Microcins, gene-encoded antibacterial peptides from enterobacteria

Microcins are gene-encoded antibacterial peptides, with molecular masses below 10 kDa, produced by enterobacteria. They are secreted under conditions of nutrient depletion and exert potent antibacterial activity against closely related species. Typical gene clusters encoding the microcin precursor, the self-immunity factor, the secretion proteins and frequently the post-translational modification enzymes are located either on plasmids or on the chromosome. In contrast to most of the antibiotics of microbial origin, which are non-ribosomally synthesized by multimodular enzymes termed peptide synthetases, microcins are ribosomally synthesized as precursors, which are further modified enzymatically. They form a restricted class of potent antibacterial peptides. Fourteen microcins have been reported so far, among which only seven have been isolated and characterized. Despite the low number of known representatives, microcins exhibit a diversity of structures and antibacterial mechanisms. This review provides an updated overview of microcin structures, antibacterial activities, genetic systems and biosyntheses, as well as of their mechanisms of action.     

Method development for a quantitative analysis performed without any standard using an evaporative light-scattering detector

A new method for quantitative analyses using an evaporative light-scattering detector (ELSD) is proposed. It is based on the preliminary determination of the calibration curve of an ELSD which correlates coefficient b and loga, the two coefficients from the equation: A=am(b), that characterize the law of the quantitative response for an ELSD. Dilutions of the mixture to be analyzed allow the determination of coefficient b for each component of the mixture. The knowledge of the b value and the experimental relationship correlating b with loga allows to determine the loga value and consequently to quantify each compound of the mixture. This method is an alternative to the quantitative method which uses an internal normalization without any response coefficient. This internal normalization method used with an ELSD provides inaccurate results and this inaccuracy increases when the analytes are in very different proportions. The relevance of the new method proposed in this work lies in the quantification of all the components present in a complex mixture when some of them are not available as standards.     

Synthesis of microsequential methylvinylsiloxane–dimethylsiloxane copolymers by nonequilibrium copolymerization

Nonequilibrium anionic ring-opening copolymerization of 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclotetrasiloxane (V₄) with hexamethylcyclotrisiloxane (D₃) was examined as a route to methylvinyl–dimethylsiloxane microsequential copolymers. The copolymerization was carried out in toluene and was initiated with Me₃SiCH₂Li using DMSO as the promoter. Distribution of siloxane units obtained from kinetic analysis based on first-order Markovian statistics was compared with that found from analysis by ²⁹Si NMR spectroscopy. Results showed that although chain transfer and back-biting play some role in this system, the kinetics of copolymerization may be described to a reasonable approximation by the first-order Markov model. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 137–145, 1998     

Molecular electrotatic potential of a triple stranded helix: Poly(dT)·poly(dA)·poly(dT)

The molecular electrostatic potential of the triple helix poly(dT)·tpoly(dA)·poly(dT) is calculated, and the results are examined in relation to those obtained for its component double and single helical parts. For the double helix presenting the standard Watson–Crick hydrogen bonds, the deepest potentials are formed on the side of the major groove, a situation similar to that observed in the A-DNA duplex. For the double helix presenting Hoogsteen-type hydrogen bonds the deepest potentials lie in the major groove, on the side of the pyrimidine strand. In the triple helix the deepest potentials are located in the major groove in a narrow zone over the thymine bases of the Watson–Crick pair.     

Copolymers of Intrinsic Microporosity Based on 2,2′,3,3′‐Tetrahydroxy‐1,1′‐dinaphthyl

A series of new copolymers with high molecular weight and low polydispersity, prepared from tetrahydroxydinaphthyl, tetrahydroxyspirobisindane, and tetrafluoroterephthalonitrile monomers, prevent efficient space packing of the stiff polymer chains and consequently show intrinsic microporosity. One copolymer, DNPIM‐33, has an excellent combination of properties with good film‐forming characteristics and gas transport performance, and exhibits higher selectivity than the corresponding spirobisindane‐based homopolymer PIM‐1 for gas pairs, such as O₂/N₂, with a corresponding small decrease in permeability. This work demonstrates that significant improvements in properties may be obtained through development of copolymers with intrinsic microporosity (CoPIMs) that extends the spectrum of high‐molecular‐weight ladder structures of poly(dibenzodioxane)s.

     

Age-dependent DNA repair and cell cycle distribution of human skin fibroblasts in response to UVA irradiation

Ageing process in cells is associated with oxidative stress. Ultraviolet A produces reactive oxygen species responsible for accumulation of DNA and cellular damage. After the evaluation of antioxidant enzyme activities and oxidative stress markers at the basal state, we have studied the responses to UVA stress of coetaneous fibroblasts, isolated from different male donors (2-88 years, n=23) in terms of cytotoxicity, genotoxicity and DNA repair capacities. For this purpose, we have determined level of DNA damage using the comet assay (single strand breaks and alkali-labile sites) and the cell cycle distribution after a 5 J/cm2 irradiation. No differences with age were observed for antioxidant enzyme activities and oxidative stress markers. DNA strand breaks after UVA irradiation (5-20 J/cm2), was found to be age-dependent. DNA repair was slow and also significantly affected by ageing. The cell cycle distribution analysis showed that high repair correlated with high proliferative capacities at basal level. Twenty-four hours after the stress, fraction of young fibroblasts blocked in G1 phase was significantly increased whereas significant modifications concerned the G2-M phase for adult and older fibroblasts. These results indicate an age-dependent decline in the DNA repair capacities correlated with modifications of the cell cycle parameters.     

Studying the signaling role of 2-oxoglutaric acid using analogs that mimic the ketone and ketal forms of 2-oxoglutaric acid

2-Oxoglutaric acid (2-OG), a Krebs cycle intermediate, is a signaling molecule in many organisms. To determine which form of 2-OG, the ketone or the ketal form, is responsible for its signaling function, we have synthesized and characterized various 2-OG analogs. Only 2-methylenepentanedioic acid (2-MPA), which resembles closely the ketone form of 2-OG, is able to elicit cell responses in the cyanobacterium Anabaena by inducing nitrogen-fixing cells called heterocysts. None of the analogs mimicking the ketal form of 2-OG are able to induce heterocysts because none of them are able to interact with NtcA, a 2-OG sensor. NtcA interacts with 2-MPA and 2-OG in a similar manner, and it is necessary for heterocyst differentiation induced by 2-MPA. Therefore, it is primarily the ketone form that is responsible for the signaling role of 2-OG in Anabaena.     

Precision study on capillary electrophoresis methods for metacycline

A CE method for metacycline (MTC) determination was investigated in an inter-laboratory experiment. Many problems were encountered in this study, most of which were related to the transfer of the method to different CE equipment. The reported problems could be classified into different categories: problems related to the precision, to the parameters in the protocol, and to the MTC peak shape. As the peak shape problem was partially responsible for the poor precision, a new CE method was developed in order to obtain a good MTC peak shape on all equipment. The precision of this new method for MTC determination was examined in an intermediate precision study, where the influence of the factors "time" and "equipment" was investigated. Although the new method could be transferred to different instruments, the precision remained poor mainly due to the contributions of the between-replicate and the between-injection variances.