A model for the fragility of the melts

The concept of fragility has been used widely to characterize the temperature dependence of the viscosity of glass forming materials. However, the physical background that determines the degree of fragility is still not well understood. In the present study an expression for the fragility is derived based on a simple model of the melt. According to the model, the fragility is determined by the relaxation of structural units that form the melt, and is described in terms of the bond strength (E ₀), coordination number (Z ₀), and their fluctuations (ΔE and ΔZ). It is shown that a strong system is characterized by large value of total bond strength (Z ₀ E ₀) and small value of its fluctuation ((ΔZ)²(ΔE)²). On the other hand, a fragile system is characterized by small value of total bond strength and large value of its fluctuation. This revised version was published online in August 2006 with corrections to the Cover Date.     

A streamlined metabolic pathway for the biosynthesis of moenomycin A

Moenomycin A (MmA) is a member of the phosphoglycolipid family of antibiotics, which are the only natural products known to directly target the extracellular peptidoglycan glycosyltransferases involved in bacterial cell wall biosynthesis. The structural and biological uniqueness of MmA make it an attractive starting point for the development of new antibacterial drugs. In order both to elucidate the biosynthesis of this unusual compound and to develop tools to manipulate its structure, we have identified the MmA biosynthetic genes in Streptomyces ghanaensis (ATCC14672). We show via heterologous expression of a subset of moe genes that the economy of the MmA pathway is enabled through the use of sugar-nucleotide and isoprenoid building blocks derived from primary metabolism. The work reported lays the foundation for genetic engineering of MmA biosynthesis to produce novel derivatives.     

A method for the preparation of corchoroside A

Chemistry of Natural Compounds -     

A concise approach for the total synthesis of pseudolaric acid A

A new strategy for the stereoselective total synthesis of natural product pseudolaric acid A (1) was accomplished in 16 steps from commercially available starting material, featuring a samarium diiodide (SmI(2))-mediated intramolecular alkene-ketyl radical cyclization and a ring-closing metathesis (RCM) reaction to stereoselectively cast the unusual trans-fused [5-7]-bicyclic core of pseudolaric acid A (1).     

Structure-activity relationships for the fluorescence of ochratoxin A: insight for detection of ochratoxin A metabolites

Ochratoxin A (OTA) is a mycotoxin produced by Aspergillus and Penicillium that is widely found as a contaminant of food products. The toxin is a renal carcinogen in male rats, the cause of mycotoxicoses in pigs and has been associated with chronic human kidney diseases. Bioactivation has been implicated in OTA-mediated toxicity, although inconsistent results have been reported, due, in part, to the difficulty in detecting OTA metabolites in vivo. Liquid chromatography (LC) coupled with fluorescence detection (FLD) is the most widely used analytical detection method for OTA. Under acidic conditions the toxin generates blue fluorescence (465 nm) that is due to an excited state intramolecular proton transfer (ESIPT) process that generates an emissive keto tautomer. Disruption of this ESIPT process quenches fluorescence intensity and causes a blue shift in emission maxima. The aim of the present study was to determine the impact of the C5-chlorine atom, the lactone moiety and the amide bond on OTA fluorescence and derive optical parameters for OTA metabolites that have been detected in vitro. Our results highlight the limitations of LC/FLD for OTA metabolites that do not undergo ESIPT. For emissive derivatives, our absorption and emission data improves the sensitivity of LC/FLD (3-4-fold increase in the limit of detection (LOD)) for OTA analogues bearing a C5-OH group, such as the hydroquinone (OTHQ) metabolite and the glutathione conjugate of OTA (OTA-GSH). This increased sensitivity may facilitate the detection of OTA metabolites bearing a C5-OH group in biological fluids and enhance our understanding of OTA-mediated toxicity.     

用于测定络合物组成的摩尔比法原理的论证

本文将Ｂｅｅｒ定律，化学平衡原理和数学方法结合起来，较为严格地论证了摩尔比法的原理。同时证明了若以试剂空白为参化液，则无论试剂在所选定的波长λ处是否有显著的吸收，从理论上讲用该法都可得到准确的结果。     

A model for the pseudorotation of cycloheptane

A very simple model for a seven-membered ring having fixed bond lengths and fixed ring bond angles is presented. The model pseudorotates in both the twist-chair/chair and the twist-boat/boat conformations and gives reasonably accurate detailed geometries.     

A ribozyme for the aldol reaction

Directed in vitro evolution can create RNA catalysts for a variety of organic reactions, supporting the "RNA world" hypothesis, which proposes that metabolic transformations in early life were catalyzed by RNA molecules rather than proteins. Among the most fundamental carbon-carbon bond-forming reactions in nature is the aldol reaction, mainly catalyzed by aldolases that utilize either an enamine mechanism (class I) or a Zn(2+) cofactor (class II). We report on isolation of a Zn(2+)-dependent ribozyme that catalyzes an aldol reaction at its own modified 5' end with a 4300-fold rate enhancement over the uncatalyzed background reaction. The ribozyme can also act as an intermolecular catalyst that transfers a biotinylated benzaldehyde derivative to the aldol donor substrate, coupled to an external hexameric RNA oligonucleotide, supporting the existence of RNA-originated biosynthetic pathways for metabolic sugar precursors and other biomolecules.     

A new method for the trifluoromethylation of aromatics

A new method is discribed for introducing a CF₃-group, by a single-step synthesis, into aromatic compounds. This trifluoromethylation is done by means of a mixture consisting of HF/CCL₄ and the aromatic compound. The reaction is thought to be of a Friedel-Crafts type and limited to aromatics which are not substituted by electron withdrawing groups.     

A Convenient Procedure for the Preparation of Adamantanone

Sinoe adamantanone is a useful and versatile startin material for secondary substituted and ring-transformed adamantanes¹we wish to report on a convenient synthesis of this ketone from adamantans with 98 percent sulfuric acid. Adamantanone may be prepared in basionally the same way, starting from 1-adamantanol, which gives abtter yield and needs a shorter reaction tine²but adamantane affords a more direot routs. The producer reported here is based on a 100-g scale, but this scale may be extended to 1 kg.     

A formula for the Kirchhoff index

We show here that the Kirchhoff index of a network is the average of the Wiener capacities of its vertices. Moreover, we obtain a closed‐form formula for the effective resistance between any pair of vertices when the considered network has some symmetries, which allows us to give the corresponding formulas for the Kirchhoff index. In addition, we find the expression for the Foster's n‐th formula.     

A model for the spherulite-to-fibril transformation

Based on an analogy with the growth of kink bands in oriented and annealed crystalline polymers, a mechanism is suggested for the deformation-induced spherulite-to-fibril transformation. Oriented and annealed material was employed as a model because it has roughly the same structure as the lamellar bundle of the spherulite. Under conditions that minimize the fibrillar quality and maximize the lamellar bundle quality, oriented and annealed crystalline polymers deform with the formation and growth of kink bands. The characteristics of kink bands are the sharpness of the kinks, occurring within zones ca. 100 Å wide, and mirror symmetry of the fibrils about the kink-zone planes. The spherulite-fibril transformation is suggested by analogy, then, to occur in the narrow zones that indeed are often found and to result in mirror symmetry of the chain axes about the transformation-zone planes, which may not exactly occur because of a tendency for the tie molecules to be pulled from the lamellae during drawing. The implications for the transformation are (a) that the microfibril backbones are derived from the tie molecules between lamellae and (b) though some unfolding occurs as blocks that were adjacent within the lamellae become separated, the crystals remain mostly intact during the transformation. The analog is supported by or at least is consistent with various other results, including those from morphological and small- and wide-angle x-ray studies.     

A simple method for the alpha-oxygenation of aldehydes

A mild, efficient and general method for the chemospecific alpha-oxygenation of aldehydes is described. Treatment of a series of aldehydes with N-tert-butyl-O-benzoyl hydroxylamine hydrochloride gives the corresponding alpha-oxygenated carbonyl via a proposed pericyclic rearrangement process.     

A three-state model for the photophysics of guanine

The nonadiabatic photochemistry of the guanine molecule (2-amino-6-oxopurine) and some of its tautomers has been studied by means of the high-level theoretical ab initio quantum chemistry methods CASSCF and CASPT2. Accurate computations, based by the first time on minimum energy reaction paths, states minima, transition states, reaction barriers, and conical intersections on the potential energy hypersurfaces of the molecules lead to interpret the photochemistry of guanine and derivatives within a three-state model. As in the other purine DNA nucleobase, adenine, the ultrafast subpicosecond fluorescence decay measured in guanine is attributed to the barrierless character of the path leading from the initially populated 1(pi pi* L(a)) spectroscopic state of the molecule toward the low-lying methanamine-like conical intersection (gs/pi pi* L(a))CI. On the contrary, other tautomers are shown to have a reaction energy barrier along the main relaxation profile. A second, slower decay is attributed to a path involving switches toward two other states, 1(pi pi* L(b)) and, in particular, 1(n(O) pi*), ultimately leading to conical intersections with the ground state. A common framework for the ultrafast relaxation of the natural nucleobases is obtained in which the predominant role of a pi pi*-type state is confirmed.     

Immunochromatographic assay for the detection of ochratoxin A

The detection of mycotoxins—toxic contaminants of fungal origin—is an important problem in the food and feed quality control. An immunochromatographic system was developed for the detection of ochratoxin A (OTA), which is one of the priority contaminants in grain. Monoclonal antibodies against OTA and their conjugates with colloidal gold nanoparticles were prepared. The detection is based on the competition of OTA in a sample and an OTA-protein conjugate immobilized on a test strip for the binding to anti-bodies on the colloidal particle surface. The method was tested in the analysis of plant extracts (maize and barley extracts). It was shown that OTA can be detected in a medium with a high content of an organic solvent (up to 35% of methanol). The disappearance of the line in the test zone is visually detected at OTA concentrations starting from 50 ng/mL. In the case of the video-digital detection of changes in the color intensity of the test zone, the limit of detection of OTA is 5 ng/mL. The duration of the assay is 10 min.     

A three-state model for the photophysics of adenine

An ab initio theoretical study at the CASPT2 level is reported on minimum energy reaction paths, state minima, transition states, reaction barriers, and conical intersections on the potential energy hypersurfaces of two tautomers of adenine: 9H- and 7H-adenine. The obtained results led to a complete interpretation of the photophysics of adenine and derivatives, both under jet-cooled conditions and in solution, within a three-state model. The ultrafast subpicosecond fluorescence decay measured in adenine is attributed to the low-lying conical intersection (gs/pipi* La)(CI), reached from the initially populated 1(pipi* La) state along a path which is found to be barrierless only in 9H-adenine, while for the 7H tautomer the presence of an intermediate plateau corresponding to an NH2-twisted conformation may explain the absence of ultrafast decay in 7-substituted compounds. A secondary picosecond decay is assigned to a path involving switches towards two other states, 1(pipi* Lb) and 1(npi*), ultimately leading to another conical intersection with the ground state, (gs/npi*), with a perpendicular disposition of the amino group. The topology of the hypersurfaces and the state properties explain the absence of secondary decay in 9-substituted adenines in water in terms of the higher position of the 1(npi*) state and also that the 1(pipi* Lb) state of 7H-adenine is responsible for the observed fluorescence in water. A detailed discussion comparing recent experimental and theoretical findings is given. As for other nucleobases, the predominant role of a pipi*-type state in the ultrafast deactivation of adenine is confirmed.     

A convergent strategy for the synthesis of beta-carba-galacto-disaccharides

[reaction in text] A convergent strategy for the synthesis of beta-carba-galacto-disaccharides is illustrated by the preparation of 1 and 4, from a central "glycone" component 22, and the corresponding "aglycone" segments, monosaccharide alcohols, 23a or 23b. The key step is the formation of the carbasugar ring via an oxocarbenium ion-enol ether cyclization.     

A stereoselective approach for the synthesis of alpha-sialosides

A highly efficient synthesis of the human melanoma associated antigen GD(3) derivative has been described. A key feature of the synthetic approach was the use of sialyl donors that were protected with a C-5 trifluoroacetamide moiety. These sialyl donors gave high yields and excellent alpha-anomeric selectivities in direct glycosylations with a wide variety of glycosyl acceptors ranging from C-8 hydroxyls of sialic acids and C-3 hydroxyls of galactosides to reactive primary alcohols.