Multiphoton ionization of pyrrole-water mixed clusters

Multiphoton ionization of the hydrogen,bonded pyrrole-water clusters (C4H5N)(n)(H2O)(m) is studied with a reflectron-time of flight mass spectrometer at 355 mn. With increasing partial concentration of pyrrole in a gas mixture source, a series of poly-pyrrole-water binary-mixed cluster ions can be observed, including unprotonated cluster ions [(C4H5N)(x)(H2O)(y)](+), protonated cluster ions [(C4H5N)(x)(H2O)(y)](+) and dehydrogenated cluster ions [(C4H4N)(C4H5N)(x)(H2O)(y)](+). Ab initio calculations of their structures, bond strengths, charge distributions and reaction energies are carried out. Stable structures of these clusters are obtained from the calculations. A probable formation mechanism of the cluster ions [(C4H5N)(x)(H2O)(y)](+), [(C4H5N)(x)(H2O)(y)]H+ and [(C4H4N)(C4H5N)(x) (H2O)(y)](+) is supposed to be the ionization of clusters followed by dissociation.

Multiphoton ionization and ab initio calculation studies of pyridine-water mixed clusters using time of flight mass spectrometer

Multiphoton ionization of binary mixed clusters (C5H5N)(x)-(H2O)(y) at 532, 355 and 266 nm laser wavelengths has been investigated using TOF mass spectrometer. The experiments showed that almost all the products were protonated ions, At 532 and 355 nm, the products were mainly protonated pyridine clusters (C5H5N)(n)-H+, while at 266 nm, mixed binary cluster ions (C5H5N)(m)- (H2O)(n)-H+ appeared. It was found that the abundance of the [(C5H5N)(3)-H2O-H](+) ions was abnormally high. The calculation indicated that the ion [(C5H5N)(3)-H2O-H](+) is Of a kind of magic number structures with C-3v symmetry. A stepwise reaction mechanism is suggested that photoionization is followed by dissociation. (C) 2001 Elsevier Science B.V. All rights reserved.

Matrix-assisted laser desorption/ionization mass spectrometry using porous silicon and silica gel as matrix

Porous silicon powder and silica gel particles have been applied as inorganic matrices for the analysis of small molecules in matrix-assisted laser desorption/ionization mass spectrometry (MALDI-TOFMS). In contrast to conventional MALDI-TOFMS, the signal interference of low-molecular analytes by the matrix has been eliminated. Almost no fragmentations of the analytes were observed. Effects of various factors, such as the particle and pore size, the suspending solution, and sample preparation procedures, on the intensity of mass spectra have been investigated. The pore structure of the inorganic matrix and penetration of the analytes into the pores must be optimized for effective desorption and ionization of the analytes. Matrices (DHB and HCCA) were covalently bound to silica gel for improvement of spectrum intensity. Copyright (C) 2001 John Wiley & Sons, Ltd.

Dynamics of excited OCS investigated by resonance-enhanced multiphoton ionization

The predissociation decay behavior of molecule carbonyl sulfide (OCS) has been investigated by resonance-enhanced multiphoton ionization spectroscopy using the pump-probe technique of dichroic femtosecond lasers in real time. The lifetime of excited OCS around 74 720 cm(-1) by two-photon absorption of 268 nm, corresponding to upsilon(1)=1 of the Rydberg state [(2)Pi(1/2)]4ppi((1)Sigma(+)), is directly determined to be tau(D)=1071+/-11 fs. This picosecond decay process indicates that the excited state is predissociative. The temporal information of dissociation enriches the knowledge of the potential-energy surface of the associative excited state.

Evaluation of flavonoids binding to DNA duplexes by electrospray ionization mass spectrometry

In this study, electrospray ionization mass spectrometry (ESI-MS) was used to investigate the binding interactions of ten flavonoid aglycones and ten flavonoid glycosides with DNA duplexes. Relative binding affinities of the flavonoids toward DNA duplexes were estimated based on the fraction of bound DNA. The results revealed that the 4'-OH group of flavonoid aglycones was essential for their DNA-binding properties. Flavonoid glycosides with sugar chain linked on ring A or ring B showed enhanced binding toward the duplexes over their aglycone counterparts, whereas glycosylation of the flavonol quercetin on ring C exhibited a less pronounced effect.

Structural characterization and identification of dibenzocyclooctadiene lignans in Fructus Schisandrae using electrospray ionization ion trap multiple-stage tandem mass spectrometry and electrospray ionization Fourier transform ion cyclotron resonance multiple-stage tandem mass spectrometry

The electrospray ionization ion trap multiple-stage tandem mass spectrometry (ESI-MSn) and electrospray ionization Fourier transform ion cyclotron resonance multiple-stage tandem mass spectrometry (ESI-FT-ICR-MSn) have been applied successfully to the direct investigation of a number of dibenzocyclooctadiene lignan constituents from the methanol extracts of the Fructus Schisandrae in the positive ion mode. The detailed structural characterization of the same skeleton and different peripheral substituents had been studied and the precise elemental compositions of ions at high mass resolution had been obtained. So the fragmentation mechanisms could be clarified.

Studies on alkaloids binding to GC-rich human survivin promoter DNA using positive and negative ion electrospray ionization mass spectrometry

Electrospray ionization mass spectrometry (ESI-MS) was used to investigate the binding of 13 alkaloids to two GC-rich DNA duplexes which are critical sequences in human survivin promoter. Negative ion ESI-MS was first applied to screen the binding of the alkaloids to the duplexes. Six alkaloids (including berberine, jatrorrhizine, palmatine, reserpine, berbamine, and tetrandrine) show complexation with the target DNA sequences. Relative binding affinities were estimated from the negative ion ESI data, and the alkaloids show a binding preference to the duplex with higher GC content. Positive ion ESI mass spectra of the complexes were also recorded and compared with those obtained in negative ion mode.

Studies of the intermolecular DNA triplexes of C+center dot GC and T center dot AT triplets by electrospray ionization Fourier-transform ion cyclotron resonance mass spectrometry

Formation and stabilities of four 14-mer intermolecular DNA triplexes, consisting of third strands with repeating sequence CTCT, CCTT, CTT, or TTT, were studied by electrospray ionization Fourier-transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS) in the gas phase. The gas-phase stabilities of the triplexes were compared with their CD spectra and melting behaviors in solution, and parallel correlation between two phases were obtained. In the presence of 20 mm NH4+ (pH 5.5), the formation of the TTT triplex was not detected in both solution and the gas phase.