Triterpenoid acids and lactones from the leaves of Fadogia tetraquetra var. tetraquetra (Rubiaceae)

Four triterpenoids isolated from the leaves of Fadogia tetraquetra var. tetraquetra, 3beta-hydroxy-11alpha, 12alpha-epoxyoleanan-28,13beta-olide (1), 3beta-hydroxyurs-11-en-28,13beta-olide (2), oleanolic acid (3), and ursolic acid (4), were evaluated for their antiviral and antibacterial properties. Compound 4 showed potent activity against the Semliki Forest virus with an IC50 of 14.7 microM, but was also found to be significantly cytotoxic (68% reduction in cell viability after 24 hours exposure at 50 microM) towards baby hamster kidney (BHK21) host cells. A viability assay on the mammalian human hepatocellular carcinoma (Huh-7) cell line showed no significant effects on intracellular ATP content after 48 hours exposure to compounds 1-4 at this concentration. Compound 4 also inhibited Staphylococcus aureus (MIC 12.5 microM), but was inactive against Enterobacter aerogenes, Escherichia coli, and Pseudomonas aeruginosa. Compounds 1-3 were inactive against all tested bacterial strains at 50 microM concentration.     

A route to the fully substituted cyclopentane unit of viridenomycin using a tandem radical cyclisation-trapping strategy

A silicon-tethered intramolecular radical cyclisation, in tandem with a radical trapping with allyltri-n-butylstannane forms the basis of a new synthetic strategy to the cyclopentane core of the antitumoral antibiotic substance viridenomycin isolated from Streptomyces viridochromogenes and S. gannmycicus.     

Expansion solution for subsonic compressible flow

An expansion solution in the physical plane is developed for subsonic compressible fluid flow past an obstacle. Assuming that the stream is inviscid, isentropic, irrotational and steady, it is shown that the velocity potential may be expressed as a series of homogeneous Heun functions and radial distance terms. The basis of this analysis is Ludford's formal discussion of corresponding singularities in Bergman's Linear Integral Operator Method. A modification of these results permits reduction of the governing nonlinear partial differential equation to an ordinary, nonhomogeneous, linear differential equation. The expansion solution is compared with the Rayleigh-Janzen method and the Prandtl-Glauert theory. The comparison indicates that this expansion gives better results than other methods currently used. The simplicity and economy of this expansion solution facilitates direct practical application.     

An expert system for the choice of factors for a ruggedness test in liquid chromatography

Method validation is an important and costly step in the development of liquid chromatographic methods, especially for pharmaceutical samples. A ruggedness test is valuable for finding the analytical conditions which give the best performance. By varying the factors that influence method performance, it is possible to design a more rugged method before a study of reproducibility is started. The choice of the factors to test is important. An expert system is described for the selection of factors in ruggedness tests. The system proved satisfactory for 10 of the 11 cases of pharmaceutical formulations tested.     

The prediction of traffic noise using a scale model

This paper describes the development of means of using a scale model of a road and its surrounding urban environment to predict L_eq, L₁₀ and other measures of traffic noise. The model described is that of the Centre Scientifique et Technique du Batiment, Grenoble, France. The problems involved in the development include allowance for relative sound absorption between real life and the model situation, the constraints on the accuracy of the results due to noise source variations on the model and the effects of the finite size of the model.     

An impulse method of measuring normal impedance at oblique incidence

An experimental method of determining the oblique incidence behaviour of sound absorbing materials is given. The method involves the measurement of acoustic impulses at the surface of an absorbing material and comparing the complex frequency components with those of a reference signal recorded remote from the surface. Amplitude and phase characteristics of the reflected signal are thus obtained and hence the normal impedance of the material is found. Comparison between measurements made by the “surface” method and standing wave tube measurements shows good agreement for normal incidence. However, at very oblique angles of incidence the method is in obvious error, the accuracy being limited by a sphericity effect.     

Insights into conformational changes of procarboxypeptidase A and B from simulations: a plausible explanation for different intrinsic activity

Different forms of carboxypeptidase proenzymes (zymogens) are observed experimentally to show different behavior: procarboxypeptidase A (proCPA, forms proCPA1 and proCPA2) exhibit some activity against small substrates, but proCPB does not. In this work, these three zymogen forms (subtypes A1, A2 and B) are investigated by means of 15-ns molecular dynamics simulations and principal component analysis to shed light on their dynamic/conformational behaviors that may be relevant to those experimental observations. The simulations revealed that proCPA (both A1 and A2) shows different conformational behavior from proCPB: the former undergoes a major conformational change (opening and closing), and the latter exhibits only a minor conformational change (remaining closed throughout the simulation). Differences center on the interface between the globular moiety of the pro-segment and the catalytic domain. Analysis of the trajectories demonstrates the importance of hydrogen bonds and salt-bridges in stabilizing the zymogen structures and shows different hydrogen-bond patterns between proCPA and proCPB: the former shows fewer strong H-bonds formed between the globular domain and the catalytic domain. The observed difference in conformational behavior between proCPA and proCPB may explain why small substrates and inhibitors can access the active sites of proCPA1 and proCPA2 but not of proCPB.     

Mechanism of C-terminal intein cleavage in protein splicing from QM/MM molecular dynamics simulations

Protein splicing is a post-translational process in which a biologically inactive protein is activated by the release of a segment denoted as an intein. The process involves four steps. In the third, the scission of the intein takes place after the cyclization of the last amino acid of the segment, an asparagine. Little is known about the chemical reaction necessary for this cyclization. Experiments demonstrate that two histidines (the penultimate amino acid of the intein, and a histidine located 10 amino acids upstream) are relevant in the cyclization of the asparagine. We have investigated the mechanism and determinants of reaction in the GyrA intein focusing on the requirements for asparagine activation for its cyclization. First, the influence that the protonation states of these two histidines have on the orientation of the asparagine side chain is investigated by means of molecular dynamics simulation. Molecular dynamics simulations using the CHARMM27 force field were carried out on the three possible protonation states for each of these two histidines. The results indicate that the only protonation state in which the conformation of the system is suitable for cyclization is when the penultimate histidine is fully protonated (positively charged), and the upstream histidine is in the His(ε) neutral tautomeric form. The free energy profile for the reaction in which the asparagine is activated by a proton transfer to the upstream histidine is presented, computed by hybrid quantum mechanics/molecular mechanics (QM/MM) umbrella sampling molecular dynamics at the SCCDFTB/CHARMM27 level of theory. The calculated free energy barrier for the reaction is 19.0 kcal mol(-1). B3LYP/6-31+G(d) QM/MM single-point calculations give a qualitatively a similar energy profile, although with somewhat higher energy barriers, in good agreement with the value derived from experiment of 25 kcal mol(-1) at 60 °C. QM/MM molecular dynamics simulations of the reactant, activated reactant and intermediate states highlight the importance of the Arg181-Val182-Asp183 segment in catalysing the reaction. Overall, the results indicate that nucleophilic activation of the asparagine for its cyclization by the upstream histidine acting as the base is a plausible mechanism for the C-terminal cleavage in protein splicing.