R/S ratios

On the basis of automated peak picking and integration procedure, after less than 2 h, the enantiomeric purity was established. R S ratio was 34.4 65.6, which is close to the values obtained from the 1H or 13C NMR spectra. 

Possible racemisation of imines, derivatives of amino acids and R(—)-myrtenal, has been examined by Dufrasne et al.1 After 72 h, no significant effect on chiral purity was observed. For imines being derivatives of chiral primary amines and the a-substituted 8-keto-aldehydes, no evidence of epimerisation has been indicated by the NMR measurements.3 For a series of imines, being derivatives of amino acids or amino acid esters and (R)-BINOL reagents, Chin et al.5 have tested the possibility of epimerization under experiment conditions. It was shown that R S ratio has changed only slightly, and after 24 h, the difference was lower than 10%. 

The R/S ratios for permethrin and phenothrin in Table 3 are 204 and 283, respectively. The Hiroyama strain showed high cross-resistance to permethrin and phenothrin, whereas the R/S ratio for natural pyrethrins was only 7. Moreover, the R/S ratio for racemic allethrin was 97, indicating the slow development of resistance compared with permethrin and phenothrin. These results agree well with the findings of Sawicki et al. [49], who reported that pyrethroids with the cyclopentenolone ring showed only a slight resistance at the International Congress of Pesticide Chemistry in Ottawa in 1986. 

In the series of allethrin homologs, the R/S ratios of racemic allethrin, dl,d -trans-allethrin (bioallethrin) and d,d-trans-allethrin were 97, 68, and 33, respectively. The closer the steric configuration of allethrin homologs comes to that of natural pyrethrins, the smaller the R/S ratio however, the R/S ratio of d,d-/nms-allethrin with the same steric configuration as natural pyrethrins is still greater than that of natural pyrethrins. 

R/S ratios decreased in the order of allethrin, allethrin II, and their mixture. 

It is noteworthy that the Obihiro strain of M. domestica showed markedly high cross-resistance to photostable pyrethroids such as permethrin and phenothrin having a benzyl group in the alcohol moiety, with their R/S ratios being 281 and 301, respectively. 

On the other hand, R/S ratios of natural pyrethrins as well as d,d-trans-allethrin and prallethrin (12) with the same steric configuration as natural pyrethrins were only as low as about 10, suggesting slow development of resistance. 

In 1H- and 19F-NMR spectroscopy studies of the metabolites of fluclox-acillin (5.52) in rat urine, the presence of both (5R)- and (5.S )-flucloxacillin penicilloic acids was demonstrated [154], During the first collection period, the concentration of the (R)-epimer largely exceeded that of the f.S )-epimer. The (R)/(S) ratio then decreased progressively until an excess of f.S )-epimer was reached. These findings are in agreement with the hypothesis that the f.S )-cpimer is formed by epimerization in urine (see above) [152],... 

If > U2, or the desired reaction is of higher order than the unwanted reaction, Eq. 3 shows that a high reactant concentration is desirable since it increases the R/S ratio. As a result, a batch or plug flow reactor would favor formation of product R and would require a minimum reactor size. 

Reversible sorption of phenolic acids by soils may provide some protection to phenolic acids from microbial degradation. In the absence of microbes, reversible sorption 35 days after addition of 0.5-3 mu mol/g of ferulic acid or p-coumaric acid was 8-14% in Cecil A(p) horizon and 31-38% in Cecil B-t horizon soil materials. The reversibly sorbed/solution ratios (r/s) for ferulic acid or p-coumaric acid ranged from 0.12 to 0.25 in A(p) and 0.65 to 0.85 in B-t horizon soil materials. When microbes were introduced, the r/s ratio for both the A(p) and B-t horizon soil materials increased over time up to 5 and 2, respectively, thereby indicating a more rapid utilization of solution phenolic acids over reversibly sorbed phenolic acids. The increase in r/s ratio and the overall microbial utilization of ferulic acid and/or p-coumaric acid were much more rapid in A(p) than in B-t horizon soil materials. Reversible sorption, however, provided protection of phenolic acids from microbial utilization for only very short periods of time. Differential soil fixation, microbial production of benzoic acids (e.g., vanillic acid and p-hydroxybenzoic acid) from cinnamic acids (e.g., ferulic acid and p-coumaric acid, respectively), and the subsequent differential utilization of cinnamic and benzoic acids by soil microbes indicated that these processes can substantially influence the magnitude and duration of the phytoxicity of individual phenolic acids (Blum, 1998). 

Five kinds of mixed crystals with different R S ratios were grown frodl aqueous methanol solutions in which the complexes with R-1 -cyanoethyl and S 1-cyanoethyl groups were dissolved in the desired portions. Crystals with R ratios 50 50, 57 43, 75 25, 87 13 and 100 0 were prepared these are abbreviated hereafter pip-1, pip-2, pip-3, pip-4, and pip-5, respectively. Of course, the racemid and chiral ones described above are pip-1 and pip-5, respectively. These crystal are isostructural to each other. The real R S ratio in each crystal was determin by x-ray crystal structure analysis. The observed ratios in the crystals of pi 1, pip-2, pip-3, pip-4, and pip-5 were 50 50, 62.38, 74 26, 87 13, and 100 0, respectively, which are very similar to the corresponding ones in solutions. 

The molecular structures of A and B in the pip-1 crystal are shown in Fig. 4. The A molecules in the crystals of pip-1 to pip-5 are substantially the same, while the B molecules have different structures of the 1-cyanoethyl groups. Figure 7 shows the conformations of the 1-cyanoethyl groups viewed along the normal to the cobaloxime plane in pip-1, pip-3, and pip-5. The structures of the 1 -cyanoethyl groups in pip-2 and pip-4 are very similar to that in pip-3 except for the R S ratio of the disordered structure. 

Figure 9 shows the crystal structure of pip-1 after 40 h exposure. The 1-cyanoethyl group of the B molecule changed to the disordered structure, while the A molecule remained unaltered. Almost the same structures were obtained for the crystals of pip-3 and pip-5. The R S ratios became 19 31, 19 31, and 21 29 for pip-1, pip-3, and pip-5, respectively. These values are identical to 20 30 within experimental error. It is noticeable that the converged value is not 25 25 but 20 30. 

The R S ratio in the racemic crystal of pip-1 before irradiation is 50 50, since the 1 -cyanoethyl groups of the A and B molecules have R and S configurations. After the irradiation, the A molecule has the R configuration, while the B molecule became the disordered structure with R S ratio of 20 30. This indicates that the R S ratio in a crystal changed from 50 50 to 70 30. This is why the specific rotatory power of the chloroform solution containing the irradiated pip-1 crystal showed +30°. 

The final question why the converged R S ratio of the B cyanoethyl group is not 25 25 but 20 30 must be explained. The reaction cavities for the B cyanoethyl groups in pip-1, pip-3, and pip-5 before and after the irradiation are shown in Fig. 11, in which each cavity is divided into two by the plane composed of the Co-C-C-N bonds. The volumes of the two parts were calculated. The left and right parts of the cavities before irradiation were 45 55, 56 44, and 63 37, respectively. These ratios became 48 52, 49 51, and 52 48, respectively after irradiation. These values are 50 50 within experimental error. This suggests that the inversion ratio of the B cyanoethyl group depends on the symmetry of the cavity. In other words, the inversion ratio is determined by the steric repulsion... 

For the cobaloxime complex with pyrrolidine as an axial base ligand, the crystalline state reaction was also observed when the crystal was exposed to a xenon lamp. Four kinds of crystals with different R S compositions were prepared. The crystals of pyrr-1 were obtained from the racemic solution. The pyrr-2 crystals were obtained from a solution that has the complex with an R S ratio of 75 25. The crystals of pyrr-3 and pyrr-4 were also obtained from solutions with the R S ratio of 80 20 and 90 10, respectively. The x-ray crystal analysis indicated that the four crystals are isostructural to each other. From solutions with the R S ratio greater than 9 1, pure enantiomeric crystals were obtained, which are not isostructural to the above crystals pyrr-1 to pyrr-4 [40]. 

Figure 12 shows the crystal structures of pyrr-1 and pyrr-2 viewed along the c axis before irradiation. There are four crystallographically independent molecules, A, B, C, and D, in a PI cell. In the crystal of pyrr-1, the A, B, C, and D molecules have R, R, 5, and S configuration, respectively therefore the crystal is racemic. In the crystal of pyrr-2 with the R S ratio of 75 25, the A, B, C, and D molecules have / ,/ ,/ + 5, and R + S configuration, respectively. The R S ratips of the C and D molecules are 10 15 and 16 9, respectively. As a whole, the pyrr-2 crystal has an R S ratio of 76 24, which is identical to the R S ratio of the solution, 75 25, within experimental error. The pyrr-3 and pyrr-4 crystals were also analyzed. The R S ratios of the A and B molecules are 25 0, whereas the R S ratios of the C and D molecules are 15 10 and 19 6 for pyrr-3 and 17 8 and 23 2 for pyrr-4, respectively. These values indicate that the R S ratios of the pyrr-3 and pyrr-4 crystals are 84 16 and 90 10, respectively, which are very similar to the R S ratios in the respective solutions. Since the crystals of pyrr-1 to pyrr-4 are all chiral, the chirality of the crystals was selected so that the A molecules have R-1 -cyanoethyl groups. 

The next question is why the R S ratio after irradiation became 72 28. The volumes of the reaction cavities for the A, B, C, and D molecules after irradiation... 

When the crystal was irradiated with a halogen lamp, the cell dimensions were gradually changed. They converge to the values observed in the photoirradiated crystal with the S isomer. The structure analyzed by x-rays after 24 h exposure revealed that it is essentially the same as that of the photoirradiated crystal with the S isomer. The molecular structure became the disordered one with the R- and S 1 -(ethoxycarbonyl)ethyl groups, which is the same as that shown in Fig. 27a. The R S ratio converged to 82 18, which is also the same as that observed in the photoirradiated crystal with the S isomer. 

The participation of a cysteine thiol in the abstraction of hydrogen from ribose C-3 coenzyme B12, which is not a feature of any other of the reactions of Table 1, leads to exchange of H between this thiol group, the 5 -methylene group of coenzyme Bi2 and water (22). It was also shown that ribonucleotide reductase catalyzes the conversion of adenosylcobalamin labeled with one deuterium atom at C-5 (initial R/S ratio = 3 1) to monodeuterated coenzyme with R/S ratio = 1. This result shows that the cobalt-carbon 0-bond is reversibly cleaved to a 5 -deoxyadenosyl radical, which permits rotation about the C-47C-5 0-bond. 

Sherman, P. and Benton, M. 1980. Influence of skim milk powder/recodan R S ratio on the viscoelasticity of groundnut oil-in-water imitation milks. 7. TextureStud. 11 1-13. 

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