Biomedical subjects

Paul M Mayer

Publications and source records attributed to Paul M Mayer.

8 recordsLinked to original sources

PubMed2006

A photoelectron and TPEPICO investigation of the acetone radical cation.

The valence shell photoelectron spectrum, threshold photoelectron spectrum, and threshold photoelectron photoion coincidence (TPEPICO) mass spectra of acetone have been measured using synchrotron radiation. New vibrational progressions have been observed and assigned in the X 2B2 state photoelectron bands of acetone-h6 and acetone-d6, and the influence of resonant autoionization on the threshold electron yield has been investigated. The dissociation thresholds for fragment ions up to 31 eV have been measured and compared to previous values. In addition, kinetic modeling of the threshold region for CH3* and CH4 loss leads to new values of 78 +/- 2 kJ mol(-1) and 75 +/- 2 kJ mol(-1), respectively, for the 0 K activation energies for these two processes. The result for the methyl loss channel is in reasonable agreement with, but slightly lower than, that of 83 +/- 1 kJ mol(-1) derived in a recent TPEPICO study by Fogleman et al. The modeling accounts for both low-energy dissociation channels at two different ion residence times in the mass spectrometer. Moreover, the effects of the ro-vibrational population distribution, the electron transmission efficiency, and the monochromator band-pass are included. The present activation energies yield a Delta(f)H298 for CH3CO+ of 655 +/- 3 kJ mol(-1), which is 4 kJ mol(-1) lower than that reported by Fogleman et al. The present Delta(f)H298 for CH3CO+ can be combined with the Delta(f)H298 for CH2CO (-47.5 +/- 1.6 kJ mol(-1)) and H+ (1530 kJ mol(-1)) to yield a 298 K proton affinity for ketene of 828 +/- 4 kJ mol(-1), in good agreement with the value (825 kJ mol(-1)) calculated at the G2 level of theory. The measured activation energy for CH4 loss leads to a Delta(f)H298 (CH2CO+*) of 873 +/- 3 kJ mol(-1).

Journal Article↗

PubMed2006

Threshold-photoelectron spectroscopic study of methyl-substituted hydrazine compounds.

The valence shell electronic structures of methylhydrazine (CH(3)NHNH(2)), 1,1-dimethylhydrazine ((CH(3))(2)NNH(2)) and tetramethylhydrazine ((CH(3))(4)N(2)) have been studied by recording threshold and conventional (kinetic energy resolved) photoelectron spectra. Ab initio calculations have been performed on ammonia and the three methyl substituted hydrazines, with the structures being optimized at the B3-LYP/6-31+G(d) level of theory. The ionization energies of the valence molecular orbitals were calculated using the Green's function method, allowing the photoelectron bands to be assigned to specific molecular orbitals. The ground-state adiabatic and vertical ionization energies, as determined from the threshold photoelectron spectra, were IE(a) = 8.02 +/- 0.16 eV and IE(v) = 9.36 +/- 0.02 eV for methylhydrazine, IE(a) = 7.78 +/- 0.16 eV and IE(v) = 8.86 +/- 0.01 eV for 1,1-dimethylhydrazine and IE(a) = 7.26 +/- 0.16 eV and IE(v) = 8.38 +/- 0.01 eV for tetramethylhydrazine. Due to the large geometry change that occurs upon ionization, these IE(a) values are all higher than the true thresholds. New features have been observed in the inner valence region and these have been compared with similar structure in the spectrum of hydrazine. The effect of resonant autoionization on the threshold photoelectron yield is discussed. New heats of formation (Delta(f)H) are proposed for the three hydrazines on the basis of G3 calculations: 107, 94, and 95 kJ/mol for methylhydrazine, 1,1-dimethyhydrazine and tetramethylhydrazine, respectively. The previously reported Delta(f)H for tetramethylhydrazine is shown to be erroneous.

Journal Article↗

PubMed2005

Delta(DeltaS) and Delta(DeltaS) for the competing bond cleavage reactions in (CH3CN)(ROH)H+ [R = CH3, C2H5, C3H7, (CH3)2CH].

Microcanonical variational transition-state theory was used to determine the entropies of activation for hydrogen-bond cleavage reactions leading to CH(3)CN + ROH(2)(+) in a series of acetonitrile-alcohol proton-bound pairs (CH(3)CN)(ROH)H(+) (where R = CH(3), CH(3)CH(2), CH(3)CH(2)CH(2), and (CH(3))(2)CH). In each case, the dissociation potential surface was modelled at the MP2/6-31 + G(d) level of theory. The dissociating configurations having the minimum sums-of-states were identified in each case and the resulting entropies of activation were calculated. Combined with previous work on the competing reaction leading to CH(3)CNH(+) + ROH, the results permitted the determination of the Delta(DeltaS) in each proton-bound pair. For the (CH(3)CN)(CH(3)OH)H(+) and (CH(3)CN)(CH(3)CH(2)OH)H(+) proton-bound pairs, the entropies of activation for the two dissociating channels are essentially the same [i.e., Delta(DeltaS) = 0], while Delta(DeltaS) for the propanol-containing pairs ranged between 40 and 45 J K(-1) mol(-1). The latter non-zero values are due to a combination of the location of the dividing surface in each dissociation and the rapidity with the frequencies of the vanishing vibrational modes go to zero as they are converted to product translations and rotations during the dissociation.

Acetonitriles↗

PubMed2005

What ion is generated when ionizing acetonitrile?

It has long been assumed that ionizing neutral acetonitrile produces ions with the same atomic connectivity, CH(3)CN(+*). Recent calculations on the C(2)H(3)N(+*) potential energy surface have suggested that it may be difficult to generate pure CH(3)CN(+*) when ionizing acetonitrile. We have probed the interconversion of CH(3)CN(+*) and its lower energy isomer CH(2)CNH(+*) by calculation, collision-induced dissociation mass spectrometry and ion-molecule reaction. The latter ion, ionized ketenimine, is co-generated upon electron or chemical ionization of neutral acetonitrile in the ion source of a mass spectrometer. An estimate of the ratio of the two isomers can be obtained from their respective ion-molecule reactions with CO(2) or COS. CH(3)CN(+*) reacts by proton-transfer with CO(2) and charge transfer with COS, whereas CH(2)CNH(+*) is unreactive.

Acetonitriles↗

PubMed2005

Air-vegetation partitioning of polychlorinated biphenyls near a point source.

We investigated polychlorinated biphenyl (PCB) emissions to the environment from a waste treatment and transfer facility over the course of three years. We show that the facility, which undertakes PCB waste consolidation and maintains a low-yield incinerator for products such as light ballasts, acted as a point source for the spatial distribution of PCBs in vegetation. Concurrent air and vegetation sampling was performed to study the relationship between air-vegetation partitioning and the octanol-air partition coefficient (KOA). We show evidence of equilibrium partitioning for lower-chlorinated congeners (log KOA between 7 and 8.5), kinetically limited deposition on plants for intermediate congeners (log KOA between 8.5 and 11), and particle-bound deposition for congeners with high log KOA values (> 11), consistent with the McLachlan partitioning model. From spring to autumn, heavier congeners become much more concentrated in samples farther away from the facility, possibly because of higher temperatures, which enhance dispersal of these congeners. Multivariate principal components analysis showed that PCB composition in vegetation near the emission source most closely resembled the Aroclor mixtures processed by the treatment facility.

Air↗

PubMed2004

Molecularly imprinted solid phase extraction-pulsed elution-mass spectrometry for determination of cephalexin and alpha-aminocephalosporin antibiotics in human serum.

A highly selective molecularly imprinted solid phase extraction (MISPE)-pulsed elution (PE) method coupled with electrospray mass spectrometry (MS) was developed for the rapid screening and determination of cephalexin in alpha-aminocephalosporin antibiotics. This method involved the solid phase extraction of cephalexin using a molecularly imprinted polymer micro-column, and pulsed elution with 1% trifluoroacetic acid in methanol, which contains sulindac as an internal standard for enhanced precision in MS detection. An LC/MS spectrometer was operated in the positive electrospray mode, and the selected-ion-recording (SIR) function was employed to detect the molecular ions of cephalexin, cefradine, cefadroxil and sulindac at m/z 348, 350, 363 and 357. Linearity was achieved in the cephalexin concentration range from 0.3 to 25microg/ml (or 5-500ng) (R(2) = 0.998). The detection limit was estimated at 0.04microg/ml (or 0.8ng) of cephalexin. Advantages of the newly developed MISPE-PE-MS, over the previously reported MISPE-DPE-FPE-UV, were evidenced in terms of detection limit, analysis time, solvent consumption, and simplicity of method development.

Cephalexin↗

PubMed2004

Confirmation of the "long-lived" tetra-nitrogen (N4) molecule using neutralization-reionization mass spectrometry and ab initio calculations.

Tetra-nitrogen (N(4)), which has been the subject of recent controversy [Cacace, d. Petris, and Troiani, Science 295, 480 (2002); Cacace, Chem. Eur. J. 8, 3839 (2002); Nguyen et al., J. Phys. Chem. A 107, 5452 (2003); Nguyen, Coord. Chem. Rev. 244, 93 (2003)] as well as of great theoretical interest, has been prepared from the N(4) (+) cation and then detected as a reionized gaseous metastable molecule with a lifetime exceeding 0.8 micros in experiments based on neutralization-reionization mass spectrometry. Moreover, we have used the nature of the charge-transfer reaction which occurs between a beam of fast N(4) (+) ions (8 keV translational energy) and various stationary gas targets to identify the vertical neutralization energy of the N(4) (+) ion. The measured value, 10.3+/-0.5, most closely matches that of the lowest energy azidonitrene (4)N(4) (+)C(s)((4)A(')) ion, resulting in the formation of the neutral bound azidonitrene (3)N(4)C(s)((3)A(")). Neutralization of the global minimum (2)N(4) (+)D( infinity h)((2)Sigma(u) (+)) ion leads to a structure 166 kJ mol(-1) above the dissociation products [N(2)((1)Sigma(g) (+))+N(2)((1)Sigma(g) (+))]; moreover, it was not possible to find a minimum on the (1)N(4) neutral potential energy surface for a covalently bonded structure. Ab initio calculations at the G3, QCISD/6-31G(d), and MP2/AUG-cc-pVTZ levels of theory have been used to determine geometries and both vertical neutralization energies of ions (doublet and quartet) and ionization energies of neutrals (singlet and triplet). In addition, we have also described in detail the EI ion source for the Ottawa VG ZAB mass spectrometer [Holmes and Mayer, J. Phys. Chem. A 99, 1366 (1995)] which was modified for high-pressure use, i.e., for the production of dimer and higher number cluster ions.

Journal Article↗

PubMed2004

Methylating peptides to prevent adduct ion formation also directs cleavage in collision-induced dissociation mass spectrometry.

We investigated the effect of N-terminal amino group and carboxyl group methylation on peptide analysis by electrospray mass spectrometry (ESI-MS) and tandem mass spectrometry (ESI-MS/MS). Permethylation of the N-terminal amino group and the carboxyl groups can reduce metal ion adducts but does not enhance sensitivity in electrospray as previously observed for matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. N-terminal trimethylated peptides exhibit collision-induced dissociation (CID) tandem mass spectra that differ from their unmodified analogs; the results support the mobile proton hypothesis of peptide fragmentation. A permanent positive charge at the N-terminus leads to competition between permanent-charge directed processes and loss of the N-terminal trimethyl amino group. Carboxyl methylation has no effect on fragmentation behavior other than to shift the mass of fragments containing methylated carboxyl groups. Comparison of regular and tandem mass spectra of different methylated peptides allowed probing the location of incomplete methylation, the proton displaced by alkali metal ions and the purity of a mass-selected methylated peptide ion.

Amino Acid Sequence↗