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R N Loeppky

Publications and source records attributed to R N Loeppky.

27 records · Page 2Linked to original sources
PubMed1975

Utilization of an Escherichia coli mutant for carbon-13 enrichment of tRNA for NMR studies.

The enrichment of tRNA at specific sites with carbon-13 has been accomplished in vivo using a mutant of Escherichia coli. A relaxed strain of E. coli auxotrophic for methionine was grown in a specifically defined medium supplemented with either [14C] or [13C]-methyl labeled methionine. Cells were collected at the end of the log-phase of growth and tRNA was extracted. Analysis of the radioactivity of the [14C]-labeled tRNA established an incorporation ratio of three labeled carbons per tRNA molecule. Incorporation of the [14C]-label in vivo was confined to the methylation of nucleotides as determined by thin layer chromatography of nucleotides resulting from a ribonuclease digestion of [14C]-labeled tRNA. The carbon-13 NMR spectrum of [13C]-enriched tRNA indicated a similar degree of incorporation into the methylated nucleotides by the substantial enhancement of [13C]-methyl NMR signals only. Assignment of signals has been made for the methyl groups of ribothymidine and N7-methylguanosine in E. coli tRNA.

Carbon Isotopes↗
PubMedDate not supplied

Pyrroles as effective agents for blocking amine nitrosation.

The ability of 10 pyrroles to block the acidic nitrosation of morpholine has been determined by using an assay that measures their effectiveness in the presence of a 10-fold excess of amine. The log (% N-nitrosomorpholine) formed is a linear function of blocking agent concentrations ranging from 0.0 to 1.5 times the equivalents of initial nitrite. The negative slopes of these plots allow a ranking of the effectiveness of the blocking agent. Several of the pyrroles have been found to be much more effective than established blocking agents such as ascorbic acid. The following order of blocking ability has been determined: 2,5-dimethylpyrrole = 1-benzyl-2,5-dimethylpyrrole much greater than 4-methylcatechol greater than ascorbic acid = 1,2-phenylenediamine = pyrrole greater than 1,2,5-tribenzylpyrrole = 1-benzylpyrrole = octamethylporphine much greater than ammonium sulfamate = hydrazine = 2,5-diphenylpyrrole much much greater than beta-nicotyrine greater than 2-pyrrolecarboxylic acid. Pyrroles give complex mixtures devoid of N-nitroso compounds upon nitrosation.

Amines↗
PubMedDate not supplied

Blocking nitrosamine formation with polymers.

A set of polymers have been synthesized in order to determine whether polymers containing nitrosation-reactive functional groups could effectively block nitrosamine formation. Polymers containing both hydrophobic backbones [poly[(chloromethyl)styrene], poly(ethylene oxide-co-epichlorohydrin), chloromethylated cross-linked polystyrene] and hydrophilic backbones [poly(ethylenimine) and poly(acryloyl chloride)] have been derivatized with pyrrole, 2,5-dimethylpyrrole, 2-[(methylamino)methyl]pyrrole, or hydroquinone. All of these polymers, as well as PEI itself, are effective at blocking the nitrosation of morpholine by nitrous acid. The hydrophilic polymers are much better able to compete for the available nitrosating agent in aqueous solution than are the hydrophobic polymers. Nevertheless, all of the polymers were highly effective in scavenging nitrous acid from aqueous solution prior to addition of the amine. The reaction products derived from the reactive, polymer-attached functional groups are retained on the polymer and are easily removed from mixtures of chemicals by physical means. Since nitrosamines and other N-nitroso compounds are known to be potent animal carcinogens, this work is of significance in the area of preventive chemical toxicology.

Binding, Competitive↗
PubMedDate not supplied

An aziridinium ion intermediate in the nitrosation of a hexetidine model.

The nitrosation chemistry of 1,3,5-trimethyl-5-aminohexahydropyrimidine (2) has been investigated as a model for the behavior of the antimicrobial agent hexetidine (1) under similar conditions. The reaction of 2 with sodium nitrite in glacial acetic acid gives 4-methyl-4-[(methylnitrosamino)methyl]-3-nitroso-1,3-oxazolidine (4) as the major nitrosamine. This compound arises from a molecular rearrangement which proceeds through the diazotization of the primary amino group followed by intramolecular displacement of nitrogen to generate an aziridinium ion. The N-nitrosooxazolidine 4 forms from the nitrosation of an imidazolidine produced from the aziridinium ring hydrolytic opening. The N-nitrosooxazolidine 4, an isomer, 5-methyl-5-[(methylnitrosamino)methyl]-3-nitroso-1,3-oxazolidine (14), which is not formed in the nitrosation of 2, and an analog 4-methyl-4-[[(2-ethylhexyl)nitrosamino]methyl]-3-nitroso-1,3-oxazolidine (22) have been independently synthesized. The N-nitrosooxazolidine 22 which would be formed from hexetidine is not present in its nitrosation mixture, suggesting the absence of reactive aziridinium ions in that case. The dissimilar nitrosation chemistry of 2 and 1 are discussed.

Aziridines↗
PubMedDate not supplied

The nitrosation of hexetidine and hexedine: characterization of the major nitrosamine from common antimicrobial agents.

The acidic nitrosation of hexetidine and hexedine, common antimicrobial agents and drug constituents, leads to a mixture of nitrosamines. The major nitrosamine product, "HEXNO", forms rapidly in yields as high as 60% over the pH range 1-4.8 at incubation times of 1 h at 37 degrees C with 40 mM NO2- and 10 mM hexetidine. On the basis of extensive spectroscopic characterization and independent synthesis HEXNO has been assigned the structure of 1-(2-ethylhexyl)-3-nitroso-4-methyl-4-[[N-(2-ethylhexyl)-N- nitrosoamino]methyl]imidazolidine (7). The synthesis of HEXNO involves the novel interception by potassium nitrite in ether/18-crown-6 of an imminium ion produced from the reaction of hexedine with benzyl chloroformate. Collapse of the alpha-amino nitrous ester produced by this reaction yields the nitrosamine containing carbamate 8, which yields HEXNO after removal of the carbamate with trimethylsilyl iodide and subsequent nitrosation. The rapid formation of HEXNO from hexetidine and hexedine supports the hypothesis that tertiary geminal diamines will produce nitrosamines rapidly by a mechanism which involves the cleavage of a nitrosammonium ion with the assistance of the neighboring nitrogen atom. This process is deemed to be of possible importance in the endogenous production of potentially carcinogenic nitrosamines because of its low nitrite requirement and high nitrosation rate. The available data suggest the probable formation of HEXNO and other nitrosamines from hexetidine under conditions of its use.

Chromatography, High Pressure Liquid↗