Lactic acid, enantiomers

Yang, Q. and Chung, T.S. (2007) Modification of the commerdal carrier in supported liquid membrane system to enhance lactic add flux and to separate L, D-lactic acid enantiomers. Journal of Membrane Science, 294, 127. 

The direct method does not provide sufficient resolution between the two lactic acid enantiomers to achieve the required detection limits. 

Fig. 5 Direct separation of lactic acid enantiomers using an LC column Daicel Chiralpak WH, 250 mm x 4.6 mm eluant CuS04 = 0.25 mmol/1, pH=5.6 flowrate 1 ml/min detection UV, X = 235 nm.

GC analytical method for lactic acid enantiomers derivatized in the form of dioxolanones... 

There are two (and only two) distinct three-dimensional representations for lactic acid enantiomers (see the figure on page 21) they are... 

Racemization. In the production of stereochemically pure lactide, formation of the other lactic acid enantiomer and wcjo-lactide is unwanted. Higher temperatures, longer reaction times, and increased catalyst levels result in increased rates of racemization [4,6,69]. Since temperature and catalyst influence the rate of lactide formation as well, controlhng the racemization rate can become quite complex. 

Figure 9.1 Formula of lactic acid enantiomers and lactide diastereoisomers...

Figure 9.2 Lactide diastereoisomers and derived homo- and stereocopolymers obtained via ring-opening polymerisation, i predominantly isotactic, s predominantly syndiotactic, c stereocomplex, and lactic acid enantiomers and homo- and stereocopolymers obtained by polycondensation (ran randomly distributed L and D lactyl units according to Bernouillian statistics)...

The relative percentages of D- and L-lactic acid enantiomers present in PLA is calculated as follows ... 

Selected lactic acid bacteria their fermentation pathways, lactic acid enantiomer produced, and major carbon sources used... 

Bacteria Fermentation pathway Lactic acid enantiomer Carbon sources substrates and complex... 

An example of a chiral compound is lactic acid. Two different forms of lactic acid that are mirror images of each other can be defined (Figure 2-69). These two different molecules are called enantiomers. They can be separated, isolated, and characterized experimentally. They are different chemical entities, and some of their properties arc different (c.g., their optical rotation),... 

Figure 2-69. The two enantiomers of lactic acid assignment of R and S configurations to the enantiomers of lactic acid after ranking the four ligands attached to the chiral center according to the Cl P rules (OH > COjH > Me > H).

Specific rotation is a physical property of a substance just as melting point boil mg point density and solubility are For example the lactic acid obtained from milk is exclusively a single enantiomer We cite its specific rotation m the form [a]o =+3 8° The temperature m degrees Celsius and the wavelength of light at which the measure ment was made are indicated as superscripts and subscripts respectively... 

Enzyme catalyzed reductions of carbonyl groups are more often than not com pletely stereoselective Pyruvic acid for example is converted exclusively to (5) (+) lactic acid by the lactate dehydrogenase NADH system (Section 15 11) The enantiomer... 

The enzyme is a single enantiomer of a chiral molecule and binds the coenzyme and substrate m such a way that hydride is transferred exclusively to the face of the carbonyl group that leads to (5) (+) lactic acid Reduction of pyruvic acid m the absence of an enzyme however say with sodium borohydride also gives lactic acid but as a racemic mixture containing equal quantities of the R and S enantiomers... 

List all the symmetry elements of the following molecules, assign each to a point group, and state whether they form enantiomers (a) lactic acid, (b) trans-[Co(ethylenediamine)2Cl2], (c) c -[Co(ethylenediamine)2Cl2], (d) cyclopropane,... 

Most of the chiral membrane-assisted applications can be considered as a modality of liquid-liquid extraction, and will be discussed in the next section. However, it is worth mentioning here a device developed by Keurentjes et al., in which two miscible chiral liquids with opposing enantiomers of the chiral selector flow counter-currently through a column, separated by a nonmiscible liquid membrane [179]. In this case the selector molecules are located out of the liquid membrane and both enantiomers are needed. The system allows recovery of the two enantiomers of the racemic mixture to be separated. Thus, using dihexyltartrate and poly(lactic acid), the authors described the resolution of different drugs, such as norephedrine, salbu-tamol, terbutaline, ibuprofen or propranolol. 

Molecules that are not identical to their mirror images are kinds of stereoisomers called enantiomers (Greek encmtio, meaning "opposite"). Enantiomers are related to each other as a right hand is related to a left hand and result whenever a tetrahedral carbon is bonded to four different substituents (one need not be H). For example, lactic acid (2-hydroxypropanoic acid) exists as a pair of enantiomers because there are four different groups (—H, -OH, - CH3, -C02H) bonded to the central carbon atom. The enantiomers are called (-i-)-lactic acid and (-)-lactic acid. Both are found in sour milk, but only the (+) enantiomer occurs in muscle tissue. 

One further point needs to be mentioned—the matter of absolute configuration. How do we know that our assignments of R,S configuration are correct in an absolute, rather than a relative, sense Since we can t see the molecules themselves, how do we know that the R configuration belongs to the dextrorotatory enantiomer of lactic acid This difficult question was finally solved in 1951, when J. M. Bijvoet of the University of Utrecht reported an X-ray spectroscopic method for determining the absolute spatial arrangement of atoms in a molecule. Based on his results, we can say with certainty that the R,S conventions are correct. 

Except for their effect on plane-polarized light, two enantiomers of a chiral compound have identical physical properties. For example, the two isomers of lactic acid shown below have the same melting point, 52°C, and density, 1.25 g/mL. 

Lactic acid buffer from, 385-386 characterization, 385,471 enantiomers, 601... 

Suppose we have two test tubes, one containing (— )-lactic acid and the other the (-I-) enantiomer. One test tube contains 34 and the other 35. How do we know which is which Chemists in the early part of this century pondered this problem and decided... 

Enantiomers have identical chemical and physical properties in the absence of an external chiral influence. This means that 2 and 3 have the same melting point, solubility, chromatographic retention time, infrared spectroscopy (IR), and nuclear magnetic resonance (NMR) spectra. However, there is one property in which chiral compounds differ from achiral compounds and in which enantiomers differ from each other. This property is the direction in which they rotate plane-polarized light, and this is called optical activity or optical rotation. Optical rotation can be interpreted as the outcome of interaction between an enantiomeric compound and polarized light. Thus, enantiomer 3, which rotates plane-polarized light in a clockwise direction, is described as (+)-lactic acid, while enantiomer 2, which has an equal and opposite rotation under the same conditions, is described as (—)-lactic acid. 

The method is based on the fact that certain bacteria, fungi, mould or yeast when allowed to grow in a racemic solution, assimilate or consume one of the enantiomers faster than the other. This is why the method is also known as selective assimilation or preferential decomposition. Thus Penicillium glaucum a species of green mould when allowed to grow in ammonium racemate solution consumes the d 0 tartaric acid and leaves the l form, but in a racemic lactic acid it assimilates the l form leaving behind the d form. 

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