The influence of hydroxyl groups on the ultraviolet spectra of substituted aromatic molecules adsorbed on silica surfaces

Infrared and uv spectroscopy have been used to study the interactions of a series of mono and di-substituted benzene molecules on both porous and nonporous high surface area silicas. It is confirmed that the strength of adsorption depends upon the presence and type of surface hydroxyl group but shown that the uv spectral shifts are not necessarily related to bond strength. Thus, when the surface OH groups reduce the effect of the electron donating side groups, stronger hydrogen bonds produce larger blue shifts in π-π* transitions. When n-π* transitions are involved, however, it is a dipole-dipole interaction which determines the magnitude of the red shift and not the strength of the hydrogen bond.     

Dehydrochlorination of monodisperse poly(vinylidene chloride) latex

The objective of the work described in this paper was to produce dispersions of small spherical carbon particles, having particle diameters in the region of 0.1 μm. To this end, the dehydrochlorination of poly(vinylidene chloride) (PVDC) latex particles was attempted. The PVDC latex was prepared by a dispersion polymerization route. Both chemical and thermal dehydrochlorination routes were attempted. Chemical dehydrochlorination, using a variety of base/solvent systems, led to nonporous, spherical black particles of the required size, but which contained only 60% carbon; most of the remainder was oxygen, introduced by nucleophilic substitution reactions. Thermal dehydrochlorination, at 700°C under a nitrogen atmosphere, using a fluidized bed arrangement, on the other hand, led to black particles, having 90% carbon and which retained their sphericity, but which were highly porous. Initial chemical dehydrochlorination, prior to thermal treatment, did not seem to reduce the porosity of the final carbons. Dispersions of the carbon particles in a variety of solvents were readily achieved.     

Mixed-metal oxide aerogels for oxidation of volatile organic compounds

Aerogels of 100% silica, 8 wt% Zr in silica, 5 wt% V in silica and 100% zirconia were synthesized and tested as oxidation catalysts in the temperature range of 300–700°C for destruction of volatile organic compounds. The silica-based aerogels were all amorphous and had surface areas above 600 m²/g after oxidation. The zirconia aerogel was crystalline with a relatively low surface area of 250 m²/g. As catalysts for oxidation (using O₂ in He) of CH₃OH to CO₂, the zirconia aerogel exhibited the highest activity and best selectivity while the silica aerogel exhibited the poorest. Inclusion of Zr at levels as low as 8 wt% into the silica aerogel framework produced activities and selectivities which were very much like the zirconia aerogel. These properties have the impact of producing a Zr based catalyst with high activity, but with thermal stability afforded by Zr–silica mixtures.     

Transient radicals in heterogeneous systems: detection by spin trapping

The application of spin trapping to the detection of transient radicals generated in heterogeneous systems is discussed. The major part of this review focuses on the interaction of light with particles in dispersion and the resultant radicals which are produced. This includes photoactive pigments such as CdS, TiO2, ZnO and phthalocyanine in a variety of solvents as well as micelles, vesicles and microemulsions containing molecules such as phthalocyanine or chlorophyll. The utility of spin trapping in heterogeneous systems which are not a function of illumination (i.e. electrochemical reactions, catalysis and biological systems) is also discussed. It is demonstrated that reaction mechanisms can be better understood by the indirect detection of the radical intermediates which are involved in the various processes. This impacts studies in solar energy utilization, electrophotography, catalysis, electrochemistry, photosynthesis and the disease process.     

Contact angle measurements with liquids consisting of bulky molecules

Well-measured contact angles with different solid-liquid systems fall approximately on smooth patterns when plotted versus liquid surface tension. However, there are deviations of 1 degrees -3 degrees , which are outside the error limits. It is the purpose of this paper to elucidate the reasons for such deviations. Two types of liquids were selected for advancing contact angle measurements on Teflon AF 1600 coated surfaces: a series of n-alkanes ranging from n-hexane to n-hexadecane and five liquids consisting of bulky molecules, octamethylcyclotetrasiloxane (OMCTS), methyl salicylate, tetralin, cis-decalin, and octamethyltrisiloxane (OMTS). It was found that contact angles of the liquids with bulky molecules fall on a perfectly smooth curve corresponding to a solid surface tension of 13.64 +/- 0.1 mJ/m2. However, contact angles of n-alkanes deviated from this curve by up to 3 degrees in a complicated fashion. The observed trend suggests that more than one mechanism is responsible for the deviations. Substrate-induced rearrangement of liquid molecules in the close vicinity of the surface in the case of long-chain n-alkanes and adsorption of vapor onto the solid surface in the case of short-chain n-alkanes are the most likely explanations. The results suggest that liquids with bulky molecules appear to be suitable for contact angle measurements to characterize energetics of polymeric surfaces.     

Selective identification and quantitative analysis of methionine containing peptides by charge derivatization and tandem mass spectrometry

To enable the development of a tandem mass spectrometry (MS/MS) based methodology for selective protein identification and differential quantitative analysis, a novel derivatization strategy is proposed, based on the formation of a "fixed-charge" sulfonium ion on the side-chain of a methionine amino acid residue contained within a protein or peptide of interest. The gas-phase fragmentation behavior of these side chain fixed charge sulfonium ion containing peptides is observed to result in exclusive loss of the derivatized side chain and the formation of a single characteristic product ion, independently of charge state or amino acid composition. Thus, fixed charge containing peptide ions may be selectively identified from complex mixtures, for example, by selective neutral loss scan mode MS/MS methods. Further structural interrogation of identified peptide ions may be achieved by subjecting the characteristic MS/MS product ion to multistage MS/MS (MS3) in a quadrupole ion trap mass spectrometer, or by energy resolved "pseudo" MS3 in a triple quadrupole mass spectrometer. The general principles underlying this fixed charge derivatization approach are demonstrated here by MS/MS, MS3 and "pseudo" MS3 analysis of side chain fixed-charge sulfonium ion derivatives of peptides containing methionine formed by reaction with phenacylbromide. Incorporation of "light" and "heavy" isotopically encoded labels into the fixed-charge derivatives facilitates the application of this method to the quantitative analysis of differential protein expression, via measurement of the relative abundances of the neutral loss product ions generated by dissociation of the light and heavy labeled peptide ions. This approach, termed "selective extraction of labeled entities by charge derivatization and tandem mass spectrometry" (SELECT), thereby offers the potential for significantly improved sensitivity and selectivity for the identification and quantitative analysis of peptides or proteins containing selected structural features, without requirement for extensive fractionation or otherwise enrichment from a complex mixture prior to analysis.     

Derivatization of protonated peptides via gas phase ion—molecule reactions with acetone

The protonated [M + H]+ ions of glycine, simple glycine containing peptides, and other simple di- and tripeptides react with acetone in the gas phase to yield [M + H + (CH3)2CO]+ adduct ion, some of which fragment via water loss to give [M + H + (CH3)2CO - H2O]+ Schiff's base adducts. Formation of the [M + H + (CH3)2CO]+ adduct ions is dependent on the difference in proton affinities between the peptide M and acetone, while formation of the [M + H + (CH3)2CO - H2O]+ Schiff's base adducts is dependent on the ability of the peptide to act as an intramolecular proton "shuttle." The structure and mechanisms for the formation of these Schiff's base adducts have been examined via the use of collision-induced dissociation tandem mass spectrometry (CID MS/MS), isotopic labeling [using (CD3)2CO] and by comparison with the reactions of Schiff's base adducts formed in solution. CID MS/MS of these adducts yield primarily N-terminally directed a- and b-type "sequence" ions. Potential structures of the b1 ion, not usually observed in the product ion spectra of protonated peptide ions, were examined using ab initio calculations. A cyclic 5 membered pyrrolinone, formed by a neighboring group participation reaction from an enamine precursor, was predicted to be the primary product.     

Can <Emphasis Type="Italic">α</Emphasis>- and <Emphasis Type="Italic">β</Emphasis>-alanine containing peptides Be distinguished based on the CID spectra of their protonated ions?

The fragmentation reactions of isomeric dipeptides containing α- and β-alanine residues (αAla-αAla, αAla-βAla, βAla-αAla, and βAla-βAla) were studied using a combination of low-energy and energy resolved collision induced dissociation (CID). Each dipeptide gave a series of different fragment ions, allowing for differentiation. For example, peptides containing an N-terminal β-Ala residue yield a diagnostic imine loss, while lactam ions at m/z 72 are unique to peptides containing β-Ala residues. In addition, MS³ experiments were performed. Structure-specific fragmentation reactions were observed for y₁ ions, which help identify the C-terminal residue. The MS³ spectra of the b₂ ions are different suggesting they are unique for each peptide. Density functional theory (DFT) calculations predict that b₂ ions formed via a neighboring group attack by the amide are thermodynamically favored over those formed via neighboring group attack by the N-terminal amine. Finally, to gain further insight into the unique fragmentation chemistry of the peptides containing an N-terminal β-alanine residue, the fragmentation reactions of protonated β-Ala-NHMe were examined using a combination of experiment and DFT calculations. The relative transition-state energies involved in the four competing losses (NH₃, H₂O, CH₃NH₂, and CH₂=NH) closely follow the relative abundances of these as determined via CID experiments.