High Molecular Mass Structures Research Articles

Thermophoretic sampling of large PAH (C ≥ 22–24) formed in flames

The study of the transition from molecular to particle-like species involved in soot inception needs experimental strategies aiming to measure high molecular mass structures that could be important also for their effect on environment and human health. In this work, we have investigated the carbon particulate selectivity involved in the thermophoretic deposition used for the first time as a direct and fast sampling of high molecular weight aromatic species formed at the inception stage in an ethylene heavily sooting flame. Gravimetric analysis along with mass spectrometry, UV–Visible and fluorescence spectroscopy showed that the dichloromethane-extract of carbon particulate matter thermophoretically deposited on the substrate was much less than that sampled by a conventional water-cooled probe as it is composed only of high molecular weight condensed phases (C ≥ 22–24). Thermophoretic deposition is demonstrated to pre-separate, already at the sampling step, soot precursors of high molecular weight allowing their further analysis without the interference of the more abundant light polycyclic aromatic compounds (C < 22–24). These high molecular weight aromatic species presented interesting optical properties in terms of size and fluorescence emission peaked in the blue-region that could be of interest for carbon dots application. Size exclusion chromatography of particulate coupled with UV–Visible spectroscopy showed the presence of some aggregates already at the beginning of soot formation to be taken into account as ruling the optical properties of the whole carbon particulate matter.

Detection of aromatic hydrocarbons and incipient particles in an opposed-flow flame of ethylene by spectral and time-resolved laser induced emission spectroscopy

Spectral and time-resolved fluorescence and incandescence measurements are used to identify aromatic compounds and particles in an atmospheric-pressure opposed-flow flame of ethylene. The fourth harmonic radiation, at 266nm, of a Nd–YAG laser is used to excite electronic transitions in aromatic intermediates resulting in different laser induced emission signals. Time-resolved analysis of the emission spectra is performed to obtain information about the structure of the compounds responsible for emission signals. Four fluorescence bands in the UV and visible range and a continuum with a maximum at larger wavelengths, attributed to incandescence radiation by solid particle, have been identified in the collected spectra. On the basis of fluorescence emission spectra and fluorescence lifetimes, two different types of high-molecular mass aromatic compounds are individuated in the flame: highly-packed, sandwich-like structures (cluster of PAHs held together by van der Waals forces) and more loose aromatic moieties linked together by σ-bonds, such as the aromatic–aliphatic linked hydrocarbons. The first class of particles has been found in the fuel zone and across the stagnation plane of the opposed-flow flame. In this region (<1500K) coagulation of aromatics is the leading mechanism forming high-molecular mass structures. The second class of particles is preferentially formed in the region close to the oxidizer side characterized by high temperatures (>1500K) and growth of aromatics enhanced by the high concentration of small radicals and gas-phase polycyclic aromatic hydrocarbons.

HxcQ Liposecretin Is Self-piloted to the Outer Membrane by Its N-terminal Lipid Anchor

Secretins are an unusual and important class of bacterial outer membrane (OM) proteins. They are involved in the transport of single proteins or macromolecular structures such as pili, needle complexes, and bacteriophages across the OM. Secretins are multimeric ring-shaped structures that form large pores in the OM. The targeting of such macromolecular structures to the OM often requires special assistance, conferred by specific pilotins or pilot proteins. Here, we investigated HxcQ, the OM component of the second Pseudomonas aeruginosa type II secretion system. We found that HxcQ forms high molecular mass structures resistant to heat and SDS, revealing its secretin nature. Interestingly, we showed that HxcQ is a lipoprotein. Construction of a recombinant nonlipidated HxcQ (HxcQnl) revealed that lipidation is essential for HxcQ function. Further phenotypic analysis indicated that HxcQnl accumulates as multimers in the inner membrane of P. aeruginosa, a typical phenotype observed for secretins in the absence of their cognate pilotin. Our observations led us to the conclusion that the lipid anchor of HxcQ plays a pilotin role. The self-piloting of HxcQ to the OM was further confirmed by its correct multimeric OM localization when expressed in the heterologous host Escherichia coli. Altogether, our results reveal an original and unprecedented pathway for secretin transport to the OM.

Particle Inception in a Laminar Premixed Flame of Benzene

Spectral optical techniques, including light extinction and laser induce fluorescence and incandescence measurements, are combined to characterize large-molecule soot precursors and soot in a slightly sooting flame of benzene at atmospheric pressure. Light absorption coupled to in-situ light scattering measurements and ex-situ Atomic Force Microscopy also allowed the evaluation of particle sizes. In the benzene flame high molecular mass structures with typical sizes of 3–4 nm are formed in the main oxidation region of the flame. The radical-rich flame environment in which these compounds are formed promotes their dehydrogenation increasing the level of their aromaticity. As a result, nanoparticles with typical sizes of about 5 nm, absorbing and fluorescing in the visible are formed. These compounds reach a maximum concentration just before the appearance of incandescent soot particles.

Nanostructures of APOBEC3G Support a Hierarchical Assembly Model of High Molecular Mass Ribonucleoprotein Particles from Dimeric Subunits

Human APOBEC3G (hA3G) is a cytidine deaminase that restricts human immunodeficiency virus (HIV)-1 infection in a vif (the virion infectivity factor from HIV)-dependent manner. hA3G from HIV-permissive activated CD4+ T-cells exists as an inactive, high molecular mass (HMM) complex that can be transformed in vitro into an active, low molecular mass (LMM) variant comparable with that of HIV-non-permissive CD4+ T-cells. Here we present low resolution structures of hA3G in HMM and LMM forms determined by small angle x-ray scattering and advanced shape reconstruction methods. The results show that LMM particles have an extended shape, dissimilar to known cytidine deaminases, featuring novel tail-to-tail dimerization. Shape analysis of LMM and HMM structures revealed how symmetric association of dimers could lead to minimal HMM variants. These observations imply that the disruption of cellular HMM particles may require regulation of protein-RNA, as well as protein-protein interactions, which has implications for therapeutic development.

HcPro, a multifunctional protein encoded by a plant RNA virus, targets the 20S proteasome and affects its enzymic activities.

The proteasome is a multicatalytic complex involved in many cellular processes in eukaryotes, such as protein and RNA turnover, cell division, signal transduction, transcription and translation. Intracellular pathogens are targets of its enzymic activities, and a number of animal viruses are known to interfere with these activities. The first evidence that a plant virus protein, the helper component-proteinase (HcPro) of Lettuce mosaic virus (LMV; genus Potyvirus), interferes with the 20S proteasome ribonuclease is reported here. LMV infection caused an aggregation of the 20S proteasome to high-molecular mass structures in vivo, and specific binding of HcPro to the proteasome was confirmed in vitro using two different approaches. HcPro inhibited the 20S endonuclease activity in vitro, while its proteolytic activities were unchanged or slightly stimulated. This ability of HcPro, a pathogenicity regulator of potyviruses, to interfere with some of the catalytic functions of the 20S proteasome suggests the existence of a novel type of defence and counter-defence interplay in the course of interaction between potyviruses and their hosts.

Characterization of Refractory Organic Substances (ROS) in Water Treatment

Due to their heterogeneity and polydispersity, refractory organic substances (ROS) play a multifunctional role in the natural environment and in water treatment processes. Those properties also complicate an authentic analytical characterization. Since ROS are ubiquitous in natural waters and as they have the potential to form toxic disinfection by-products research aims at the characterization of their behavior in water treatment processes. Gel chromatography in combination with UV/Vis and sensitive dissolved organic carbon detection is shown to be useful, yielding information on molecular absorbance, molecular mass distribution, and reactivity. Additional experiments under defined conditions lead to information about the fate of ROS during biological, oxidation and adsorption processes. It is shown that ozonation, peroxonation, and chlorination result in partial decomposition of high molecular mass structures to low molecular hydrophilic compounds which are more bioavailable for microorganisms. In contrast to this, adsorption using activated carbon removes organic fractions of lower molecular mass, whereas the high molecular mass compounds of ROS showing relative high UV absorbance remain in solution. Based on this partial decomposition to smaller and therefore better bioavailable compounds oxidative processes can be used to improve the efficiency of waste water treatment.

Characterization of refractory organic substances (ROS) in water treatment

Due to their heterogeneity and polydispersity, refractory organic substances (ROS) play a multifunctional role in the natural environment and in water treatment processes. Those properties also complicate an authentic analytical characterization. Since ROS are ubiquitous in natural waters and as they have the potential to form toxic disinfection by-products research aims at the characterization of their behavior in water treatment processes. Gel chromatography in combination with UV/Vis and sensitive dissolved organic carbon detection is shown to be useful, yielding information on molecular absorbance, molecular mass distribution, and reactivity. Additional experiments under defined conditions lead to information about the fate of ROS during biological, oxidation and adsorption processes. It is shown that ozonation, peroxonation, and chlorination result in partial decomposition of high molecular mass structures to low molecular hydrophilic compounds which are more bioavailable for microorganisms. In contrast to this, adsorption using activated carbon removes organic fractions of lower molecular mass, whereas the high molecular mass compounds of ROS showing relative high UV absorbance remain in solution. Based on this partial decomposition to smaller and therefore better bioavailable compounds oxidative processes can be used to improve the efficiency of waste water treatment.

Insertion mutagenesis of XpsD, an outer-membrane protein involved in extracellular protein secretion in Xanthomonas campestris pv. campestris.

XpsD is an outer-membrane protein required for extracellular protein secretion in Xanthomonas campestris pv. campestris. Cross-linking and gelfiltration chromatography analyses have suggested that it forms a multimer. To determine its structure-function relationship, linker-insertion mutants were constructed in an xpsD gene carried on a plasmid. To assay for secretion function, each mutant gene was introduced into an xpsD::Tn5 mutant strain (XC1708) and assayed for alpha-amylase secretion on starch plates. To test whether the mutant genes exerted a dominant-negative effect, each was introduced into the parental strain XC1701 and examined for secretion interference. Nine functional, one semi-functional and eleven non-functional mutants were obtained. All the non-functional mutants, except two for which the mutant proteins were undetectable on immunoblots, showed interference of normal secretion. The insertion sites in the different mutant proteins are randomly distributed throughout the entire sequence of the XpsD protein. All the permissive insertion sites are located where beta-turn or coiled secondary structure is predicted. Over half of the non-permissive sites are located within predicted helical or beta-sheet regions. By pretreating total membranes of XC1701 in SDS at 50 degrees C, an immunoreactive band with high molecular mass (HMM) could be detected that remained in the stacking gel during SDS-PAGE. The semi-functional and all functional mutant proteins formed HMM complexes that were as SDS-resistant as those of the wild-type, whereas all except three of the non-functional mutant proteins formed HMM structures that were less resistant to SDS than the wild-type. By analysing the appearance of SDS-resistant HMM complexes, we were able to detect conformational alterations in XpsD that are too subtle to be detected by other assays.

Properties of carbonaceous nanoparticles in flat premixed C 2H 4/air flames with C/O ranging from 0.4 to soot appearance limit

The presence of very fine carbonaceous particles in the ambient causes concern either for their toxic properties or for their influence on atmospheric chemistry and physics. Previous work has shown that carbonaceous nanoparticles with a typical size of about 2–3 nm are formed in the reaction zone of rich premixed sooting flames and may act as soot precursors. This paper ascertains the presence of nanoparticles in rich laminar C 2 H 4 /air premixed flames with C/O ratio ranging over a wide interval below the soot limit. Ultraviolet (UV)-visible absorption starting from 200-nm, laser-induced fluorescence (LIF), and laser light scattering (LLS) methods were used for the in situ measurements and for the characterization of the material collected from the flames as hydrosol in water traps. It was found that the absorption spectra from 200 to 300 nm were due both to hot CO 2 , whose absorption cross sections are orders of magnitude higher than those found at room temperature, and to carbonaceous high molecular mass structures. The absorption spectra of those structures, obtained by subtracting the CO 2 contribution, and the fluorescence spectra show the same behavior as those measured on the hydrosol suspension. The monochromatic absorption coefficient in the UV due to high molecular mass structures have their onset at C/O=0.4, whereas the fluorescence excited at λ 0 =266 nm appears around C/O=0.5, where also light scattering coefficients, in the UV and visible, in excess with respect to those due to burned gases, start to be measurable. These results are interpreted in terms of a carbonaceous network of one-and two-ring aromatic functionalities, connected by aliphatic bonds, with typical size around 2 nm. The high-temperature formation of these carbonaceous nanoparticles below soot formation limit is considered an important route for explaining the presence of organic carbon aerosols in the atmosphere.

The optical band gap model in the interpretation of the UV-visible absorption spectra of rich premixed flames

A spectral analysis of the light extinction of near-sooting premixed benzene/air flames at atmospheric pressure has been performed between 200 and 700 nm, together with UV laser-excited fluorescence and light scattering. The absorption spectra, attributed to high molecular mass structures, are interpreted in the framework of the optical properties of solid state matter, according to the Tauc band-gap model. This is a powerful approach to characterize carbonaceous material; in fact, theoretical computations on carbon structures containing conjugated π bonds indicate that the extension of aromatic clusters or islands in side the structures can be obtained from the measured optical band-gap energies. The absorption measurements, performed both in the soot-preinception and soot-inception regions, give a clear indication that a spread of structures, with a different degree of aromatization, is present, since at least three optical band gaps are identified. The soot inception itself is characterized by a sharp decrease of the band gap caused by the progressive aromatization of the structures. This process takes place before the particles grow by surface reactions. Finally, the extinction spectra of fully grown soot particles, so far expressed by an empirical dispersion law, are reinterpreted in terms of the optical band gap, thus allowing a more rational correlation between the optical and chemical properties of the particles.

The optical characterization of high molecular mass carbonaceous structures produced in premixed laminar flames across the soot threshold limit

The consistence of the dark material in interstellar space is still under discussion. Reasonable candidates are structures containing silicon or carbon. In that light it is interesting to know the physical and chemical properties of carbonaceous structures produced in flames and to compare them with the spectroscopical properties of interstellar dust. Light scattering, fluorescence induced by UV laser and light absorption have been employed to analyse phenomenological aspects of formation; destruction and chemical transformation of carbonaceous structures formed in rich premixed Laminar methane-oxygen and ethylene-oxygen flames at atmospheric pressure with different C/O ratio, at and above the soot threshold limit. Light scattering measurements also show that in non-sooting conditions, i.e. in the non-sooting zone of sooting flames or in flames below the soot threshold limit, high molecular mass structures are formed. Absorption spectra measured in non-sooting conditions show a continuous decay from 200 to 300 Dill, while in fully sooting conditions an additional peak at around 235 nm appears. Further information is obtained by laser-induced fluorescence investigations. Here. a broad-band peak with a maximum at around 320 nm is always measurable. Approaching the soot nucleation zone a further broad-band fluorescence emission in the visible. in addition to that in the UV, is observed. All the experimental results produce evidence that the early formation of high molecular mass carbonaceous molecules containing aromatic functionalities with not more than two rings occurs. Their role as soot precursors seems to be connected to their change into higher polycondensed aromatic structures. Our results give a hint that these soot precursors and the interstellar dust are members of the same family of carbonaceous structures.

Optical and Spectroscopic Characterization of Rich Premixed Flames across the Soot Formation Threshold

ABSTRACT Phenomenological aspects of formation, destruction, and coagulation of high molecular mass structures formed in the main oxidation zone of rich premixed flames and in rich flames well below the soot threshold limit have been examined. High molecular mass structures transparent to the visible radiation, previously detected in the preinception region of soot forming flames, are also present in flames below the soot formation limit. The onset of ultra-violet fluorescence within the main oxidation zone implies that the formation of these species is a very fast process and can be considered as a “polymerization” of small aromatic groups activated by the presence of oxidizing agents. The final concentration of this material falls down as the C/O ratio is decreased and below C/O = 0.35 it is not anymore present. It appears that rich premixed flames present two critical C/O ratios: a first one for soot formation and a lower second one for high molecular mass structure formation. Ultra-violet scattering/e...

Optical characterization of rich premixed CH4/O2 flamesacross the soot formation threshold

Light scattering, fluorescence, and C2 emission induced by an ultraviolet laser source and uv-visibleabsorption have been used to study the phenomenological aspects of formation, destruction, and chemical transformation of high molecular mass structures formed in rich premixed methane flames at atmospheric pressure. High molecular mass structures are formed in CH4/O2 flames with C/O ratios down to 0.35. This value, corresponding to their formation threshold, is noticeably lower than the soot threshold limit (C/O=0.45). Different flames have been analyzed across the soot threshold limit, varying the C/O ratio and the maximum flame temperature, and no dramatic changes in the properties of the high molecular mass structures have been found in the passage from nonsooting to sooting conditions. Their role as soot precursors seems to be connected to a kinetically controlled passage from aromatic functionalities with one or two rings to higher polycondensed aromatic functionalities more than to a carbonization process of these structures. High molecular mass structures are easily photofragmented by an ultraviolet laser source with high-energypulses, and the consequent C2 emission has been attributed to these structures more than to soot particles. From a practical point of view, the spatial distribution of these potential pollutants, whose toxic properties are largely unknown, can be easily followed in practical diffusion flames by monitoring the 2D fluorescence due to C2 photofragments employing an ultraviolet laser source.

Optical Characterization of Soot

AbstractThe chemistry of fuel‐rich hydrocarbon flames is extremely complex because different types of high molecular mass structures are produced. In fact, Polycyclic Aromatic Hydrocarbons (PAH) and submicronic turbostratic graphitic‐like particles (soot) are not the only products present but also partially aromatic polymers, gums, fullerenes and diamond‐like particles may be formed during pyrolysis and rich combustion. In addition, the heavy fractions of liquid fuels like the asphaltenes or the large polar‐aromatic compounds pyrolyze to specific forms of solid carbon. The tight requirement of a high space‐time resolution of the measurements during the high temperature combustion privileges the optical and spectroscopic diagnostics. This contribution will shortly review the main optical effects employed in this field namely absorption and fluorescence spectroscopy and elastic light scattering.