Application of matrix-assisted laser desorption/ionization to on-line aerosol time-of-flight mass spectrometry

Matrix-assisted laser desorption/ionization (MALDI) mass spectra were obtained from single biological aerosol particles using an aerosol time-of-flight mass spectrometer (ATOFMS). The inlet to the ATOFMS was coupled with an evaporation/condensation flow cell that allowed the aerosol to be coated with matrix material as the sampled stream entered the spectrometer. Mass spectra were generated from aerosol composed either of gramicidin-S or erythromycin, two small biological molecules, or from aerosolised spores of Bacillus subtilis var niger. Three different matrices were used: 3-nitrobenzyl alcohol, picolinic acid and sinapinic acid. A spectrum of gramicidin-S was generated from approximately 250 attomoles of material using a molar ratio of 3-nitrobenzyl alcohol to analyte of approximately 20:1. A single peak, located at 1224 Da, was obtained from the bacterial spores. The washing liquid and extract solution from the spores were analyzed using electrospray mass spectrometry and subsequent MS/MS product ion experiments. This independent analysis suggests that the measured species represents part of the B. subtilis peptidoglycan. The on-line addition of matrix allows quasi-real-time chemical analysis of individual, aerodynamically sized particles, with an overall system residence time of less than 5 seconds. These results suggest that a MALDI-ATOFMS can provide nearly real-time identification of biological aerosols. Copyright 2000 John Wiley & Sons, Ltd.     

Norbornane: an investigation into its valence electronic structure using electron momentum spectroscopy, and density functional and Green's function theories

We report on the results of an exhaustive study of the valence electronic structure of norbornane (C(7)H(12)), up to binding energies of 29 eV. Experimental electron momentum spectroscopy and theoretical Green's function and density functional theory approaches were all utilized in this investigation. A stringent comparison between the electron momentum spectroscopy and theoretical orbital momentum distributions found that, among all the tested models, the combination of the Becke-Perdew functional and a polarized valence basis set of triple-zeta quality provides the best representation of the electron momentum distributions for all of the 20 valence orbitals of norbornane. This experimentally validated quantum chemistry model was then used to extract some chemically important properties of norbornane. When these calculated properties are compared to corresponding results from other independent measurements, generally good agreement is found. Green's function calculations with the aid of the third-order algebraic diagrammatic construction scheme indicate that the orbital picture of ionization breaks down at binding energies larger than 22.5 eV. Despite this complication, they enable insights within 0.2 eV accuracy into the available ultraviolet photoemission and newly presented (e,2e) ionization spectra, except for the band associated with the 1a(2) (-1) one-hole state, which is probably subject to rather significant vibronic coupling effects, and a band at approximately 25 eV characterized by a momentum distribution of "s-type" symmetry, which Green's function calculations fail to reproduce. We note the vicinity of the vertical double ionization threshold at approximately 26 eV.     

The nature of the electronic factor governing diastereofacial selectivity in remotely substituted (X) 2‐adamantyl cations: 5‐X versus 4‐X substitution

A limited series of 4^eq‐substituted (X) 2‐methyleneadamantanes ( 6 , Y?CH₂, X?F, Cl, Br, I, and SnMe₃) has been synthesized and diastereoselectivities for their hydrochlorination (HCl/CH₂Cl₂) have been determined. Diastereoselectivities for the fluorination (DAST/CH₂Cl₂) of secondary alcohol mixtures, obtained from the hydride reduction of the precursor ketones ( 6 ,Y?O) to the alkenes, have also been measured. A comparison of this selectivity data for nucleophilic trapping of 4^eq‐substituted (X) 2‐adamantyl cations ( 4 , R?H and Me) with the corresponding information for 5‐substituted (X) 2‐adamantyl cations ( 1 , R?H and Me) has revealed important distinctions between the two series. In particular, whereas extended hyperconjugative effects appear to be the predominant electronic effect governing facial selectivity in the 5,2‐series, electrostatic influences prevail in the 4,2‐disposition. Copyright © 2007 John Wiley & Sons, Ltd.     

Methodologies for detection of hemoglobin-based oxygen carriers

Blood substitutes based on hemoglobin or hemoglobin-based oxygen carriers (HBOCs) are oxygen-carrying therapeutic agents developed for use in operations and emergencies in place of donated blood. Increased oxygen-carrying capacity through the use of blood substitutes could help elite athletes to lengthen endurance capacity and improve their performance. As blood substitutes become more readily available, it is essential that a qualitative detection method for their abuse in sport is available. Ideally, such a method would be simple and inexpensive. This study investigates methods that could be used as screening procedures to easily detect HBOCs in plasma and develops tests that can unequivocally confirm their presence. The investigation into the screening method indicates that the direct visual screening of plasma discoloration is the most appropriate with detection limits of less than 1% HBOC in plasma. Two methods are shown to confirm the presence of exogenous hemoglobin in plasma samples, size-exclusion chromatography with photodiode array detection and high-performance liquid chromatography analysis of enzymatic digests with detection by electrospray mass spectrometry. This work emphasizes the need for cooperation between drug developers and sports testing laboratories to ensure that methods for the detection of putative doping agents are available prior to product release.     

First principles computational study for understanding the interactions between ssdna and gold nanoparticles: adsorption of methylamine on gold nanoparticulate surfaces

We conducted a computational adsorption study of methylamine on various surface models of a gold nanoparticle which is facetted by multiple [111] and [100] planes. In addition to these flat surfaces, our models include the stepped surfaces (ridges) formed along the intersections of these planes. Binding on the flat surfaces was fairly weak, but substantially stronger on the ridges by an average of 4.4 kcal/mol. This finding supports the idea that ssDNA's interaction with gold nanoparticles occurs through the amines on the purine/pyrimidine rings. Also, this typically undesirable interaction between DNA and gold nanoparticles is expected to increase as the particle size decreases. Our analysis suggests that particle size is an important controlling parameter to reduce this interaction.     

Origin of diastereofacial selectivity in tertiary 2-adamantyl cations.

The intrinsic factors governing the diastereofacial selectivity of 2-methyl-5-X-2-adamantyl cations (X = F (I(F)), Si(CH(3))(3) (I(Si))) toward a representative nucleophile, i.e., methanol, have been investigated in the gas phase at 750 Torr and in the 20-80 degrees C temperature range. The kinetic results indicate that CH(3)OH addition to I(F) proceeds through tight transition structures (TS(F)(syn) and TS(F)(anti)) characterized by advanced C-O bonding. The same interactions are much less pronounced in the comparatively loose transition structures involved in the CH(3)OH addition to I(Si) (TS(Si)(syn) and TS(Si)(anti)). The experimental evidence indicates that the activation barriers for the anti addition to I(F) and I(Si) are invariably lower than those for the syn attack. Large adverse entropic factors account for the preferred syn diastereoselectivity observed in the reaction with I(F). Entropy plays a minor role in the much looser transition structures involved in the reaction with I(Si), which instead exhibits a preferred anti diastereoselectivity. Comparison of the above gas-phase results with related theoretical and solution data suggests that the diastereofacial selectivity of I(F) and I(Si) measured in solution arises in part from the differential solvation of the two faces of the pyramidalized ions.     

Modified ligand-exchange for efficient solubilization of CdSe/ZnS quantum dots in water: a procedure guided by computational studies

One of the methods to render CdSe/ZnS core-shell quantum dots(QDots) water-soluble is to functionalize the surface with carboxylate groups by the use of heterobifunctional ligands such as 3-mercaptopropionic acid, where the thiolic end binds onto the outer ZnS shell. However, currently available ligand-exchange procedures starting with TOPO-capped quantum dots often lead to significant loss of quantum yields and poor stability of the colloids in water. As part of our efforts to overcome these problems, we used computational methods to understand the nature of binding between alkyl thiols and ZnS wurtzite surfaces. Guided by the computational results, we modified the ligand-exchange method and increased the reactivity of 3-mercaptopropionic acid toward the ZnS surface in chloroform. The functionlization reaction required only mild reaction conditions and led to QDot nanoparticles that were individually dispersed in water with good colloidal stability. Importantly, the photoluminescence performance of the QDots was highly preserved.     

Surface design for controlled crystallization: the role of surface chemistry and nanoscale pores in heterogeneous nucleation

Current industrial practice for control of primary nucleation (nucleation from a system without pre-existing crystalline matter) during crystallization from solution involves control of supersaturation generation, impurity levels, and solvent composition. Nucleation behavior remains largely unpredictable, however, due to the presence of container surfaces, dust, dirt, and other impurities that can provide heterogeneous nucleation sites, thus making the control and scale-up of processes that depend on primary nucleation difficult. To develop a basis for the rational design of surfaces to control nucleation during crystallization from solution, we studied the role of surface chemistry and morphology of various polymeric substrates on heterogeneous nucleation using aspirin as a model compound. Nucleation induction time statistics were utilized to investigate and quantify systematically the effectiveness of polymer substrates in inducing nucleation. The nucleation induction time study revealed that poly(4-acryloylmorpholine) and poly(2-carboxyethyl acrylate), each cross-linked by divinylbenzene, significantly lowered the nucleation induction time of aspirin while the other polymers were essentially inactive. In addition, we found the presence of nanoscopic pores on certain polymer surfaces led to order-of-magnitude faster aspirin nucleation rates when compared with surfaces without pores. We studied the preferred orientation of aspirin crystals on polymer films and found the nucleation-active polymer surfaces preferentially nucleated the polar facets of aspirin, guided by hydrogen bonds. A model based on interfacial free energies was also developed which predicted the same trend of polymer surface nucleation activities as indicated by the nucleation induction times.     

Application of the cell potential method to predict phase equilibria of multicomponent gas hydrate systems

We present the application of a mathematical method reported earlier by which the van der Waals-Platteeuw statistical mechanical model with the Lennard-Jones and Devonshire approximation can be posed as an integral equation with the unknown function being the intermolecular potential between the guest molecules and the host molecules. This method allows us to solve for the potential directly for hydrates for which the Langmuir constants are computed, either from experimental data or from ab initio data. Given the assumptions made in the van der Waals-Platteeuw model with the spherical-cell approximation, there are an infinite number of solutions; however, the only solution without cusps is a unique central-well solution in which the potential is at a finite minimum at the center to the cage. From this central-well solution, we have found the potential well depths and volumes of negative energy for 16 single-component hydrate systems: ethane (C2H6), cyclopropane (C3H6), methane (CH4), argon (Ar), and chlorodifluoromethane (R-22) in structure I; and ethane (C2H6), cyclopropane (C3H6), propane (C3H8), isobutane (C4H10), methane (CH4), argon (Ar), trichlorofluoromethane (R-11), dichlorodifluoromethane (R-12), bromotrifluoromethane (R-13B1), chloroform (CHCl3), and 1,1,1,2-tetrafluoroethane (R-134a) in structure II. This method and the calculated cell potentials were validated by predicting existing mixed hydrate phase equilibrium data without any fitting parameters and calculating mixture phase diagrams for methane, ethane, isobutane, and cyclopropane mixtures. Several structural transitions that have been determined experimentally as well as some structural transitions that have not been examined experimentally were also predicted. In the methane-cyclopropane hydrate system, a structural transition from structure I to structure II and back to structure I is predicted to occur outside of the known structure II range for the cyclopropane hydrate. Quintuple (L(w)-sI-sII-L(hc)-V) points have been predicted for the ethane-propane-water (277.3 K, 12.28 bar, and x(eth,waterfree) = 0.676) and ethane-isobutane-water (274.7 K, 7.18 bar, and x(eth,waterfree) = 0.81) systems.     

Electronic composition-property relationship applied to SO2 chemisorption on Pt111 surfaces, alloys, and overlayers

We have developed a new electronic composition-property relationship between the adsorption energy of molecules binding unequally to multiple atoms on metal surfaces and the electronic properties of the surface. This relationship allows the estimation of the relative stability of adsorbates on various surfaces, assuming that the adsorbate adopts the same local configuration on each surface, and therefore also allows the estimation of the adsorption energy of molecules through large regions of parameter space in alloy systems with data from only a few explicit calculations. We have applied this relationship to the adsorption of SO(2) on Pt surfaces alloyed with Pd, Cu, Ru, and Ni. Using a new formula for weighting the metal d-band, we found a strong linear relationship between the weighted positions of the d states of surfaces and the most stable molecular adsorption energies. The consequences of our electronic composition-property relationship for catalyst design are also discussed.