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Cited by 8 publications
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The vancomycin–ristocetin family of glycopeptides is a subclass of linear sugar‐containing peptides composed of seven amino acids cross‐linked to generate a specific stereochemical configuration. This configuration forms the basis of a particular mechanism of action, eg, complexation with the D ‐alanyl‐ D ‐alanine terminus of bacterial cell wall components. The term dalbaheptide, from D ‐al (anyl‐ D ‐alanine)b(inding)a(ntibiotics) having hept(apept)ide structure, has been proposed to distinguish them within the larger and diverse groups of glycopeptide antibiotics. About 40 different naturally produced dalbaheptides have been reported. Among them, vancomycin and teicoplanin are used clinically as a result of high activity against gram‐positive pathogens such as many coagulase‐negative Staphylococci (CNS), corynebacteria, Clostridium difficile , multiresistant Staphylococcus aureus , and highly gentamicin‐resistant Enterococci which are refractory to established drugs. Eremomycin is under clinical evaluation. Many patents claim feed‐utilization efficiency increase and growth promotion in domestic animals for several dalbaheptides but only avoparcin (Avotan) is commercially used. Knowledge of the mechanism of action and investigations on the physicochemical characteristics has stimulated the transformation of natural antibiotics into new dalbaheptide derivatives using both chemical and biosynthetic modification. All the dalbaheptides are produced by strains belonging to the order of Actinomycetales . Dalbaheptides are produced by submerged fermentation. Pure dalbaheptides are obtained as colorless or whitish amorphous powders that usually retain water and solvents. Dalbaheptides are generally water‐soluble. Teicoplanin can be obtained as an internal salt or as a partial monoalkaline (sodium) salt. Other dalbaheptides are obtained as acidic salts. Dalbaheptides are levorotatory. The basis of the antibacterial action of dalbaheptides is the ability to form a complex with the terminal D ‐Ala‐ D ‐Ala residues of growing peptidoglycan chains, thus preventing transglycosylation, eg, chain elongation, and transpeptidation, eg, cross‐linking. The consequent defective cell wall stops bacterial growth and eventually leads to cell death. Only three dalbaheptides are commercialized: vancomycin and teicoplanin for human health, and avoparcin for animal usage. Vancomycin and teicoplanin as formulated drugs are lyophilized powders to be reconstituted with sterile water for injection.
The vancomycin–ristocetin family of glycopeptides is a subclass of linear sugar‐containing peptides composed of seven amino acids cross‐linked to generate a specific stereochemical configuration. This configuration forms the basis of a particular mechanism of action, eg, complexation with the D ‐alanyl‐ D ‐alanine terminus of bacterial cell wall components. The term dalbaheptide, from D ‐al (anyl‐ D ‐alanine)b(inding)a(ntibiotics) having hept(apept)ide structure, has been proposed to distinguish them within the larger and diverse groups of glycopeptide antibiotics. About 40 different naturally produced dalbaheptides have been reported. Among them, vancomycin and teicoplanin are used clinically as a result of high activity against gram‐positive pathogens such as many coagulase‐negative Staphylococci (CNS), corynebacteria, Clostridium difficile , multiresistant Staphylococcus aureus , and highly gentamicin‐resistant Enterococci which are refractory to established drugs. Eremomycin is under clinical evaluation. Many patents claim feed‐utilization efficiency increase and growth promotion in domestic animals for several dalbaheptides but only avoparcin (Avotan) is commercially used. Knowledge of the mechanism of action and investigations on the physicochemical characteristics has stimulated the transformation of natural antibiotics into new dalbaheptide derivatives using both chemical and biosynthetic modification. All the dalbaheptides are produced by strains belonging to the order of Actinomycetales . Dalbaheptides are produced by submerged fermentation. Pure dalbaheptides are obtained as colorless or whitish amorphous powders that usually retain water and solvents. Dalbaheptides are generally water‐soluble. Teicoplanin can be obtained as an internal salt or as a partial monoalkaline (sodium) salt. Other dalbaheptides are obtained as acidic salts. Dalbaheptides are levorotatory. The basis of the antibacterial action of dalbaheptides is the ability to form a complex with the terminal D ‐Ala‐ D ‐Ala residues of growing peptidoglycan chains, thus preventing transglycosylation, eg, chain elongation, and transpeptidation, eg, cross‐linking. The consequent defective cell wall stops bacterial growth and eventually leads to cell death. Only three dalbaheptides are commercialized: vancomycin and teicoplanin for human health, and avoparcin for animal usage. Vancomycin and teicoplanin as formulated drugs are lyophilized powders to be reconstituted with sterile water for injection.
91Cf + -9f c,, = -If the Langmuir isotherm expression is used to describe the equilibrium relationship, Eq. 9 is given as Nitrogen Pollutant Formation in a Pulverized Coal Combustor: Effect of Secondary Stream SwirlConcentrations of NO, NH3, and HCN, together with coal particle burnout and gas composition, were measured during combustion of a bituminous coal. Control of incoming secondary gas swirl level and overall stoichiometric ratio led to significant reductions in nitrogen oxide pollutant concentration. In-situ measurements showed that coal particles were confined near the reactor center during rapid particle reaction. This took place in a locally fuel-rich environment, producing nearquantitative conversion of fuel-nitrogen to NH3 and HCN, with some NO. Subsequent gas phase reactions of these nitrogen species were identified as important in establishing the ultimate NO concentration. SCOPEIt has been shown (e.g., Pershing et al., 1975;Rees et al., 1980) that control of nitrogen oxide can be achieved through control of the pulverized coal mixing and combustion processes. It has also been shown (Pershing and Wendt, 1979) that oxidation of the nitrogen in the coal is the major source of the oxides of nitrogen, and that NO can be controlled through staging (Chen et al., 1980; Wendt, 1980). However, work is continuing to understand the NO formation and to further reduce NO levels. The objective of this work was to investigate the extent and causes of NO formation in the presence of secondary inlet air swirl level. Experiments were conducted in a cylindrical, laboratory N. S. Harding. Jr , is currently with Conoco, Inc., Library, PA. pulverized coal combustor (13.7 kg coal/h) with samples being withdrawn 145 cm aft of the fuel-inlet port. Pulverized coal and air were injected through a cylindrical primary port while the secondary air stream was injected in a swirling fashion surrounding the primary stream. Measurements were made on gas composition (Ar, CO, COz, CHI, Hz, Nz, Oz), elemental particulate composition (ash, C, H, N), and pollutant concentration (NO, NHs, HCN). Effects of secondary stream swirl, stoichiometric ratio, primary stream velocity and solids loading level, coal particle size, and secondary stream preheat temperature were tested on coal burnout and N-pollutant level. Experiments were also conducted at three swirl levels with gas-particulate samples obtained at various axial and radial positions. Resultant data provided NO concentration maps for interpreting the test results. July, 1982Page 573 AlChE Journal (Vd. 28, No. 4) CONCLUSIONS AND SIGNIFICANCEStoichiometric ratio (SR) and secondary swirl (S) significantly influenced N-pollutant concentrations. An 88% reduction in NO was obtained while operating fuel-rich (SR = 0.66) at high secondary swirl compared to fuel-lean operation (SR = 1.32) without a swirling secondary stream. A reduction in NO by a factor of nearly 4 was obtained through use of the lower SR value, while an additional factor of 2 was obtained through use of secondary swirl (F...
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