The growth of branched polymer structures up to the gel point has been examined in a polyester system at two different branch agent concentrations. Several independent measurements of the size and molecular weight of these polymers were made using elastic light scattering, quasi-elastic light scattering, intrinsic viscosity [SI, and size-exclusion chromatography with low-angle light scattering detection. In all cases, scaling relationships between these various properties were displayed for the whole range of molecular weights examined. The weight-average molecular weight scaled as @,p ) T , where p is the extent of reaction and pc is the extent of reaction at the gel point. The exponent y was found to be 1.8 f 0.3. Scaling exponents from the radius-M, and [+M, relationships were evaluated for unfractionated samples. Using these exponents, another critical exponent of gelation, T, and the exponent relating size to molecular weight for a branched polymer in a good solvent, $, could be evaluated. These were compared with values for the same exponents obtained through size-exclusion chromatography of the polymers. The critical exponent 7 was obtained from this fractionation experiment through the shape of the observed distribution function and from the scaling relation between the molecular weight M-, corresponding to the fraction making the largest contribution to light scattering in this separation, and M , for each sample. Good agreement was observed between these two separate measures of T and the one from the unfractionated samples, as well as from both fractionated and unfractionated samples. The measured values were T = 2.29 f 0.03 and $ = 0.48 f 0.02. The distribution functions for polymers were described by a single universal distribution function. The critical exponents for gelation were compared to percolation and Flory-Stockmayer predictions and were found to favor the percolation results, but the agreement with percolation for the exponent 7 was marginal.
New and improved strategies are eagerly sought for the rapid identification of microorganisms, particularly in mixtures. Mass spectrometry remains a powerful tool for this purpose. Small acid-soluble proteins (SASPs), which are relatively abundant in Bacillus spores, represent potential biomarkers for species characterization. Despite sharing extensive sequence homology, these proteins differ sufficiently in sequence for discrimination between species. This work focuses on the differences in sequence between SASPs from various Bacillus species. Compilation of SASP sequences from protein database searches, followed by in silico trypsin digestion and analysis of the resulting fragments, identified several species-specific peptides that could be targeted for analysis using mass spectrometry. This strategy was tested and found to be successful in the characterization of Bacillus spores both from individual species and in mixtures. Analysis was performed using an ion trap mass spectrometer with an atmospheric pressure MALDI source. This instrumentation offers the advantage of increased speed of analysis and accurate precursor ion selection for tandem mass spectrometric analysis compared with vacuum matrix-assisted laser desorption/ionization and time-of-flight instruments. The identification and targeting of species-specific peptides using this type of instrumentation offers a rapid, efficient strategy for the identification of Bacillus spores and can potentially be applied to different microorganisms.
SynopsisPoly(Ltyrosine) [(L-Tyr),] has been characterized in aqueous solution using circular dichroism (CD) and infrared (ir) spectroscopy, and ultracentrifugal analysis. Most of the experiments were carried out a t 0.017, polymer or less to avoid the complications caused by precipitation previously encountered by others. This permitted us to study fiolutions of (L-Tyr), a t lower pH values than had been attained previously. Our results show that a transition to an antiparallel-0 conformation occurs at pH 11.32 upon titration from higher pH. The p structure is intramolecular when first formed and aggregates with time or upon titration below pH 11. Ultracentrifugal analysis of the intramolecular p conformation shows that it is quite compact, with a frictional coefficient ratio, f/fmln, of 1.09. In addition to the p structure, a nonordered form of the polymer has been obtained below pH 11 by rapid titration of the ionized polyelectrolyte. This form is nonaggregated and waFj found to have an f/fmln of 1.01, and is therefore almost spherical. The aggregated ( 3 form was found to be thermodynamically more stable than the nonordered form a t pH 10.7.
The analysis of poly(ethylene glycol) (PEG)‐containing particles by online single particle aerosol mass spectrometers equipped with laser desorption/ionization (LDI) is reported. We demonstrate that PEG‐containing particles are useful in the development of aerosol mass spectrometers because of their ease of preparation, low cost, and inherently recognizable mass spectra. Solutions containing millimolar quantities of PEGs were nebulized and, after drying, the resultant micrometer‐sized PEG‐containing particles were sampled. LDI (266 nm) of particles containing NaCl and PEG molecules of average molecular weight <500 Da generated mass spectra reminiscent of mass spectra of PEG collected by other mass spectrometer platforms including the characteristic distribution of positive ions (Na+ adducts) separated by the 44 m/z units of the ethylene oxide units separating each degree of polymerization. PEGs of average molecular weight >500 Da were detected from particles that also contained the tripeptide tyrosine‐tyrosine‐tyrosine or 2,5‐dihydroxybenzoic acid, which were added to nebulized solutions to act as matrices to assist LDI using pulsed 266 nm and 355 nm lasers, respectively. Experiments were performed on two aerosol mass spectrometers, one reflectron and one linear, that each utilize two time‐of‐flight mass analyzers to detect positive and negative ions created from a single particle. PEG‐containing particles are currently being employed in the optimization of our bioaerosol mass spectrometers for the application of measurements of complex biological samples, including human effluents, and we recommend that the same strategies will be of great utility to the development of any online aerosol LDI mass spectrometer platform. Copyright © 2007 John Wiley & Sons, Ltd.
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