Isomer specificity

Enzymatic Method. L-Amino acids can be produced by the enzymatic hydrolysis of chemically synthesized DL-amino acids or derivatives such as esters, hydantoins, carbamates, amides, and acylates (24). The enzyme which hydrolyzes the L-isomer specifically has been found in microbial sources. The resulting L-amino acid is isolated through routine chemical or physical processes. The D-isomer which remains unchanged is racemized chemically or enzymatically and the process is recycled. Conversely, enzymes which act specifically on D-isomers have been found. Thus various D-amino acids have been... 

For details of the clean-up of the pyrolysate of the DIN oven see reference 12. Identification and quantification of PBDD/F was performed by GC/MS (refs. 8-12). This was done for all brominated PBDD and PBDF from mono- through octabromo compounds using external standards which were either prepared (refs. 11-13) or purchased. There exists a total of 210 brominated compounds of PBDD/F. Since not all isomers are available a complete isomer-specific determination could not be performed. 

Because of its high toxicity, there is concern about very low levels of 2,3,7,8-TCDD in biota. This raises analytical problems, and high resolution capillary gas chromatography (GC) is needed to obtain reliable isomer-specific analyses at low concentrations. In the analysis of herring gull eggs collected from the Great Lakes, Hebert et al. (1994)... 

Kannan, K., Tanabe, S., and Borrell, A. et al. (1993). Isomer specific analysis and toxic evaluation of PCBs in striped dolphins affected by an epizootic in the western Mediterranean sea. Archives Environmental Contamination and Toxicology 25, 227-233. 

Ofiwald, P. et al.. Isomer-specific fuel destruction pathways in rich flames of methyl acetate and ethyl formate and consequences for the combustion chemistry of esters, /. Phys. Chem. A, 111, 4093,2007. 

Murohoshi, T. Kaneda, K. Ikegami,M. Arai, T. Photoisomerization and isomer-specific addition of water in hydroxystilhenes. Photochem. Photobiol. Sci. 2003, 2, 1247-1249. 

Lewis, F. D. Crompton, E. M. Hydroxystilbene isomer-specific photoisomerization versus proton transfer. J. Am. Chem. Soc. 2003, 125, 4044-4045. 

LC-MS or LC-MS-MS using ESI or APCI are versatile tools for the isomer-specific determination of trace levels of HBCDs, monitoring the specific transitions mlz 640.6 to mlz 78.9 and 80.9. Budakowsky and Tomy [117] showed that APCI has lower intensities than in a similar experiment with ESI. Consequently, the ESI mode was preferred for determining diastereoisomers in several studies. Different methods for the analysis of diastereoisomeric HBCD using LC-ESI-MS-MS and SRM were developed, obtaining LODs of 0.5-6 pg on-column [117, 118]. 

All amino acids except glycine exist in these two different isomeric forms but only the L isomers of the a-amino acids are found in proteins, although many D amino acids do occur naturally, for example in certain bacterial cell walls and polypeptide antibiotics. It is difficult to differentiate between the D and the L isomers by chemical methods and when it is necessary to resolve a racemic mixture, an isomer-specific enzyme provides a convenient way to degrade the unwanted isomer, leaving the other isomer intact. Similarly in a particular sample, one isomer may be determined in the presence of the other using an enzyme with a specificity for the isomer under investigation. The other isomer present will not act as a substrate for the enzyme and no enzymic activity will be demonstrated. The enzyme L-amino acid oxidase (EC 1.4.3.2), for example, is an enzyme that shows activity only with L amino acids and will not react with the D amino acids. 

Application of Soft Independent Method of Class Analogy (SIMCA) in Isomer Specific Analysis of Polychlorinated Biphenyls... 

A principal components multivariate statistical approach (SIMCA) was evaluated and applied to interpretation of isomer specific analysis of polychlorinated biphenyls (PCBs) using both a microcomputer and a main frame computer. Capillary column gas chromatography was employed for separation and detection of 69 individual PCB isomers. Computer programs were written in AMSII MUMPS to provide a laboratory data base for data manipulation. This data base greatly assisted the analysts in calculating isomer concentrations and data management. Applications of SIMCA for quality control, classification, and estimation of the composition of multi-Aroclor mixtures are described for characterization and study of complex environmental residues. 

STALLING ET AL. Isomer Specific Analysis of PCBs Table I. Continued Chlorines Peak RRT Total Ortho Structure 203 Cone. uk/k... 

To illustrate the environmental application of the SIMCA method we examined a set of isomer specific analyses of sediment samples. The data examined were derived from more than 200 sediment samples taken from a study site on the Upper Mississippi River (41). These analytical data were transferred via magnetic tape from the laboratory data base to the Cyber 175 computer where principal component analysis were conducted on the isomer concentration data (ug/g each isomer). 

Analytical model, assumptions and practical implications, 52-57 Analytical performance, correlation chromatography, 108 Analytical process, steps of, 7 Aroclors, isomer-specific analysis of, application of SIMCA, 195-232 Atomic absorption spectrometry, determination of iron in water, 116... 

Pattern recognition studies on complex data from capillary gas chromatographic analyses were conducted with a series of microcomputer programs based on principal components (SIMCA-3B). Principal components sample score plots provide a means to assess sample similarity. The behavior of analytes in samples can be evaluated from variable loading plots derived from principal components calculations. A complex data set was derived from isomer specific polychlorinated biphenyl (PCBS) analyses of samples from laboratory and field studies. 

More extensive use of isomer specific analysis, when combined with chemometric techniques, should improve insight into how residues in the environment relate to their sources. This approach could lead to a quantitative description of changes in the composition of these chemicals as they pass through the food chain and are distributed in the environment. 

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