Racemization kinetics

A detailed study involving racemization kinetics and spin states of a series of nine related complexes (507) has been undertaken.1357 In an X-ray crystallographic study of a series of complexes (R Pr1, R2 = Me, R3 = H, R4 = Me, Ph, Bu ) all complexes were found to possess pseudotetrahedral geometry.1358... 

It may also be surprising how easily this racemization may occur. Friedman and Liardon (126) studied the racemization kinetics for various amino acid residues in alkali-treated soybean proteins. They report that the racemization of serine, when exposed to 0.1M NaOH at 75°C, is nearly complete after just 60 minutes. However, caution must be used when examining apparent racemization rates for protein-bound amino acids. Liardon et al. (127) have also reported that the classic acid hydrolysis, employed to liberate constituent amino acids, causes amino acids to racemize to various degrees. This will necessarily result in D-isomer determinations that are biased high. Widely applicable correction factors are not possible since the racemization behavior of free amino acids is different from that of amino acid residues in proteins (which can be further affected by sequence). Of course, this is not a problem for free amino acid isomer determinations since the acid hydrolysis is unnecessary. Liardon et al. also describe an isotopic labeling/mass spectrometric method for determining true racemization rates unbiased by the acid hydrolysis. For an extensive and excellent review of the nutritional implications of the racemization of amino acids in foods, the reader is directed to a review article written by Man and Bada (128). 

M Friedman, R Liardon. Racemization kinetics of amino acid residues in alkali-treated soybean proteins. J Agric Food Chem 33 666-672, 1985. 

Figure 9. Racemization kinetics of fll)-l-chloro-2,2-dimethylaziridine at 82°C by monitoring the increase of the (S)-enantiomer with time by complexation GC (experimental conditions cf. Figure 8).

X.-L. Lu and S. K. Yang, Resolution of enantiomeric lorazepam and its acyl and O-methyl derivatives and racemization kinetics of lorazepam enantiomers, J. Chromatogr., 535 229 (1990). 

M. M. (2001) Chiral studies in amorphous solids The effect of the polymeric glassy state on the racemization kinetics of bridged paddled binaphthyls. Journal of the American Chemical Society, 123,... 

See footnotes b) and °) to Table 2. 6) Data from epimerization kinetics. c) Data from racemization kinetics. 

Although the first demonstration that amino acid racemization took place in fossils used mollusc shells (23), the application of this reaction in dating these materials has been extensively investigated only recently (19,20). Work on Mercenaria (19), Chione (20), and other species (24) has tested the application of racemization dating to fossil mollusc shells from geological contexts and Indian shell middens. These and other studies have shown that there are problems with amino acid racemization dating of carbonaceous fossils which are not encountered with bone. Reversible first-order racemization kinetics which are observed in bone... 

Even if chirality is not a primary interest, the chemist should be aware of its presence, notably in the interpretation of NMR data for structure determination. In kinetically labile systems formed by self-assembly, the study of racemization kinetics offers vital information on the robustness of the system, and the possible mechanisms of decomposition or flagmentation. In short, what appears at first to be an extra complication in an already complex system does in fact offer the chemist new means of investigation and control over the properties of these compounds. 

The racemization kinetics of isoleucine are slightly complicated by the fact that this amino acid is asymetric at both the a- and /3-carbons. The racemization reaction for isoleucine can be written as... 

Racemization kinetics from CPL measurements. Experiments concerned with the enantiomer interconversion processes were carried out on the [Eu(TTHA)] compound in neutral aqueous solution (Mondry et al., 1994). It has been demonstrated that the CPL spectrum of this system shows only a very slight temperature dependence from 283 to 353 K,... 

The effects of the stationary phase and solvent on the racemization of 2,2 -Z)is-phosphino- and 2,2 -Z)is(phosphinyl)-l,l -biphenyls (BIPHEP) and the racemization kinetics of the 3,3 -disubstitued BIPHEP ligands have been determined by chiral dynamic HPLC (DHPLC) using a novel three-column continuous flow technique. Also, phosphatidylcholine has been determined by ultra performance liquid chromatography (UPLC) with MS/MS detection. ... 

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