The effects of electrospray ionization (ESI) solvent and source temperature on the relative abundance of the preferred solution-phase (N-protonated; i.e. amine) versus preferred gas-phase (O-protonated; i.e., acid) isomers of p-aminobenzoic acid (PABA) were investigated. When PABA was electrosprayed from protic solvents (i.e., methanol/water), the infrared multiple photon dissociation (IRMPD) spectrum recorded was consistent with that for O-protonation, according to both calculations and previous studies. When aprotic solvent (i.e., acetonitrile) was used, a different spectrum was recorded and was assigned to the N-protonated isomer. As the amine is the preferred protonation site in solution, this suggests that an isomerization takes place under certain conditions. Photodissociation at the diagnostic band for the O-protonated isomer (NH2 stretching mode) was used to quantify the relative contributions of each isomer to ion signal as a function of ESI conditions. For mixtures of methanol and acetonitrile, the relative contribution of the O-protonated gas-phase structure increased as a function of methanol content. Yet, substituting methanol for water resulted in a marked decrease of isomerization to the O-protonated structure. The source temperature (i.e., temperature of a heated desolvation capillary) was found to play a key role in determining the extent of isomerization, with higher temperatures yielding increased presence of gas-phase structures. These results are consistent with a protic bridge mechanism, in which the ESI droplet temperatures, dependent on endothermic desolvation and radiative heating from the capillary, may determine the isomerization yield.

No abstract

Ionic liquids are used for myriad applications, including as catalysts, solvents, and propellants. Specifically, 2-hydroxyethylhydrazinium nitrate (HEHN) has been developed as a chemical propellant for space applications. The gas-phase behavior of HEHN ions and clusters is important in understanding its potential as an electrospray thruster propellant. Here, the unimolecular dissociation pathways of two clusters are experimentally observed, and theoretical modeling of hydrogen bonding and dissociation pathways is used to help rationalize those observations. The cation/deprotonated cation cluster [HEH - H], which is observed from electrospray ionization, is calculated to be considerably more stable than the complementary cation/protonated anion adduct, [HEH + HNO], which is not observed experimentally. Upon collisional activation, a larger cluster [(HEHN)HEH] undergoes dissociation via loss of nitric acid at lower collision energies, as predicted theoretically. At higher collision energies, additional primary and secondary loss pathways open, including deprotonated cation loss, ion-pair loss, and double-nitric-acid loss. Taken together, these experimental and theoretical results contribute to a foundational understanding of the dissociation of protic ionic liquid clusters in the gas phase.

We report a high-spatial resolution imaging technique to measure optical absorption and detect chemical and physical changes on surfaces embedded in thick tissue. Developing sensors to measure chemical concentrations on implanted surfaces through tissue is an important challenge for analytical chemistry and biomedical imaging. Tissue scattering dramatically reduces the resolution of optical imaging. In contrast, X-rays provide high spatial resolution imaging through tissue but do not measure chemical concentrations. We describe a hybrid technique which uses a scanning X-ray beam to irradiate Gd(2)O(2)S scintillators and detect the resulting visible luminescence through the tissue. The amount of light collected is modulated by optical absorption in close proximity to the luminescence source. By scanning the X-ray beam, and measuring total amount of light collected, one can measure the local absorption near scintillators at a resolution limited by the width of luminescence source (i.e. the width of the X-ray excitation beam). For proof of principle, a rectangular 1.7 mm scanning X-ray beam was used to excite a single layer of 8 μm Gd(2)O(2)S particles, and detect the absorption of 5 nm thick silver island film through 10 mm of pork. Lifetime and spectroscopic measurements, as well changing the refractive index of the surroundings indicate that the silver reduces the optical signal through attenuated total internal reflection. The technique was used to image the dissolution of regions of the silver island film which were exposed to 1 mM of H(2)O(2) through 1 cm of pork tissue.

We have compared analgesia during labour provided by two epidural drug regimens, in a double-blind, randomized, controlled study. Group A received 10-ml bolus doses of 0.1% bupivacaine with fentanyl 2 micrograms ml-1 while group B received 0.25% plain bupivacaine 10 ml. Analgesia provided by both techniques was similar, but women in group A retained motor power in their legs and 60% chose to get out of bed. Duration of labour and time from insertion of the epidural to delivery was similar in both groups, but in group A, duration of the second stage was significantly shorter (P = 0.0003; 95% confidence interval (CI) -1.17, -0.27 h) and the incidence of forceps delivery was lower (P = 0.032). Maternal satisfaction with epidural analgesia, as assessed by VAS, was higher in group A (P = 0.04; 95% CI -0.001, 10.001).

No abstract

The post-translational modifications sulfation and phosphorylation pose special challenges to mass spectral analysis due to their isobaric nature and their lability in the gas phase, as both types of peptides dissociate through similar channels upon collisional activation. Here, we present resonant infrared photodissociation based on diagnostic sulfate and phosphate OH stretches, as a means to differentiate sulfated from phosphorylated peptides within the framework of a mass spectrometry platform. The approach is demonstrated for a number of tyrosine-containing peptides, ranging from dipeptides (YG, pYG, and sYG) over tripeptides (GYR, GpYR, and GsYR), to more biologically relevant enkephalin peptides (YGGFL, pYGGFL, and sYGGFL). In all cases, the diagnostic ranges for sulfate OH stretches are established as 3580-3600 cm(-1) and can thus be distinguished from other characteristic hydrogen stretches, such as carboxylic acid OH, alcohol OH, and phosphate OH stretches.

We present an infrared laser-based mass spectrometric strategy to differentiate peptides that are phosphorylated (i.e., containing pS, pT, or pY) from those that are nonphosphorylated (i.e., containing S, T, or Y), and those peptides that have none of these moieties (i.e., containing neither pS, pT, pY nor S, T, Y). This is demonstrated for a series of tripeptides and for two larger octapeptides, showing that the diagnostic phosphate OH stretch (indicative for pS, pT, or pY) can be distinguished from the alcohol OH stretch (indicative for S, T, or Y). In addition, the infrared multiple photon dissociation (IRMPD) spectra of multiple peptide analytes are recorded simultaneously in a multiplexed fashion. This is achieved by complexing each peptide precursor with a noncovalently bound 18-crown-6 ether, which is detached upon resonant infrared photon absorption.