Optical characterization of methanol compression-ignition combustion in a heavy-duty engine

In the search for renewable fuels, there are very few candidates as compelling as methanol. It can be derived from refuse material and industrial waste, while the infrastructure exists worldwide to support broad and fast adoption, potentially even as a “drop-in” fuel for existing vehicles with only minor modifications. The most efficient engines currently available are compression-ignition engines, however they often come with high emissions or compromises like the soot-NO_x trade-off. Methanol however, is a low sooting fuel that can potentially be used in such engines despite its high resistance to auto-ignition and reduce emissions while maintaining high engine efficiency. Due to the auto-ignition resistance, few studies of methanol compression-ignition exist and even fewer are conducted in an optically accessible engine. Here, two cases of premixed combustion and two of spray-driven combustion of methanol are studied in a Heavy-Duty optically accessible engine. Ignition and combustion propagation are characterized with a combination of time-resolved natural flame luminosity measurements and single-shot, acetone fuel-tracer, laser induced fluorescence. Additionally, Mie-scattering is used to identify the interaction between liquid spray and ignition sites in spray-driven methanol combustion. Results show that methanol combusts drastically different compared to conventional fuels, especially in spray-driven combustion. The evaporative cooling effect of methanol appears to play a major role in the auto-ignition characteristics of the delivered fuel. Ignition sites appear right at the end of injection when the evaporative cooling effect is withdrawn or at liquid length oscillations where, again the effect is momentarily retracted. To the authors’ knowledge, this has not been documented before.     

Exponential irreducible neighborhoods for combinatorial optimization problems

This paper deals with irreducible augmentation vectors associated with three combinatorial optimization problems: the TSP, the ATSP, and the SOP. We study families of irreducible vectors of exponential size, derived from alternating cycles, where optimizing a linear function over each of these families can be done in polynomial time. A family of irreducible vectors induces an irreducible neighborhood; an algorithm for optimizing over this family is known as a local search heuristic. Irreducible neighborhoods do not only serve as a tool to improve feasible solutions, but do play an important role in an exact primal algorithm; such families are the primal counterpart of a families of facet inducing inequalities.     

Electrothermal atomic absorption spectrometric method for the determination of vanadium in diesel and asphaltene prepared as detergentless microemulsions

A method based on electrothermal atomic absorption spectrometry for the determination of vanadium in diesel and asphalthene fractions is proposed. In order to avoid analyte losses observed at the microgram per liter range for metal traces in organic solutions, diesel samples were stabilized as detergentless microemulsions by mixing with propan-1-ol and nitric acid solution. The solid asphaltene oil fraction was separated and dissolved in dichloromethane before mixing these solution with propan-1-ol and nitric acid solution. Wall atomization as well as no modifier was used. For diesel, aqueous analytical solutions could be used for calibration. For asphaltene, calibration was performed with analytical solutions prepared at the dichloromethane+propan-1-ol+nitric acid medium, spiked with inorganic standard solution. Linear ranges up to 200 μg l⁻¹ were observed, as well as limit of detection of 5 μg l⁻¹ and 4 μg g⁻¹ for diesel and asphaltene, respectively. Good recoveries were obtained for V-cyclohexanebutyrates spiked diesel samples, as well as coherent results for the asphaltene fraction of the NIST 1634c (trace elements in fuel oil) certified reference material.     

Leader Peptide‐Free In Vitro Reconstitution of Microviridin Biosynthesis Enables Design of Synthetic Protease‐Targeted Libraries

Microviridins are a family of ribosomally synthesized and post‐translationally modified peptides with a highly unusual architecture featuring non‐canonical lactone as well as lactam rings. Individual variants specifically inhibit different types of serine proteases. Here we have established an efficient in vitro reconstitution approach based on two ATP‐grasp ligases that were constitutively activated using covalently attached leader peptides and a GNAT‐type N‐acetyltransferase. The method facilitates the efficient in vitro one‐pot transformation of microviridin core peptides to mature microviridins. The engineering potential of the chemo‐enzymatic technology was demonstrated for two synthetic peptide libraries that were used to screen and optimize microviridin variants targeting the serine proteases trypsin and subtilisin. Successive analysis of intermediates revealed distinct structure–activity relationships for respective target proteases.     

Biocatalytic Properties and Structural Analysis of Phloroglucinol Reductases

Phloroglucinol reductases (PGRs) are involved in anaerobic degradation in bacteria, in which they catalyze the dearomatization of phloroglucinol into dihydrophloroglucinol. We identified three PGRs, from different bacterial species, that are members of the family of NAD(P)H‐dependent short‐chain dehydrogenases/reductases (SDRs). In addition to catalyzing the reduction of the physiological substrate, the three enzymes exhibit activity towards 2,4,6‐trihydroxybenzaldehyde, 2,4,6‐trihydroxyacetophenone, and methyl 2,4,6‐trihydroxybenzoate. Structural elucidation of PGRcl and comparison to known SDRs revealed a high degree of conservation. Several amino acid positions were identified as being conserved within the PGR subfamily and might be involved in substrate differentiation. The results enable the enzymatic dearomatization of monoaromatic phenol derivatives and provide insight into the functional diversity that may be found in families of enzymes displaying a high degree of structural homology.     

Discovery of the Lanthipeptide Curvocidin and Structural Insights into its Trifunctional Synthetase CuvL

Lanthipeptides are ribosomally-synthesized natural products from bacteria featuring stable thioether-crosslinks and various bioactivities. Herein, we report on a new clade of tricyclic class-IV lanthipeptides with curvocidin from Thermomonospora curvata as its first representative. We obtained crystal structures of the corresponding lanthipeptide synthetase CuvL that showed a circular arrangement of its kinase, lyase and cyclase domains, forming a central reaction chamber for the iterative substrate processing involving nine catalytic steps. The combination of experimental data and artificial intelligence-based structural models identified the N-terminal subdomain of the kinase domain as the primary site of substrate recruitment. The ribosomal precursor peptide of curvocidin employs an amphipathic α-helix in its leader region as an anchor to CuvL, while its substrate core shuttles within the central reaction chamber. Our study thus reveals general principles of domain organization and substrate recruitment of class-IV and class-III lanthipeptide synthetases.     

Clustering in the absence of attractions: density functional theory and computer simulations

We numerically investigate the formation of stable clusters of overlapping particles in certain systems interacting via purely repulsive, bounded pair potentials. In close vicinity of a first-order phase transition between a disordered and an ordered structure, clusters are encountered already in the fluid phase which then freeze into crystals with multiply occupied lattice sites. These hyper-crystals are characterized by a number of remarkable features that are in clear contradiction to our experience with harshly repulsive systems: upon compression, the lattice constant remains invariant, leading to a concomitant linear growth in the cluster population with density; further, the freezing and melting lines are to high accuracy linear in the density-temperature plane, and the conventional indicator that announces freezing, that is, the Hansen-Verlet value of the first peak of the structure factor, attains for these soft systems much higher values than for their hard-matter counterparts. Our investigations are based on the generalized exponential model of index 4 (i.e., Phi(r) approximately exp[-(r/sigma)4]). The properties of the phases involved are calculated via liquid state theory and classical density functional theory. Monte Carlo simulations for selected states confirm the theoretical results for the structural and thermodynamic properties of the system. These numerical data, in turn, fully corroborate an approximate theoretical framework that was recently put forward to explain the clustering phenomenon for systems of this kind (Likos, C. N.; Mladek, B. M.; Gottwald, D.; Kahl, G. J. Chem. Phys. 2007, 126, 224502).     

Synthesis and Host-abilities of some New Corands Bearing Uncommon Chiral Spacer Units

Benzo- and naphtho-ethylene glycols are prepared from dihydroxy-benzenes or -naphthalenes and the corresponding chloroethoxyethanols using known procedures. Cyclocondensation reactions of these open-chain diols with the chiral and concave tetra-t-butyl-2,6,9-trioxabicyclo[3.3.1]nonadiene (“bridged bisdioxine”) bisacid chloride afford several new corands. In one case, besides the expected macrocycle, an open-chain podand is obtained and further cyclized to a large 2:2 macrocycle. The bridged bisdioxine spacer may also be converted into the tetraoxaadamantane scaffold as shown with an example. Extraction experiments employing selected corands towards various metal ions show a significant host-property towards Hg(II) salts for those macrocycles having o-disubstituted aryl groups incorporated, thus forming 1:1 complexes. NMR titration studies with sodium thiocyanate as well as with some organic guests do not exhibit significant host–guest interactions, in the case of Hg(II) thiocyanate a weak complexation was determined.     

Toward the discovery of inhibitors of babesipain-1, a Babesia bigemina cysteine protease: in vitro evaluation, homology modeling and molecular docking studies

Babesia bigemina is a protozoan parasite that causes babesiosis, a disease with a world-wide distribution in mammals, principally affecting cattle and man. The unveiling of the genome of B. bigemina is a project in active progress that has already revealed a number of new targets with potential interest for the design of anti-babesiosis drugs. In this context, babesipain-1 has been identified as a proteolytically active enzyme whose three-dimensional structure has not been resolved yet, but which is known to be inhibited by cysteine proteases inhibitors such as E64, ALLN, leupeptin, and vinyl sulfones. In this work, we introduce (1) a homology model of babesipain-1; (2) a comparison between babesipain-1 and falcipain-2, a cysteine protease of the malaria parasite Plasmodium falciparum; (3) in vitro data for babesipain-1 inhibition by HEDICINs and HECINs, previously reported as modest inhibitors of falcipain-2; and (4) the docked binding conformations of HEDICINs and HECINs in the model of babesipain-1. HEDICINs presented similar preferred binding conformations for both babesipain-1 and falcipain-2. However, in vitro bioassay shows that HEDICINs and HECINs are better inhibitors of babesipain-1 than of falcipain-2, which could be explained by observed differences between the active pockets of these proteins in silico. Results presented herein provide a valuable contribution to future computer-aided molecular design of new babesipain-1 inhibitors.