Stereoisomer Composition Research Articles

Inconsistent Transcriptomic Responses to Hexabromocyclododecane in Japanese Quail: A Comparative Analysis of Results From Four Different Study Designs.

Efforts to use transcriptomics for toxicity testing have classically relied on the assumption that chemicals consistently produce characteristic transcriptomic signatures that are reflective of their mechanism of action. However, the degree to which transcriptomic responses are conserved across different test methodologies has seldom been explored. With increasing regulatory demand for New Approach Methods (NAMs) that use alternatives to animal models and high-content approaches such as transcriptomics, this type of comparative analysis is needed. We examined whether common genes are dysregulated in Japanese quail (Coturnix japonica) liver following sublethal exposure to the flame retardant hexabromocyclododecane (HBCD), when life stage and test methodologies differ. The four exposure scenarios included one NAM: Study 1-early-life stage (ELS) exposure via a single egg injection, and three more traditional approaches; Study 2-adult exposure using a single oral gavage; Study 3-ELS exposure via maternal deposition after adults were exposed through their diet for 7 weeks; and Study 4-ELS exposure via maternal deposition and re-exposure of nestlings through their diet for 17 weeks. The total number of differentially expressed genes (DEGs) detected in each study was variable (Study 1, 550; Study 2, 192; Study 3, 1; Study 4, 3) with only 19 DEGs shared between Studies 1 and 2. Factors contributing to this lack of concordance are discussed and include differences in dose, but also quail strain, exposure route, sampling time, and HBCD stereoisomer composition. The results provide a detailed overview of the transcriptomic responses to HBCD at different life stages and routes of exposure in a model avian species and highlight certain challenges and limits of comparing transcriptomics across different test methodologies. Environ Toxicol Chem 2024;00:1-11. © 2024 The Author(s). Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Astaxanthin Bioactivity Is Determined by Stereoisomer Composition and Extraction Method.

Astaxanthin (ASX) is a natural product and one of the most powerful antioxidants known. It has significant effects on the metabolism of many animals, increasing fecundity, egg yolk volume, growth rates, immune responses, and disease resistance. A large part of the bioactivity of ASX is due to its targeting of mitochondria, where it inserts itself into cell membranes. Here, ASX stabilizes membranes and acts as a powerful antioxidant, protecting mitochondria from damage by reactive oxygen species (ROS). ROS are ubiquitous by-products of energy metabolism that must be tightly regulated by cells, lest they bind to and inactivate proteins, DNA and RNA, lipids, and signaling molecules. Most animals cannot synthesize ASX, so they need to acquire it in their diet. ASX is easily thermally denatured during extraction, and its high hydrophobicity limits its bioavailability. Our focus in this review is to contrast the bioactivity of different ASX stereoisomers and how extraction methods can denature ASX, compromising its bioavailability and bioactivity. We discuss the commercial sources of astaxanthin, structure of stereoisomers, relative bioavailability and bioactivity of ASX stereoisomers, mechanisms of ASX bioactivity, evolution of carotenoids, and why mitochondrial targeting makes ASX such an effective antioxidant.

A Highly Selective and Sensitive Chiral Derivatization Method for High- Performance Liquid Chromatographic Determination of the Stereoisomer Composition of Natural Products With Chiral Branched Alkyl Chains.

In the study of chiral biologically active compounds such as pheromones, the analysis of the stereoisomer composition is essential to gain more insight into their stereochemical diversity, which affects the pheromone communication channels and therefore the diversification of species. This mini-review summarizes the development of fluorescence derivatization reagents for high-performance liquid chromatographic (HPLC) determination of the absolute configuration and stereoisomer composition of natural products with a chiral branched alkyl chain. The diastereomeric separation of anteiso fatty acids bearing a branched methyl group up to the C-26 position was achieved by reversed-phase HPLC under very low column temperature conditions using (1S,2S)-2-(2,3-anthracenedicarboximido)cyclohexanol as a derivatization reagent, enabling fluorescent detection of these compounds at femtomole levels. This method was also applicable to chiral alcohols and amines with chiral branched methyl groups using similar reagents containing a carboxyl group. These reagents were successfully applied to determine the absolute configurations and stereoisomer composition of the chiral alkyl chain of natural compounds including some insect pheromones, miyakosyne A, and plakoside A. The combination of these reagents and two-dimensional HPLC constitutes a very powerful tool for the analysis of the stereoisomers of natural crude samples. Furthermore, the analysis of some natural bioactive substances using this method demonstrated that natural substances are not always optically pure, consisting instead of stereoisomer mixtures exhibiting stronger activity than optically pure enantiomers. These results cast doubts on the concept of biological homochirality and demonstrate that natural pheromones do not always show the highest activity among all stereoisomers.

Glycerol Phosphate Cytidylyltransferase Stereospecificity Is Key to Understanding the Distinct Stereochemical Compositions of Glycerophosphoinositol in Bacteria and Archaea.

Compatible solutes of hyperthermophiles show high efficacy in thermal protection of proteins in comparison with solutes typical of mesophiles; therefore, they are potentially useful in several biotechnological applications. Glycerophosphoinositol (GPI) is synthesized from CDP-glycerol and l-myo-inositol 1-phosphate in a few hyperthermophiles. In this study, the molecular configuration of the GPI stereoisomers accumulated by members of the Bacteria and Archaea was established. The stereospecificity of glycerol phosphate cytidylyltransferase (GCT), the enzyme catalyzing the synthesis of CDP-glycerol, is crucial to the stereochemistry of GPI. However, the stereospecific properties of GCTs have not been investigated thus far. We devised a method to characterize GCT stereospecificity which does not require sn-glycerol 1-phosphate, a commercially unavailable substrate. This led us to understand the biochemical basis for the distinct GPI stereoisomer composition observed in archaea and bacteria.

Stereoselective Metabolism of the Sterol Biosynthesis Inhibitor Fungicides Fenpropidin, Fenpropimorph, and Spiroxamine in Grapes, Sugar Beets, and Wheat

Metabolism of chiral pesticides in crops is typically studied using achiral analytical methods and, consequently, the stereoisomer composition of residues is unknown. In this study, we developed an enantioselective GC-MS/MS method to quantify residues of the fungicides fenpropidin, fenpropimorph, and spiroxamine in plant matrices. In field trials, the fungicides were applied to grapevines, sugar beets, or wheat. Fenpropidin was metabolized with no or only weak enantioselectivity. For fenpropimorph, slightly enantioselective metabolism was observed in wheat but more pronounced in sugar beets. This enantioselectivity was due to different rates of metabolism and not due to interconversion of enantiomers. The four stereoisomers of spiroxamine were also metabolized at different rates, but selectivity was only found between diastereomers and not between enantiomers. trans-Spiroxamine was preferentially degraded in grapes and cis-spiroxamine in wheat. These findings may affect the consumer dietary risk assessment because toxicological end points were determined using racemic test substances.

Transfer of α-tocopherol stereoisomers in laying hens from feed to egg yolk and tissues of hatched chicken

Alpha-tocopherol (vitamin E) derived from natural source consists solely of a single stereoisomer (i.e. RRR-α-tocopherol) while synthetic vitamin E consists of a mixture of eight stereoisomers (all-rac-α-tocopherol). These stereoisomers are not equally retained in the body and their efficiency of utilization differs amongst animal species. A study with laying hens was carried out to determine the transfer of different α-tocopherol stereoisomers from feed to egg yolk and various tissues of newly hatched chicken. The results of this study show that the α-tocopherol stereoisomer composition of egg yolk and various tissues of newly hatched chicken are closely related to dietary vitamin E sources. When all-rac-α-tocopheryl acetate is supplied to feed, 2R forms are preferentially transferred to egg yolk and tissues as reflected by an increase in the 2R/2S ratio. Discrimination of different isomers is related to dose level and target tissues. Brain turned out to be the most discriminating tissue amongst tissues tested in this study. Low dose levels of all-rac α-tocopherol result in relatively higher RRR isomer retention in tissues. The degree of stereoisomer discrimination is less pronounced in laying hens compared to pigs and ruminants. However, a tissue specific discriminatory mechanism for selecting different α-tocopherol isomers appears to exist also in layers.

Composition of α-tocopherol and fatty acids in porcine tissues after dietary supplementation with vitamin E and different fat sources

The present study evaluated the effect of increasing supplementation of all-rac-α-tocopheryl acetate and dietary fatty acid composition during a four week period after weaning on porcine tissue composition of α-tocopherol stereoisomers and fatty acids, and on hepatic expression of genes involved in transfer of α-tocopherol, and oxidation and metabolism of fatty acids. From day 28 to 56 of age, pigs were provided 5% of tallow, fish oil or sunflower oil and 85, 150, or 300mg/kg of all-rac-α-tocopheryl acetate. Samples of liver, heart, and adipose tissue were obtained from littermates at day 56. Tissue fatty acid composition was highly influenced by dietary fat sources. Dietary fatty acid composition (P<0.001) and vitamin E supplementation (P<0.001) influenced the α-tocopherol stereoisomer composition in liver, i.e. less proportion of the RRR-α-tocopherol was observed in pigs provided fish oil and the highest dose of vitamin E in comparison with other dietary treatments. In addition, the stereoisomer composition of α-tocopherol in heart, and adipose tissue was influenced by dietary treatments. Expression of genes in liver involved in the regulation of FA conversion, SCD (P=0.002) and D6D (P=0.04) were lower in pigs fed fish oil compared to other treatments, whereas the fatty acid oxidation, as indicated by the expression of PPAR-α, was higher when sunflower and fish oil was provided (P=0.03). Expression of α-TTP in liver was higher in pigs fed fish oil (P=0.01). Vitamin E supplementation did not influence significantly the hepatic gene expression.

Variations of the equilibrium make-up and spatial orientation of a molecular layer of polymethine dyes

Molecular layers of polymethine dyes on glass substrates have been investigated. As the surface concentration of molecules increases, the number of cis isomers of various structures decreases, and the relative concentration of all-trans isomers increases. The increased electron-donorability of the end groups and the lengthening of the conjugation chain increases the number of cis isomers of various structures. The equilibrium stereoisomer composition of the molecular layer is determined by the degree of asymmetry of the intramolecular electron-density distribution induced by its interaction with the substrate. A model is proposed for the reconstruction of the structure of the layer under the action of heating and photoexcitation. Reversible changes in the make-up of the layer are caused by stereoisomerization when a segment of a molecule rotates around the second bond of the conjugation chain; irreversible changes are caused by transformations in the asymmetry of the electron-density distribution in the molecule due to irreversible spatial reorientation of the components of the layer.

Inoculation of weaned pigs with E. coli reduces depots of vitamin E

This study was designed to investigate the effect of vitamin E supplementation on vitamin E depots and immune responses in weaned pigs after an E. coli inoculation. The design was a 2×2 factorial with vitamin E supplementation (150mg/kg RRR-α-tocopheryl acetate versus a control diet containing 60mg all-rac-α-tocopheryl acetate) and E. coli 0 149 inoculation (inoculation of 1×108CFU on day 2 and 3 after weaning versus inoculation of vehicle). The pigs were housed individually during the experiment which lasted for 10days from weaning at 7weeks of age. Blood was sampled on day 1 (day of weaning) and 9 of the experiment, and serum was analyzed for α-tocopherol concentration. On day 10 of the experiment, pigs were killed and samples of liver, heart, muscle, adipose tissue and intestinal epithelium were obtained, and immune cells (alveolar macrophages) were harvested, and analyzed for α-tocopherol concentration. Immune cells were furthermore analyzed for PGE2 synthesis after in vitro stimulation. The concentration of IgA and IgM was analyzed in samples obtained from the bile, and in mucosal and intestinal content from three sites of the intestine. The results showed that E. coli inoculation reduced the concentration of liver α-tocopherol with 30–37%, increased the concentration of IgA in bile, and reduced the concentration of IgM in intestinal content of pigs. The vitamin E supplementation increased the concentration of α-tocopherol in serum, organs and tissue samples except the adipose tissue. The stereoisomer composition of α-tocopherol in serum, liver and immune cells was highly influenced by the dietary provision of natural vitamin E. In conclusion, dietary natural vitamin E supplementation increased the α-tocopherol depots of the pigs, and notably the RRR-form of α-tocopherol, but had no influence on the measured immune responses. Irrespective of dietary supplementation with vitamin E, short-term inoculation of pigs with E. coli led to a decreased liver α-tocopherol status.

On the Practical Limits of Determining Isolated Product Yields and Ratios of Stereoisomers: Reflections, Analysis, and Redemption

This paper examines the limits of accuracy in reporting isolated product yields (i.e., recovery of total mass from chromatography or extractions) as well as ratios of isomers determined by HPLC, GC, or NMR methods. Attention is directed to the magnitude of errors encountered in the HPLC or GC measurements of such ratios when these measurements are conducted without accurate calibrations or determinations of response factors for the particular isomers. Accurately defined mixtures of compounds (prepared by volumetric means) were examined by the above methods and the obtained measurements compared with the actual composition. The relative errors between actual content and the measured values are listed for all comparisons. In addition, accuracy in the determination of weight of a sample as a function of scale was also examined. The results are tabulated for comparison and suggestions are made to the reader as to how to avoid inaccurate reporting of experimental parameters. The authors hope that disclosure of these facts will result in new editorial policies requiring that a phrase ‘dr or er ratios reported in this paper have not been validated by calibration’ be inserted into general experimental descriptions. In addition, such editorial policy should also discourage the use of terms ‘de’ and ‘ee’, as these descriptors do not provide accurate and meaningful information about stereoisomer composition. This latter issue has already been suggested in the literature on several occasions.

Integration of metabolomics and in vitro metabolism assays for investigating the stereoselective transformation of triadimefon in rainbow trout

Triadimefon is a systemic agricultural fungicide of the triazole class whose major metabolite, triadimenol, also a commercial fungicide, provides the majority of the actual fungicidal activity, i.e., inhibition of steroid demethylation. Both chemicals are chiral: triadimefon has one chiral center with two enantiomers while its enzymatic reduction to triadimenol produces a second chiral center and two diastereomers with two enantiomers each. All six stereoisomers of the two fungicides were separated from each other using a chiral BGB-172 column on a GC-MS system so as to follow stereospecificity in metabolism by rainbow trout hepatic microsomes. In these microsomes the S-(+) enantiomer of triadimefon was transformed to triadimenol 27% faster than the R-(-) enantiomer, forming the four triadimenol stereoisomers at rates different from each other. The most fungi-toxic stereoisomer (1S,2R) was produced at the slowest rate; it was detectable after 8 h, but below the level of method quantitation. The triadimenol stereoisomer ratio pattern produced by the trout microsomes was very different from that of the commercial triadimenol standard, in which the most rat-toxic pair of enantiomers (known as "Diastereomer A") is about 85% of the total stereoisomer composition. The trout microsomes produced only about 4% of "Diastereomer A". Complementary metabolomic studies with NMR showed that exposure of the separate triadimefon enantiomers and the racemate to rainbow trout for 48 h resulted in different metabolic profiles in the trout liver extracts, i.e., different endogenous metabolite patterns that indicated differences in effects of the two enantiomers.

Do the Terms “%ee” and “% de” Make Sense as Expressions of Stereoisomer Composition or Stereoselectivity?

Do the Terms “%ee” and “% de” Make Sense as Expressions of Stereoisomer Composition or Stereoselectivity?

Intrageneric differences in the four stereoisomers of stenusine in the rove beetle genus, Stenus (Coleoptera, Staphylinidae)

Most species of the rove beetle genus Stenus employ the spreading alkaloid stenusine as an escape mechanism on water surfaces. In the case of danger, they emit stenusine from their pygidial glands, and it propels them over the water very quickly. Stenusine is a chiral molecule with four stereoisomers: (2'R,3R)-, (2'S,3R)-, (2'S,3S)-, and (2'R,3S)-stenusine. The percentile ratio of these four isomers is only known for the most common species of the genus: Stenus comma. With the intention of determining the stereoisomer ratios of five additional species from the two subgenera, Stenus and Hypostenus, we used GC/mass spectrometry measurements with a chiral phase. The results showed that the ratio differs among the genus. These findings can be a basis for chemotaxonomy. It is also possible that the biological function of stenusine, e.g., as antibiotic or fungicide, varies with changing stereoisomer composition.

Stereoisomer quantification of the [formula omitted]-blocker drugs atenolol, metoprolol, and propranolol in wastewaters by chiral high-performance liquid chromatography–tandem mass spectrometry

A chiral liquid chromatography–tandem mass spectrometry (HPLC-MS–MS) method was developed and validated for measuring individual enantiomers of three β -blocker drugs (atenolol, metoprolol, and propranolol) in wastewater treatment plant (WWTP) influents and effluents. Mean recoveries of the pharmaceuticals ranged from 67 to 106%, and the limits of detection of the analytes were 2–17 ng/L in wastewater effluents. The method was demonstrated by measuring, for the first time, the stereoisomer composition of target analytes in raw and treated wastewaters of two Canadian WWTPs. In these trials, racemic amounts of the three drugs were observed in influent of one wastewater treatment plant, but nonracemic amounts were observed in another. Effluents of the two plants contained nonracemic amounts of the drugs. These results indicate that biologically-mediated stereoselective processes that differ among WWTPs had occurred to eliminate individual enantiomers of the target analytes.

α-Tocopherol concentration and stereoisomer composition in plasma and milk from dairy cows fed natural or synthetic vitamin E around calving

The aim of this study was to compare the effects of supplementing dairy cows with 1000 IU/day of all-rac-alpha-tocopheryl acetate (SynAc), RRR-alpha-tocopheryl acetate (NatAc), or RRR-alpha-tocopherol (NatAlc), from approximately 3 weeks before estimated calving until 2 weeks after calving, on the concentration of alpha-tocopherol and its stereoisomers (RRR-, RSS-, RRS-, RSR- and the four 2S-forms of alpha-tocopherol) in blood and milk. An unsupplemented group was included as control. Blood samples were collected at 3, 2 and 1 weeks before estimated calving, at calving, and 3, 7 and 14 days after calving, while milk samples were taken twice within 24 h after calving and at 7 and 14 days in milk. Overall, time and treatment had significant effects on plasma alpha-tocopherol with higher concentrations in NatAc than in the other groups. In addition, SynAc had higher concentrations than Control, and NatAlc tended to be higher than Control. The lowest plasma concentrations were observed at calving and 3 days after calving. Independent of treatment, the concentration was higher in colostrum than in milk day 7 and 14 after calving. Analyses of the stereoisomer distribution in plasma and milk showed that, irrespective of dietary treatment, RRR-alpha-tocopherol was the most predominant form, constituting more than 86%, whereas the remaining part of alpha-tocopherol was made up by the three synthetic 2R isomers, while the 2S isomers only contributed less than 1% of the total alpha-tocopherol. In control cows and cows supplemented with natural vitamin E, the proportion of RRR-alpha-tocopherol in plasma and milk constituted more than 98% of the total alpha-tocopherol. In conclusion, the results indicate that daily oral supplementation of dairy cows with RRR-alpha-tocopheryl acetate gives the highest blood concentrations of alpha-tocopherol in the periparturient period. Analyses of the distribution of the individual stereoisomers of alpha-tocopherol further indicate that the bioavailability of RRR-alpha-tocopherol relative to synthetic stereoisomers in cattle is considerably higher than officially accepted until now.

Do the Terms “% ee” and “% de” Make Sense as Expressions of Stereoisomer Composition or Stereoselectivity?

Enantiomeric excess (ee) was originally defined as a term to describe enantiomeric composition and was equated with optical purity. More recently, ee and its cousin de (diastereomeric excess) have been used (inappropriately) to quantitate stereoselectivity. The quantity ee has been used in equations describing processes such as kinetic resolutions, but these equations are unnecessarily complex because it is enantiomer ratio, not enantiomeric excess, that directly reflects relative rates. A historical summary of the development of ee as an expression of enantiomer composition and enantioselectivity is presented, along with new equations and figures defining and illustrating the stereoselectivity factor, s, kinetic resolutions versus % conversion, and linear correlations of enantiomer composition of catalysts and products. New figures illustrating nonlinear effects versus enaniomer composition are presented, and Kagan's index of amplification for positive nonlinear effects is discussed and illustrated. A case is made for the discontinuance of ee and de as descriptors of stereoisomer composition and stereoselectivity.

Influence of supplementation of all-rac-α-tocopheryl acetate preweaning and vitamin C postweaning on α-tocopherol and immune responses of piglets1

This study was designed to test whether dietary maternal supplementation of all-rac-alpha-tocopheryl acetate during lactation and dietary vitamin C supplementation after weaning could increase the alpha-tocopherol status pre- and postweaning and the immune responses of piglets postweaning. The experiment involved 12 crossbred sows that were fed increasing levels of all-rac-alpha-tocopheryl (70, 150, and 250 IU/kg, as-fed basis) during lactation. After weaning (d 28 of age), litters were divided into two groups that were supplemented with or without vitamin C (500 mg/kg of feed, as-fed basis). Milk and blood samples were obtained from the sows during lactation. Pigs were bled at 4, 16, 28, 35, 42, and 49 d of age. Liver, heart, muscle, and s.c. adipose tissues were collected (on 28, 35, 42, and 49 d of age) and analyzed for alpha-tocopherol. On the same days, alveolar macrophages of the lungs were collected, and analyzed for the concentration of alpha-tocopherol and its stereoisomer composition, fatty acid composition, and release of prostaglandin E2, leukotriene B4, and thromboxane B2. Increasing dietary all-rac-alpha-tocopheryl acetate concentration increased the concentration of alpha-tocopherol in plasma (P = 0.02) and milk (P = 0.007) of sows, and the sow milk concentrations of alpha-tocopherol and vitamin A were greater on d 2 of lactation than later on during lactation. The plasma concentration of alpha-tocopherol in piglets decreased from d 4 to later on during suckling (P < 0.001) and again as the postweaning period progressed (P < 0.001). When lipid-standardized, plasma alpha-tocopherol was increased in piglets of sows fed 250 IU of all-rac-alpha-tocopheryl acetate compared with other sow-groups (P = 0.005). At 28 d of age, alpha-tocopherol concentrations in tissues were increased with supplementation of the high dietary all-rac-alpha-tocopheryl acetate levels to the sows; however, after weaning, a decrease in alpha-tocopherol concentration in most tissues (except liver) was observed, but the decrease tended to be less in the muscle (P = 0.099) and adipose tissue (P = 0.11) of piglets suckling sows fed 150 and 250 IU of all-rac-alpha-tocopheryl acetate. Vitamin C supplementation after weaning increased liver alpha-tocopherol (P = 0.01) and serum immunoglobulin M concentration (P = 0.04), and vitamin C supplementation increased the proportion of the RRR-alpha-tocopherol (P = 0.03) at the expense of the RRS-stereoisomer form (P = 0.05) of alpha-tocopherol in alveolar macrophages of the piglets. In conclusion, this study on maternal all-rac-alpha-tocopheryl acetate and postweaning vitamin C supplementation suggests a nutritional strategy for increasing alpha-tocopherol status and immune responses of weaned piglets.

Stereoisomer Composition of the Chiral UV Filter 4-Methylbenzylidene Camphor in Environmental Samples

4-Methylbenzylidene camphor (4-MBC) is an important organic UV filter used in many personal care products such as sunscreens and cosmetics. After use, 4-MBC may enter the aquatic environment due to its release from skin during recreational activities (swimming, bathing) and from personal hygiene measures (washing, laundering of cloths) via wastewater treatment plants (WWTPs). In fact, 4-MBC has been detected in wastewater, in surface waters, and even in fish. 4-MBC can exist as distinct cis-(Z)- and trans-(E)-isomers, both of which are chiral. Despite the fact that stereoisomers often show a different biological behavior, the stereochemistry of 4-MBC has hardly ever been considered in environmental or biological studies. In this study, enantioselective gas chromatography-mass spectrometry (GC-MS) was used to determine the stereoisomer composition of 4-MBC. For stereoisomer assignment, the pure enantiomers of (E)-4-MBC were synthesized from (+)- and (-)-camphor. The photochemical isomerization (sunlight) of these (E)-isomers to the corresponding (Z)-isomers eventually allowed the configurational assignment of all four stereoisomers of 4-MBC. In a technical material and in a major brand sun lotion, 4-MBC was shown to consist entirely (>99%) of (E)-isomers and to be racemic (R/S, 1.00 +/- 0.02). Wastewater showed the presence of both (E)- and (Z)-4-MBC with a clear excess of (E)-isomers (E > Z). Untreated wastewater showed a nearly racemic composition (R/S= 0.95-1.09), suggesting that most if not all commercial 4-MBC is racemic. Treated wastewater indicated some excess of (R)- or (S)-stereoisomers (R/S, 0.89-1.17), likely as a result of some enantioselective (bio)degradation in WWTPs. Residues of 4-MBC in lakes and in a river with inputs from WWTPs and/or recreational activities consisted mainly of (E)-4-MBC and, with exception of one lake (Greifensee), showed a small enantiomer excess (R/S, 1.04-1.16). In Greifensee, 4-MBC showed a higher enantiomer excess (R/S, 1.70-1.83), probably as a result of more extensive biotic degradation in this lake. The analysis of 4-MBC in a small number of fish from these lakes indicated residues with nearly racemic compositions or a moderate enantiomer excess (R/S, approximately 1.0-1.2) in roach (Rutilus rutilus), whereas in perch (Perca fluviatilis) a much higher enantiomer excess (R/S, approximately 5) was observed. The data indicate that the stereoisomer composition of 4-MBC in environmental samples is not only a function of initial product composition but is also modified by enantioselective processes in lakes and biota (fish).

X-Ray Analysis of Methyl Cis-1-chloro-2-trichloromethylcyclopentane-1-carboxylate

Recently we have found that the products of 4,3-addition of trichloroacetic acid derivatives to 1,1-dichloro-4-methyl1,3-pentadiene in the presence of complex catalysts based on univalent copper undergo intramolecular cyclization, forming cyclobutanes with functional substituents [1, 2]. It was assumed that the stereoisomer composition of the isolated cyclobutanes depends on the ability of the functional group of the starting acid to be coordinated to the transition metal ion [3]. Further evidence in support of the observed tendencies was provided by cyclization of methyl 2,2,7,7-tetrachloro6-heptenate (prepared from methyl dichloroacetate and 1,1-dichloro-5-bromo-1-pentene by the procedure of [4], yield 72%) to 1-chloro-trichloromethylcyclopentane-1-carboxylic acid ester in the presence of amine complexes of univalent copper.

1H NMR Spectroscopy in the Analysis and Characterization of Poly(lactide)

High resolution 500 MHz 1H NMR spectroscopy is a powerful tool for the analysis and characterization of poly(lactide) (PLA). It accurately provides information about the distribution of a few stereosequences in the polymer whose resonances are well-resolved in the NMR spectrum. Here, the splitting of the methine resonance due to coupling to the methyl protons in the polymer is removed by homonuclear decoupling. 13C NMR provides complementary stereosequence information, but due to the poor signal-to-noise ratio its accuracy is not comparable to that of 1H NMR. Through the analysis of the stereosequence distribution in a number of PLA spectra, it was determined that there is a preference for syndiotactic addition during lactide stereo copolymerization. It was shown that the normalized intensity of a few well-resolved resonances in the 1H spectrum of PLA can be used to quantitatively determine the lactide stereoisomer composition incorporated in the polymer. The change in stereosequence distribution with polymerization can also be conveniently monitored by 1H NMR. By following the reversible polymerization of D,L-lactide (racemic lactide) in this manner, it was found that the stereo-specificity (or reactivity ratio) for syndiotactic addition reduced with increasing polymerization. This increasingly random polymerization is due to the interplay of kinetic and thermodynamic effects. Kinetic effects control the stereochemistry during the early stages of polymerization while equilibrium effects dominate at later stages. The viscosity changes during the melt polymerization additionally influence the stereochemistry.