desorption ionization (MALDI-TOF-PD-TOF) using a linear-plus-quadratic potential reflectron

A tandem time-of-flight mass spectrometer for the study of photodissociation of biopolymer ions generated by matrix-assisted laser desorption ionization was designed and constructed. A reflectron with linear and quadratic (LPQ) potential components was used. Characteristics of the LPQ reflectron and its utility as the second stage analyzer of the tandem mass spectrometer were investigated. Performance of the instrument was tested by observing photodissociation of [M + H](+) from angiotensin II, a prototype polypeptide. Quality of the photodissociation tandem mass spectrum was almost comparable to that of the post-source decay spectrum. Monoisotopic selection of the parent ion was possible, which was achieved through the ion beam-laser beam synchronization. General theoretical considerations needed for a successful photodissociation of large biopolymer ions are also presented.     

Sequence- and site-specific photodissociation at 266 nm of protonated synthetic polypeptides containing a tryptophanyl residue

Photodissociation at 266 nm of protonated synthetic polypeptides containing a tryptophanyl residue was investigated using a homebuilt tandem time-of-flight mass spectrometer equipped with a matrix-assisted laser desorption/ionization source. Efficient photodissociation of the protonated peptides was demonstrated. Most of the intense peaks in the laser-induced tandem mass spectra were sequence ions. Furthermore, sequence ions due to cleavages at all the peptide bonds were observed; this is a feature of the technique that is particularly useful for peptide sequencing. Fragmentations at both ends of the tryptophanyl residue were especially prevalent, which can be useful for location of the tryptophanyl chromophore in a peptide.     

Electron impact excitation of ions from larger organic molecules

[M + Na]⁺ ions (m/z 1164) from gramicidin S can be dissociated in a Fourier-transform mass spectrometer with ～ 85% efficiency using a 50 μA beam of 70 eV electrons for 500 μs. Although the fragment ions are smaller, on average, than those from collisionally activated dissociation or 193 nm photodissociation, these also provide extensive sequence information.     

飞秒脉冲作用下氯丙烯的多光子解离和电离动力学

The dissociation and photoionization dynamics of C₃H₅Cl were studied at 200, 400, and 800 nm with femtosecond laser pulses. The time-of-flight mass spectra, laser power index and photoelectron images were recorded. At short wavelength (200 nm), ionization of the parent molecule was found to be the dominant channel, while other ions were generated by the dissociation of C₃H₅Cl⁺. With the shift to long wavelength (e.g., 800 nm), fragment ions became dominant, and were generated through the multiphoton ionization of neutral fragments after the photodissociation of C₃H₅Cl. These results imply that photodissociation plays a significant role at long wavelength, because neutral fragments are supposed to be generated from the intermediate states reached by 800 nm photons. At 400 nm, the dissociation on the intermediate states is also critical, but is not as high as that at 800 nm. Taken together, our results demonstrate that the dissociation/ionization behaviors of allyl chloride are wavelength-dependent, and reveal the complex dynamics of allyl chloride at 200, 400 and 800 nm.     

不同电荷态泛素蛋白离子的193 nm紫外光解离质谱

将193 nm激光与傅里叶变换离子回旋共振(FT-ICR)质谱仪结合, 研究了不同价态的泛素蛋白离子的紫外光解离质谱. 结果表明, 在光解离过程中向分析池内引入适量的碰撞气, 不仅能增加母体离子的裂解率, 也能提高碎片离子的捕获效率. 相对于碰撞辅助解离(CAD)中产生的b和y离子, 紫外光解离(UVPD)方法能够产生更为丰富的不同种类碎片离子. 其中, 对于+11价泛素离子, 蛋白质序列的覆盖率接近80%, 远高于对应的CAD实验结果. 与已往报道不同的是, 裂解覆盖率呈现出较强的电荷相关性. 因此, 如何进一步提高较低电荷态的蛋白离子的解离效率和序列覆盖率还需深入研究.     

铅、硫团簇的形成、反应与光解

用激光直接溅射和串级溅射两种方法产生铅／硫二元团簇,并用串级飞行时间质谱仪研究了二元团簇的组份及光解规律,用激光直接溅射铜＋硫混合样品时,组成为PbnSn-1 和PbnSn-的团簇丰度最大,是二元团簇的结构骨架和稳定组份,而用激光串级溅射铅样品和硫样品,通过铅团簇与硫团簇的反应,则可得到PbnSm (n=1-3,m=0-9)和PbnSm-(n=1-7,m=0-9)。这两种二元团簇的产生方法对应两种不同的团簇形成机理。     

Mass resolved MPI spectra of methyl iodide in the 430-490 nm region

The mass resolved multiphoton ionization (MPI) spectra of methyl iodide were obtained in the 430-490 nm region using a time-of-flight (TOF) mass spectrometer. They have the same vibrational structure, which testifies that the fragment species, in the wavelength region under study, are from the photodissociation of multiphoton ionized molecular parent ions. Some features in the spectra are identified as three-photon excitations to 6p and 7s Rydberg states of methyl iodide. Two new vibrational structures of some Rydberg states are observed. The mechanism of ionization and dissociation is also discussed.     

Photoinduced dissociation of electrospray-generated ions in an ion trap/time-of-flight mass spectrometer using a pulsed CO2 laser

An ion trap/time-of-flight (IT/TOF) mass spectrometer was developed and applied to infrared multiphoton dissociation (IRMPD) studies of ions generated by electrospray ionization. A pulsed 10.6- micro m laser beam from a CO(2) laser was used for excitation of trapped ions. Results from IRMPD of peptide ions show that this method provides useful information related to the amino acid sequence of analyzed peptides. Comparative studies show that IRMPD spectra are similar to those obtained using a 266-nm UV laser beam for excitation. However, in contrast to multiple-pulse excitation required at 266 nm, the energy of a single laser pulse in IRMPD is sufficient to induce dissociation of peptide ions. The laser power is practically an exclusive parameter that must be controlled in order to obtain IRMPD spectra that will provide the optimal structural information. It is further demonstrated that the IRMPD IT/TOF technique has the potential to probe the structural features of larger ions that cannot be readily fragmented by collision-induced dissociation (CID). A multiply charged ion of equine cytochrome c is successfully fragmented in a single laser pulse experiment. The IRMPD IT/TOF technique is also shown to be a promising tool for studying dissociation kinetics of peptide and protein ions. Unlike other methods that usually monitor the dissociation ion kinetics in a dissociation time frame of greater than milliseconds, the IT/TOF can promptly detect all product ions generated by the dissociation process, and thus monitor the dissociation process of peptides and proteins in a sub-millisecond time frame. This instrument allows us to determine the dissociation rates of cytochrome c ions using high-energy photoexcitation. It is found that the charge state of the protein ion has a significant effect on dissociation kinetics, which is consistent with that found under low-energy excitation experiments. It is shown that the increase in energy of a laser pulse from 130 to 180 mJ changes the dissociation rate constant for the +12 ion from k = 2.4 x 10(3) x s(-1) to k = 7.3 x 10(4) x s(-1). The +8 ion following excitation at 130 mJ dissociates slower with a rate constant of k = 2.6 x 10(2) x s(-1). The rate difference observed is attributed to conformational differences among the ions with different charge states.     

High Efficiency Photo-Induced Dissociation of Precursor Ions in a Tandem Time-of-Flight Mass Spectrometer

High efficiency photo-induced dissociation (PID) has been demonstrated in a tandem time-of-flight mass spectrometer. This instrument focuses isomass ion packets to temporal and spatial dimensions similar to those of the focused laser pulses from a high power excimer laser. This high density overlap of photons and ions yields highly efficient fragmentation and also provides high resolution selection of specific precursor ion mass-to-charge ratio values. Using 193 nm photon excitation of the molecular ion of bromobenzene (m/z = 1561, fragmentation, collection, and PID efficiencies af 79%, 132%, and 104%, respectively, were obtained. Characteristic fragmentations of toluene, nitrobenzene, acetophenone, triethylamine, N,N-diethylformamide, N-methylacetamide, and cyclohexene have also been demonstrated.