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Racemate, true

Chemical development Proof of structure and configuration are required as part of the information on chemical development. The methods used at batch release should be validated to guarantee the identity and purity of the substance. It should be established whether a drug produced as a racemate is a true racemate or a conglomerate by investigating physical parameters such as melting point, solubility and crystal properties. The physicochemical properties of the drug substance should be characterized, e.g. crystallinity, polymorphism and rate of dissolution. [Pg.325]

Aliphatic acids such as butyric acid have been previously implicated as being allelopathic compounds (46, 47, 23). Chou and Patrick (23) isolated butyric acid from soil amended with rye and showed that it was phytotoxic. Hydroxy acids have also been shown to possess phytotoxic properties (48) but have not been implicated in any allelopathic associations. Since SHBA is a stereo isomer, and the enantiomer was not identified because of impurity, all bioassays were run using a racemic mixture. The D-(-) stereo isomer of SHBA has been isolated from both microorganisms and root nodules of legumes and is suspected to be a metabolic intermediate in these systems (49). It is likely that only one enantiomer was present in the extract therefore, the true phytotoxic potential of this compound awaits clarification of the phytotoxicity of the individual enantiomers. [Pg.264]

In the bulk crystalline phases, large differences exist in the properties of the racemic mixture and the pure enantiomers. X-ray powder diffraction patterns showed that the racemic mixture was a true racemate, and the melting transition points and heats of fusion of the racemate were markedly different from those of the pure enantiomers [which were identical (Arnett and Thompson, 1981)]. [Pg.71]

A quasi-racemate is a molecular compound that is related to a true racemic compound by a small structural change in one of the enantiomers (Fredga, 1944). [Pg.103]

Strictly speaking, a chiral species cannot correspond to a true stationary state of the time-dependent Schrodinger equation H

time scale for such spontaneous racemization is extremely long. The wavefunction of practical interest to the (finite-lived) laboratory chemist is the non-stationary Born-Oppenheimer model Eq. (1.2), rather than the true T of Eq. (1.1). [Pg.42]

A quasi-racemate or pseudo-racemate is a true racemate like molecular compound formed between optical antipode of different (but related) compounds. The quasi-racemate also has a melting point ciin c resembling the curve of a true racemate but with quasi-racemic compounds the curves are unsymmetrical, because the melting points of the components are different as shown in Fig. (9.3). The curve A represents the melting point of a true-racemate formed by mixing (+) mandelic acid XXII and (-) hexahydromandelic acid XXIII while B represents that of a mixture of (+) XXII and (+) hexa hydro-mandelic acid XXIII. [Pg.142]

Fredga believes that substances capable of forming true racemates are also liable to form quasiracemates. [Pg.143]

A D Aniello, L Petrucelli, C Gardner, G Fisher. Improved method for hydrolyzing proteins and peptides without inducing racemization and for determining their true D-amino acid content. Anal Biochem 213, 290, 1993. [Pg.122]

As was true in the case of mean sequence lengths for meso and racemic dyads, the necessary relationships can be used to develop corresponding equations for any particular n-ad distribution. A favorable point concerning the concept of "like" configurations is that it attaches a physical significance to the racemic distribution. [Pg.310]

A typical example that illustrates the method concerns the lipase- or esterase-catalyzed hydrolytic kinetic resolution of rac-1-phenyl ethyl acetate, derived from rac-1-phenyl ethanol (20). However, the acetate of any chiral alcohol or the acetamide of any chiral amine can be used. A 1 1 mixture of labeled and non-labeled compounds (S)- C-19 and (f )-19 is prepared, which simulates a racemate. It is used in the actual catalytic hydrolytic kinetic resolution, which affords a mixture of true enantiomers (5)-20 and (J )-20 as well as labeled and non-labeled acetic acid C-21 and 21, respectively, together with non-reacted starting esters 19. At 50% conversion (or at any other point of the kinetic resolution), the ratio of (5)- C-19 to (1 )-19 correlates with the enantiomeric purity of the non-reacted ester, and the ratio of C-21 to 21 reveals the relative amounts of (5)-20 and (J )-20 (98). [Pg.24]

As already mentioned, the secondary alcohols that are obtained are optically active. It should be stressed that the reduction of ketones to carbinols by means of fermenting yeast is completely different from the method of resolution of racemic alcohols by treatment with living microorganisms (Pasteur). In the latter case one of the enantiomorphs is removed by oxidation during metabolism in the former it is produced by true asymmetric hydrogenation, without the intermediate formation of the inactive form, (Cf. Mayer and Levene and Walti. )... [Pg.83]

Put another way, epimerization is the mechanistic event, racemization is the observation. True racemization, the actual production of a racemic mixture, is rarely seen in peptide synthesis. Instead, it is the extent of epimerization that defines the stereochemical outcome of a peptide-bond-forming reaction. In order to assess the probability of epimerization under a given set of conditions, one must be aware of the mechanisms of epimerization, as well as the thermodynamic and kinetic factors that affect this process. [Pg.657]

The optical rotation of the mixture approaches zero (a racemic mixture) over time, with apparent first-order kinetics. This observation was supported by the semi-log plot [ln(a°D/ aD) vs time], which is linear (Figure 1). It has been shown that this racemization process does in fact follow a true pseudo-first-order rate equation, the details of which have been described by Eliel.t30 Therefore, these processes can be described by the first-order rate constant associated with them, which reflects precisely the intrinsic rate of racemization. Comparison of the half-lives for racemization under conditions of varying amino acid side chain, base, and solvent is the basis for this new general method. [Pg.664]

In this chapter the question of homochirality has also been considered according to Meir Lahav breaking of symmetry is not the problem. I do not know how many scientists would agree with him, but it is certainly true that in the laboratory chiral compounds can be obtained starting from racemic mixtures -and this by simple means, without invoking subtle effects of parity violation. Of course we do not know how homochirality really evolved in nature however, it is comforting to know that there is in principle an experimental solution to the problem. [Pg.57]

The observation that chains tend to grow in a homochiral way, is also true for polypeptides. For example in the polymerization of NCA-activated a-amino acids there is a natural tendency to form homochiral chains (Blocher et al., 2001 Hitz and Luisi, 2004). In particular when a racemic mixture of NCA-activated amino acids is polymerized, a mixture of chain lengths up to heptamers or decamers is obtained, which, as expected, is a racemate. However, in these products there is a significant excess of homochiral stretches. One typical result is shown in Figure 4.11. [Pg.80]

The precision of enantiomeric purity determinations by gas chromatography is high123 124-1 >s. This statement holds not only for small enantiomeric purities ( 0% ee), e.g., in the differentiation of a true racemate from enantiomerically slightly enriched mixtures (in reactions devoted to the amplification of optical activity under prebiotic conditions), but also for very high enantiomeric purities (— 100% ee), with detection of 1.0 to 0.1% (and less) of enantiomeric impurities (see Section 3.1.5.8). It is always advantageous if the enantiomer present as an impurity is eluted as the first peak from the gas chromatographic column (Section 3.1.5.3.). This is achieved by the proper selection of the chirality of the nonracemic stationary phase147-188 which, unfortunately, is not possible for the cyclodextrin phases. [Pg.179]

Figure 21. A1) True racemic composition for enantiomer separation196 of frons-2,3-dimethyloxirane by complexation gas chromatography on nickel(II) bis[3-heptafluorobutanoyl-(D )-camphorale] at 80CC. Integration with a Spectra-Physics SP4100 instrument (peak areas are equal). Figure 21. A1) True racemic composition for enantiomer separation196 of frons-2,3-dimethyloxirane by complexation gas chromatography on nickel(II) bis[3-heptafluorobutanoyl-(D )-camphorale] at 80CC. Integration with a Spectra-Physics SP4100 instrument (peak areas are equal).

See other pages where Racemate, true is mentioned: [Pg.332]    [Pg.168]    [Pg.199]    [Pg.233]    [Pg.1172]    [Pg.341]    [Pg.380]    [Pg.142]    [Pg.380]    [Pg.203]    [Pg.278]    [Pg.294]    [Pg.143]    [Pg.44]    [Pg.123]    [Pg.188]    [Pg.260]    [Pg.223]    [Pg.229]    [Pg.229]    [Pg.234]    [Pg.242]    [Pg.78]    [Pg.137]    [Pg.348]    [Pg.161]    [Pg.7]    [Pg.114]    [Pg.132]    [Pg.345]    [Pg.152]    [Pg.247]   
See also in sourсe #XX -- [ Pg.332 ]




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