The unimolecular chemistry of [1,2-propanediol]+˙: a rationale in terms of hydrogen-bridged radical cations

By combining results from a variety of mass spectrometric techniques (metastatle ion, collisional activation, collision-induced dissociative ionization, neutralization–reionization spectrometry and appearance energy measurements) and the classical method of isotopic labelling, a unified mechanism is proposed for the complex unimolecular chemistry of ionized 1,2-propanediol. The key intermediates involved are the stable hydrogen-bridged radical cations [CH₂?C(H)? H…?O…?O(H)CH₃]⁺˙, which were generated independently from [4-methoxy, 1-butanol]⁺˙ (loss of C₂H₄) and [1-methoxyglycerol]⁺˙ (loss of CH₂O), [CH₃? C?O…?H…?O(H)CH₃]⁺˙ and the related ion-dipole complex [CH₂?C(OH)CH₃/H₂O]⁺˙. The latter species serves as the precursor for the loss of CH³˙ and in this reaction the same non-ergodic behaviour is observed as in the loss of CH³˙ from the ionized enol of acetone.     

TRANSIENT INTERMEDIATES IN INTRAMOLECULARLY PHOTOSENSITIZED PYRIMIDINE DIMER SPLITTING BY INDOLE DERIVATIVES

Abstract— Intramolecularly photosensitized pyrimidine dimer splitting can serve as a model for some aspects of the monomerization of dimers in the enzyme-substrate complex composed of a photolyase and UV-damaged DNA. We studied compounds in which a pyrimidine dimer was covalently linked either to indole or to 5-methoxyindole. Laser flash photolysis studies revealed that the normally observed photoejection of electrons from the indole or the 5-methoxyindole to solvent was diminished by an order of magnitude for indoles with dimer attached (dimer-indole and dimer-methoxyindole). The fluorescence lifetime of dimer-indole in aqueous methanol was 0.85 ns, whereas that of the corresponding indole without attached dimer (tryptophol) was 9.7 ns. Similar results were obtained for the dimer-methoxyindole (0.53 ns) and 5-methoxytryptophol (4.6 ns). The quantum yield of dimer splitting for the dimer-methoxyindole (φ₂₈₇K7 = 0.08) was only slightly greater than the value found earlier for the dimer bearing the unsubstituted indole (4>_2K7= 0.04). Transient absorption spectroscopy also revealed lower yields of indole radical cations following laser flash photolysis of dimer-indole compared to the indole without attached dimer. Dimer-methoxyindole behaved similarly. These results are interpreted in terms of an enhanced rate of radiationless relaxation of the indole and methoxyindole excited singlet states in dimer-indoles. The possible quenching of the indole and methoxyindole excited states via electron abstraction by the covalently linked dimer is discussed.     

PHOTOMODIFICATION OF HUMAN ERYTHROCYTES: EXTERNAL HEAVY ATOM EFFECT, SELECTIVE PERMEABILITY AND PROPERTIES OF THE ANION PERMEATION PATHWAY

Abstract— Anion permeability was studied in normal and photomodified erythrocytes and the role of anion species in photomodification and singlet oxygen generation was evaluated. Relative permeability to halides and nitrate was assessed using lysis rates in valinomycin-treated cells to make anion permeation rate-limiting. In non-photomodified cells the normal high temperature dependence (E:, = 67 kJ/mol; 16 kcalhol), selectivity sequence (NO, > I > Br > F > Cl) and sensitivity to block by the stilbene derivative HzDIDS were confirmed. In cells photomodified by illumination in the presence of phloxine B the anion permeability was severely perturbed. The temperature dependence was strongly reduced, the permeability to ions other than fluoride increased, leading to a reversal of the F/CI selectivity, and the capacity to be blocked by HIDIDS was lost. Pretreatment with NEM and posttreatment with DTE did not affect rates of photohemolysis. The amount of photomodification varied with species of anion present in the reaction medium, being greatest for F and smallest for I. Rates of singlet oxygen generation in aqueous solution measured by the RNO bleaching method followed the same anion sequence, suggesting a strong external heavy atom effect.     

Synthesis of 2-oxo and 2-thioxo-3(2H)-benzothiazoleethanimic acid anhydride with acetic acid and related products

The reaction of the appropriate 2-benzothiazolinone with 2-chloroacetamide under basic conditions afforded the 2-oxo-3(2H)-benzothiazolineacetamides 6–9. The 2-thioxo-3(2H)-benzothiazolineacetamide ( 10 ) was prepared by the reaction of 3-(carbethoxymethyl)benzothiazoline-2-thione with ammonium hydroxide. The reaction of acetamides 6–10 with the appropriate anhydride containing a catalytic amount of the sodium salt of the acid corresponding to the anhydride afforded the titled compounds 11–18 in excellent yields. The omission of the catalyst in the same reaction furnished a mixture containing 57% of the titled compound, 37% of the nitrile and 6% of an unknown. Possible mechanism and supporting nmr, ir and mass spectral data are discussed.     

Surface morphological and impedance spectroscopic studies on the interaction of polyethylene glycol (PEG) and polyvinyl pyrrolidone (PVP) with mild steel in acid solutions

The inhibition of mild steel corrosion in aerated acid mixture of 0.5 N H₂SO₄ and 0.5 N HCl solution was investigated using potentiodynamic polarization studies, linear polarization studies, electrochemical impedance spectroscopy, adsorption, and surface morphological studies. The effect of inhibitor concentration on corrosion rate, degree of surface coverage, adsorption kinetics, and surface morphology is investigated. The inhibition efficiency increased markedly with increase in additive concentration. The presence of PEG and PVP decreases the double-layer capacitance and increases the charge-transfer resistance. The inhibitor molecules first adsorb on the metal surface following a Langmuir adsorption isotherm. Both PEG and PVP offer good inhibition properties for mild steel and act as mixed-type inhibitors. Surface analysis by scanning electron microscopy (SEM) and atomic force microscopy (AFM) shows that PVP offers better protection than PEG.     

Covalent Immobilization of β-Glucosidase on Magnetic Particles for Lignocellulose Hydrolysis

β-Glucosidase hydrolyzes cellobiose to glucose and is an important enzyme in the consortium used for hydrolysis of cellulosic and lignocellulosic feedstocks. In the present work, β-glucosidase was covalently immobilized on non-porous magnetic particles to enable re-use of the enzyme. It was found that particles activated with cyanuric chloride and polyglutaraldehyde gave the highest bead-related immobilized enzyme activity when tested with p-nitrophenyl-β-D-glucopyranoside (104.7 and 82.2 U/g particles, respectively). Furthermore, the purified β-glucosidase preparation from Megazyme gave higher bead-related enzyme activities compared to Novozym 188 (79.0 and 9.8 U/g particles, respectively). A significant improvement in thermal stability was observed for immobilized enzyme compared to free enzyme; after 5 h (at 65 °C), 36 % of activity remained for the former, while there was no activity in the latter. The performance and recyclability of immobilized β-glucosidase on more complex substrate (pretreated spruce) was also studied. It was shown that adding immobilized β-glucosidase (16 U/g dry matter) to free cellulases (8 FPU/g dry matter) increased the hydrolysis yield of pretreated spruce from ca. 44 % to ca. 65 %. In addition, it was possible to re-use the immobilized β-glucosidase in the spruce and retain activity for at least four cycles. The immobilized enzyme thus shows promise for lignocellulose hydrolysis.     

Ligand binding and unfolding of tryptophan synthase revealed by ion mobility-tandem mass spectrometry employing collision and surface induced dissociation

Understanding protein tertiary and quaternary structures is crucial to a better understanding of their biological functions. Here we illustrate for tryptophan synthase that tandem mass spectrometry (MS/MS) reveals not only protein subunit architectures, but also protein unfolding behavior when coupled with ion mobility (IM). In the present study, we verified the subunit arrangement with surface induced dissociation (SID). We are able to correlate experimental results by IM with those obtained in unfolding simulations for the hetero-tetramer Tryptophan Synthase (TS) protein complex by identifying the presence of at least three stable intermediates (I₁, I₂, and I₃) during the unfolding process in collision induced dissociation (CID). We illustrate that the unfolding of the TS complex is likely due to the initial unfolding of an α-monomer subunit, followed by the unfolding of the second α-monomers. We also illustrate the ability of this combination of techniques to not only identify conformational changes of TS upon addition of D,L-α-glycerol phosphate (GP), but also to determine the location of the ligand, which is buried within the α-monomer of the TS.     

A quantitative and comprehensive method to analyze human milk oligosaccharide structures in the urine and feces of infants

Human milk oligosaccharides (HMOs), though non-nutritive to the infant, shape the intestinal microbiota and protect against pathogens during early growth and development. Infant formulas with added galacto-oligosaccharides have been developed to mimic the beneficial effects of HMOs. Premature infants have an immature immune system and a leaky gut and are thus highly susceptible to opportunistic infections. A method employing nanoflow liquid chromatography time-of-flight mass spectrometry (MS) is presented to simultaneously identify and quantify HMOs in the feces and urine of infants, of which 75 HMOs have previously been fully structurally elucidated. Matrix-assisted laser desorption/ionization Fourier transform ion cyclotron resonance MS was employed for high-resolution and rapid compositional profiling. To demonstrate this novel method, samples from mother–infant dyads as well as samples from infants receiving infant formula fortified with dietary galacto-oligosaccharides or probiotic bifidobacteria were analyzed. Ingested oligosaccharides are demonstrated in high abundance in the infant feces and urine. While the method was developed to examine specimens from preterm infants, it is of general utility and can be used to monitor oligosaccharide consumption and utilization in term infants, children, and adults. This method may therefore provide diagnostic and therapeutic opportunities.

The effects of season and soil type on microbial degradation of gasoline residues from incendiary devices

The primary task of a fire debris chemist is to determine if there is an ignitable liquid present in a fire debris sample and, if so, to classify it according to its boiling point and carbon number range. However, in organic-rich substrates such as soil, the ignitable liquid residue is subject to microbial degradation due to the ease with which bacteria can metabolize the various hydrocarbons present. This is a rapid process which is problematic in many forensic laboratories as fire debris is often stored for extended periods of time due to case backlog. Although microbial degradation has been studied in laboratory samples, it has not been well-studied in “real-world” samples, which have not only been exposed to microbial degradation but have also suffered the effects of weathering due to the intense heat of the fire. In this work, the effects of microbial degradation of gasoline from an incendiary device have been evaluated over time. In addition to visually monitoring chromatographic changes, this work also utilizes multivariate statistical techniques to simplify the complex data set and elucidate trends that might not otherwise be observed. Results indicate a clear difference between glass samples, which suffered the loss of low boiling compounds, and soil, which suffered the loss of the normal alkanes and lesser substituted aromatics. Also, devices deployed on lawn soil and in the winter season appear to show the most extensive degradation of gasoline. Finally, while the ratio of the C₃-alkylbenzenes is significantly altered in soil samples recovered from large devices, the overall chromatographic profile of gasoline recovered from smaller incendiary devices is significantly lower.

Electrophilic aromatic prenylation via cascade cyclization

To gain access to prenylated hexahydroxanthenes, tandem cascade cyclization–electrophilic aromatic substitution reactions have been studied on substrates bearing allylic and propargylic substituents. Both BF₃·OEt₂ and TMSOTf can be used to initiate this reaction sequence, resulting in different ratios of the C-2 and C-6 substitution products. Even though allylic transposition is observed in some cases, the results of a crossover experiment are consistent with an intramolecular reaction sequence. Taken together, these studies now allow preparation of either the C-2 or C-6 prenylated hexahydroxanthene products.