Induction of Cryptococcus laurentii α-galactosidase

The surface activity of Cryptococcus laurentii ??-galactosidase can be significantly (up to 80 times) influenced by a carbon source present in a cultivation medium. Induction of this enzyme is stimulated by the presence of saccharides containing bound galactose. The highest activity observed when the cells grew in a lactose medium was probably a consequence of the absence of cleavage products serving as repressors (glucose). This idea is supported by the poor growth of cells in the medium with lactose as the carbon source. The induction of surface ??-galactosidase was accompanied by increased activities in cytosole. The membrane fraction also contained this enzyme, but the influence of the carbon source was not proportional. The induction of ??-galactosidase may play an important role in galactose metabolism of the genus Cryptococcus with a direct influence on the virulence of these capsular yeasts.     

Restricted variation in the glycosylation of human α1-acid glycoprotein

Summary The glycosylation patterns of multiglycosylated proteins reflect both the cellular control of glycosyl transferase activity and the local control of the transfer event. Little information is available concerning these mechanisms. Changes in glycosylation occur in the disease state and provide a convenient way of examining the control mechanism(s) operating at individual glycosylation sites. Chromatographic methods have been applied to the human orosomucoid (OMD) isolated from seventeen different pathologies. Based on the distribution of the glycans at the individual sites it is clear that the major protein-glycan interactions that restrict glycosylation in normal OMD remain intact in the disease state in the presence of changing glycosyl transferase activity.     

The Preparation of α-Phosphonovinylzirconocenes and their Application in the Stereospecific Synthesis of α-Halo-l-alkenylphosphonates

Hydrozirconation of 1-alkylnylphosphonates gives the organozirconium(Ⅳ) complexes 2 in syn-addition way.Complexs 2 was trapped with NCS,NBS or I2 to afford stereodifined α-halo-1-alkenylphosphonates in moderate to high yields.     

Formation of α-enaminoketones from α-azidoketones: An elimination of molecular nitrogen from α-azidoketones

α-Enaminoketones are readily accessible through reaction of α-bromoketones with sodium azide followed by an elimination of molecular nitrogen. The reaction is base catalyzed and proceeds under relatively mild conditions. An anionic mechanism is proposed.     

The source of oxygen in cerium-catalyzed α-hydroxylation of β-dicarbonyl compounds

An investigation into the mechanism of the cerium-catalyzed α-hydroxylation of β-dicarbonyl compounds is presented. By ¹⁸O-labeling studies it was proved that dioxygen is the direct source of the HO-groups in the product. It was excluded that the HO-groups originate from nucleophilic attack of water to an electrophilic radical intermediate. Based on these results and on the stoichiometry of the reaction, a new hypothesis for the mechanism is proposed.     

The A53T mutation is key in defining the differences in the aggregation kinetics of human and mouse α-synuclein

Despite a 95% sequence similarity, the aggregation of human and mouse α-synuclein is remarkably different, as the human form is slower than the mouse form in forming fibrils but is associated with Parkinson's disease in both humans and transgenic mice. Here, the amino acid code underlying these differences is investigated by comparing the lag times, growth rates, and secondary structure propensities of a systematic series of eight human-mouse chimeras. Fluorescence analysis of these variants shows that the A53T substitution dominates the growth kinetics, while the lag phase is affected by a combination of the A53T and S87N substitutions. The secondary structure propensities derived from an NMR chemical shift analysis of the monomeric forms of the human-mouse variants enable us to establish a link between the changes in the conformational properties in the region of position 53 upon mutation and the corresponding changes in growth rates. These results suggest that the presence of an alanine residue at position 53 may be an evolutionary adaptation to minimize Parkinson's disease in humans and indicates that effective drug development efforts may be directed to target this N-terminal region of the sequence.     

The influence of simulation conditions in molecular dynamics investigations of model β-sheet peptides

The influence of simulation methods, cutoff based and particle mesh Ewald (PME) on the accuracy by which experimentally derived nuclear Overhauser effect (NOE) data are reproduced, has been investigated using 500-ns-long molecular dynamics simulations on a model -sheet peptide in explicit solvent. The structural and conformational features under the different conditions were evaluated in terms of flexibility, secondary structure content, hydrogen-bonding pattern and percent of native contacts as a function of time. It was found that the different simulation methods strongly influence the dynamics of the peptide, confirming previous observations based on ideal peptide models simulated for much shorter times. Moreover, the results of our simulations prove once more that it is necessary to reach extremely long time scales to obtain enough statistics to accurately reproduce experimental NOE restraints even in the case of the PME method, despite its tendency to the stabilization of conformations which are structurally closely related to the ones derived through experiment. Possible implications regarding the stabilization and folding mechanisms, together with their relationship to the experimental study of peptide models, are discussed.     

The Preparation of α-Phosphonovinylzirconocenes and their Application in the Stereospecific Synthesis of α-Halo-1 -alkenylphosphonates

Hydrozirconation of 1-alkylnylphosphonates gives the organozirconium(Ⅳ) complexes 2 in syn-addition way. Complexs 2 was trapped with NCS, NBS or I2 to afford stereodifined α-halo-1-alkenylphosphonates in moderate to high yields.     

The rate of intramolecular end-to-end photodimerisation of α,ω-dianthrylpolystyrene in solution

To study the effect of molecular weight on the rate of the intramacromolecular end-to-end reaction of a flexible polymer, the intramolecular photodimerisation of α,ω-dianthrylpolystyrene has been carried out by successive illumination with a 337 nm pulsed nitrogen laser of a dilute solution of butanone at 30°C. The extent of dimerisation against the number of pulses (measured fluorometrically) was converted, based on several assumptions, to the apparent diffusion-controlled rate constants for polymer cyclisation. The rate constants for polymer cyclisation decrease in proportion to DP^−0·3 for the range of DP = 110–2600. The implications of this result in the chain dynamics of polymers are briefly discussed.     

The gas-phase ozonolysis of α-humulene

α-Humulene contains three double bonds (DB), and after ozonolysis of the first DB the first-generation products are still reactive towards O(3) and produce second- and third-generation products. The primary aim of this study consisted of identifying the products of the three generations, focusing on the carboxylic acids, which are known to have a high aerosol formation potential. The experiments were performed in a 570 litre spherical glass reactor at 295 K and 730 Torr. Initial mixing ratios were 260-2090 ppb for O(3) and 250-600 ppb for α-humulene in synthetic air. Reactants and gas-phase products were measured by in situ FTIR spectroscopy. Particulate products were sampled on Teflon filters, extracted with methanol and analyzed by LC-MS/MS-TOF. Using cyclohexane (10-100 ppm) as an OH-radical scavenger and by monitoring the yield of cyclohexanone by PTR-MS, an OH-yield of (10.5 ± 0.7)% was determined for the ozonolysis of the first DB, and (12.9 ± 0.7)% of the first-generation products. The rate constant of the reaction of O(3) with α-humulene is known as k(0) = 1.17 × 10(-14) cm(3) molecule(-1) s(-1) [Y. Shu and R. Atkinson, Int. J. Chem. Kinet., 1994, 26, 1193-1205]. The reaction rate constants of O(3) with the first-generation products and the second-generation products were, respectively, determined as k(1) = (3.6 ± 0.9) × 10(-16) and k(2) = (3.0 ± 0.7) × 10(-17) cm(3) molecule(-1) s(-1) by Facsimile-simulation of the observed ozone decay by FTIR. A total of 37 compounds in the aerosol phase and 5 products in the gas phase were tentatively identified: 25 compounds of the first-generation products contained C13-C15 species, 9 compounds of the second-generation products contained C8-C11 species, whereas 8 compounds of the third-generation products contained C4-C6 species. The products of all three generations consisted of a variety of dicarboxylic-, hydroxy-oxocarboxylic- and oxo-carboxylic acids. The formation mechanisms of some of the products are discussed. The residual FTIR spectra indicate the formation of secondary ozonides (SOZ) in the gas phase, which are formed by the intramolecular reaction of the Criegee moiety with the carbonyl endgroup. These SOZ revealed to be stable over several hours and its formation was shown not to be affected by the addition of Criegee-radical scavengers such as HCOOH or H(2)O. This suggests that in the ozonolysis of α-humulene at atmospheric pressures the POZ will decompose rapidly, and that a large fraction of the formed exited Criegee Intermediate will be stabilized to form stable SOZ, while the formation of OH-radicals via the hydroperoxide channel will be a minor process.     

The thermolysis of α-pinene and verbenone epoxides in supercritical solvents

Thermal transformations of α-pinene and verbenone epoxides were studied in supercritical solvents with complex compositions, including CO₂, lower alcohols (ethanol and isopropanol), and water, over the temperature and pressure ranges 387–575 K and 135–215 atm. The main product from α-pinene epoxide in a supercritical solvent containing water was campholenic aldehyde and pinocamphone; the total content of these products in the reaction mixture was 80%. Suggestions concerning the mechanism of the thermal isomerization of α-pinene epoxide depending on the acidity of supercritical solvents were made. The direction of verbenone epoxide transformations was independent of the presence of water in the mixture. The main identified products were ketoalcohols with para-menthane and camphane frameworks.     

The Fascinating Chemistry of α-Haloamides

The aim of this review is to highlight the rich chemistry of α-haloamides originally mainly used to discover new C−N, C−O and C−S bond forming reactions, and later widely employed in C−C cross-coupling reactions with C(sp³), C(sp²) and C(sp) coupling partners. Radical-mediated transformations of α-haloamides bearing a suitable located unsaturated bond has proven to be a straightforward alternative to access diverse cyclic compounds by means of either radical initiators, transition metal redox catalysis or visible light photoredox catalysis. On the other hand, cycloadditions with α-halohydroxamate-based azaoxyallyl cations have garnered significant attention. Moreover, in view of the important role in life and materials science of difluoroalkylated compounds, a wide range of catalysts has been developed for the efficient incorporation of difluoroacetamido moieties into activated as well as unactivated substrates.     

The effect of molecular distortions on spin–orbit coupling in simple hydrocarbons

Recent work [D.N.S. Parker et al., Chem. Phys. Lett. 469 (2009) 43–49] has found intersystem crossing (ISC) on an ultrafast timescale in electronically excited benzene, a surprise as hydrocarbons generally have small spin–orbit coupling. In this paper, the effect of molecular distortions on spin–orbit coupling (SOC) is calculated for cyclobutadiene and benzene. At equilibrium the SOC in both molecules is negligible, and therefore terms arising from molecular distortions must play a significant role in any fast ISC. We show that out-of-plane C–H bends, which leads to the hybridisation of σ and π orbitals, are responsible for the most significant effect. The S₁/S₀ conical intersection is an important feature for understanding the photochemistry of these molecules. We examine the SOC along the vector from the Franck–Condon point to the lowest energy point on the crossing seam and discuss the potential importance of the SOC to the ultrafast dynamics.     

The chemist’s concept of molecular structure

The concept of molecular structure is fundamental to the practice and understanding of chemistry, but the meaning of this term has evolved and is still evolving. The Born–Oppenheimer separation of electronic and nuclear motions lies at the heart of most modern quantum chemical models of molecular structure. While this separation introduces a great computational and practical simplification, it is neither essential to the conceptual formulation of molecular structure nor universally valid. Going beyond the Born–Oppenheimer approximation introduces new paradigms, bringing fresh insight into the chemistry of fluxional molecules, proteins, superconductors and macroscopic dielectrics, thus opening up new avenues for exploration. But it requires that our ideas of molecular structure need to evolve beyond simple ball-and-stick-type models.

Effect of Temperature on The Solubility of α-Amino Acids and α,ω-Amino Acids in Water

The aqueous solubilities of glycine, dl-α-alanine (2-aminopropanoic acid), dl-α-aminobutyric acid (2-aminobutanoic acid), dl-α-norvaline (2-aminopentanoic acid), dl-α-norleucine (2-aminohexanoic acid), β-alanine (3-aminopropanoic acid), γ-aminobutyric acid (4-aminobutanoic acid), 5-aminovaleric acid (5-aminopentanoic acid), and 6-aminocaproic acid (6-aminohexanoic acid) were determined from 293.15 to 323.15 K at intervals of 5.00 K using the gravimetric method. The temperature dependence of the solubility of α-amino acids and α,ω-amino acids in water is well described by the van’t Hoff equation. Linear van’t Hoff plots were used to determine the differential enthalpy of solution. The results obtained are compared with reported values in literature and are discussed in terms of the position of the ionic groups in the hydrocarbon chain.     

The molecular electrochemistry of metal–organic metallamacrocycles

Considering that most metallamacrocycles possess redox-active metal nodes as well as redox-active linker ligands, the number of studies aimed at investigating that inherent property is astoundingly small. This microreview summarizes the most relevant, recent work on the electrochemistry of organometallic macrocycles. We hope that this article encourages further incentives to not only explore, but also exploit the inherent redox-activity of metal–organic macrocycles.     

How does human odorant binding protein bind odorants? The case of aldehydes studied by molecular dynamics

Human Odorant Binding Protein (OBPIIa) has a strong affinity for odorants belonging to the family of aldehydes. After having built the initial structures based on the protein sequence, we have performed molecular dynamics simulations on human OBP, free and bound to citral and undecanal to examine the reasons for this affinity from a dynamic point of view. The formation of a Schiff base between a Lysine residue and the aldehyde function could be responsible for this strong affinity.     

Radical chain mechanism of the oxidation of α-terpinene by molecular oxygen

The oxidation of -terpinene by molecular oxygen is shown to occur by a radical chain mechanism involving hydroperoxy radicals. The principal molecular reaction products are H₂O₂ and 1-methyl-4-isopropylbenzene. The rate constants for chain propagation and chain breaking measured at 35–70°C are (liter/mol·s) lgk₃ = 7.10– (6700±400)/4.57T and lgk₅ = 9.80– (2700±300)/4.57T respectively.Translated from Teoreticheskaya Éksperimental'naya Khimiya, Vol. 30, No. 3, pp. 143–146, May–June, 1994The authors thank the Ukraine National Committee for the Basic Research Fund for financing this work.     

The role played by head–tail configuration on the molecular weight distribution of α-cyclodextrin tubes

Supramolecular complexes consisting of cyclic molecules, such as cyclodextrins (CD), and polymeric chains have attracted considerable attention, being addressed in literature as novel molecular assembly. The so-called molecular tube (MT), synthesized by cross-linking adjacent α-CD in a polyrotaxane, is expected to act as host for large molecules in inclusion processes. In addition, these tubes can also be used as building-blocks in the formulation of novel materials. Molecular tubes constructed with α-cyclodextrin are obtained as a mixture containing entities with various molecular weights, and the molecular features determining the tube size distribution are not completely understood. In this paper, we propose the use of a statistical procedure based on binary numbers to examine the MT formation process. A complete analysis of the distinct orientations between cyclodextrin’s units was made and, in the light of the approximations of our model, we pointed out, on quantitative basis, that the molecular weight distribution of α-cyclodextrin MTs can be explained assuming imperfections in the cross-linking process due to the existence of head-to-tail (HT) arrangements in the polyrotaxanes employed in synthesis.