Lactic acid isomers

Q Effects of Natural Moisturizing Factor and Lactic Acid Isomers on Skin Function... 

Rawlings, A.V. etal. Keratinocyte ceramide synthesis, effect of lactic acid isomers on stratum corneum lipid levels and stratum corneum barrier function, Arch. Dermatol. Res., 288, 383, 1996. 

SLe mimics 46 to 48, which had the GlcNAc unit replaced by (R,/ )-cyclohexan-l,2-diol and sialic acid substituted by glycolic acid or (5)-cyclohexyl lactic acid, were active as antagonists (Figure 16.21). These observations are in agreement with predictions based on computational studies that suggested these compounds are able to adopt the bioactive conformation. These computational simulations further predicted that the (/f)-lactic acid isomers were inactive because of their inability to properly orient the pharmacophores. Indeed, replacement of sialic acid by... 

Not all LAB produce the same lactic acid isomer. For example, Lactococcus and Carnobaderium are known to produce L-lactic acid, whereas Leuconostoc produces D-lactic acid (Liu, 2003 Gravesen et al., 2004). The levels and also the type of organic acids that are produced during any fermentation process are, therefore, dependent on LAB species or strains,... 

Lactic acid (LAK-tik AS-id) is a colorless, odorless, syrupy liquid that occurs in two isomeric forms, D-lactic acid and L-lactic acid. Isomers are two or more forms of a chemical compound with the same molecular formula, but different structural formulas and different chemical and physical properties. The D form is produced during metabolic reactions that take place in muscle tissue, while the L form is produced by yeast cells. The synthetic production of lactic acid results in a product consisting of equal amounts of the D and L forms, a mixture known as a racemic mixture. 

Rawlings AV, Davies A, Carlomusto M, Pillai S, Zhang K, Kosturko R, Verdejo P, Feinberg C, Nguyen L, Chandar P (1996) Effect of lactic acid isomers on keratinocyte ceramide synthesis, stratum corneum lipid levels and stratum corneum barrier function. Arch Dermatol Res 288 383-390 Reed JT, Ghadially R, Elias PM (1995) Skin type, but neither race nor gender, influence epidermal permeability barrier recovery. Arch Dermatol 131 1134-1138... 

The two lactic acid isomers shown in Figure 10.15 are labeled o-lactic acid and L-lactic acid (d is for dextro, meaning right l is for levo, meaning left). They might be expected to have quite similar properties, and they do. They have identical melting points, boiling points, and solubilities, for instance. And if we try to prepare lactic add in the... 

Many of the physical properties are not affected by the optical composition, with the important exception of the melting poiat of the crystalline acid, which is estimated to be 52.7—52.8°C for either optically pure isomer, whereas the reported melting poiat of the racemic mixture ranges from 17 to 33°C (6). The boiling poiat of anhydrous lactic acid has been reported by several authors it was primarily obtained duriag fractionation of lactic acid from its self-esterification product, the dimer lactoyUactic acid [26811-96-1]. The difference between the boiling poiats of racemic and optically active isomers of lactic acid is probably very small (6). The uv spectra of lactic acid and dilactide [95-96-5] which is the cycHc anhydride from two lactic acid molecules, as expected show no chromophores at wavelengths above 250 nm, and lactic acid and dilactide have extinction coefficients of 28 and 111 at 215 nm and 225 nm, respectively (9,10). The iafrared spectra of lactic acid and its derivatives have been extensively studied and a summary is available (6). 

Lactic acid is generally recognized as safe (GRAS) for multipurpose food use. Lactate salts such as calcium and sodium lactates and esters such as ethyl lactate used in pharmaceutical preparations are also considered safe and nontoxic (7). The U.S. Food and Dmg Administration fists lactic acid (all isomers) as GRAS and sets no limitations on its use in food other than current good manufacturing practice (46). 

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. 

The solution to this problem has been to isolate the lactide and to polymerize this directly using a tin(ii) 2-(ethyl)hexanoate catalyst at temperatures between 140 and 160 °C. By controlling the amounts of water and lactic acid in the polymerization reactor the molecular weight of the polymer can be controlled. Since lactic acid exists as d and L-optical isomers, three lactides are produced, d, l and meso (Scheme 6.11). The properties of the final polymer do not depend simply on the molecular weight but vary significantly with the optical ratios of the lactides used. In order to get specific polymers for medical use the crude lactide mix is extensively recrystallized, to remove the meso isomer leaving the required D, L mix. This recrystallization process results in considerable waste, with only a small fraction of the lactide produced being used in the final polymerization step. Hence PLA has been too costly to use as a commodity polymer. 

Lactide was polymerized by lipase PC in bulk at high temperature (80-130°C) to produce poly(lactic acid) with Mw up to 2.7 x 105 [64, 65]. The molecular weight of the polymer from the D,L-isomer was higher than that from the d,d- and L,L-ones. Protease (proteinase K) also induced the polymerization of lactide, however, the catalytic activity was relatively low. 

In general, the maximum number of optically active isomers is given by 2n where n represents the number of asymmetric carbon atoms. Thus for a compound where n = 1, as in lactic acid, there would be two stereoisomers, one the dextro and the other the laevo. For a compound with two asymmetric carbon atoms, there would be 22 = 4 stereoisomers. But if the two asymmetric carbon atoms carry exactly identical groups, as in tartaric acid, the number would be fewer than four and we know that it exists in three forms, the d the 1 and the meso. 

Some times the sign and extent of rotation help in determining which isomer has which configuration. This happens because rotation of structurally related compounds of identical configuration undergoes analogous changes under the influence of temperature, solvent or other factors. Let us study the molecular rotations of L(+) lactic acid and alanine. 

As we have seen a stereoselective reaction is one in which there is a preponderance of one isomer irrespective of the stereochemistry of the reactant. The enzymatic reduction of pyruvic acid is stereoselective when the chiral molecules of the enzyme complexes with achiral pyruvic acid, they given a preponderance of one form of pyruvic acid-enzyme complex which then gives a single form of lactic acid. 

Each isomer is the mirror image of the other and they are known as optical isomers or enantiomers. But all molecules have mirror images, yet they do not all exhibit optical isomerism. What makes lactic acid different is that its two isomers are non-superimposable. You should make molecular models of these optical isomers to convince yourself that one isomer cannot be superimposed on the other. 

Bacterial strains were isolated that could grow on R-CPA [(R)-2-chloropropanoic acid], but not on S-CPA, because although both isomers were dehalogenated by different R and S specific dehalogenase isozymes, the -lactic acid formed from the S-CPA could not be metabolised. 

In the case of lactic acid and trimethylacetaldehyde the crude product is a ca. 4 1 mixture of the cis- and the trans-isomers, 4 and 5 (R = /-Bu). respectively75,77 from which pure 4 can be isolated with d.r. 98 2 after one recrystallization at — 78 °C11. Similarly, with 2-/er/-butyl-5-phenyl-1,3-dioxolan-4-one (1, Rl = C6H5 Rz = f-Bu) the crude product is already formed with a d.r. >99 1 75 77 in favor of the m-isomer, giving the pure cw-compound after recrystallization77,79. 

By the action of water upon the chloride of fi chlorpropionyl, a body of the composition of chloropropionic add is obtained but inasmuch os this body yields paralactic acid by ebullition with potash, whilst chloropropionic acid gives under the same drcumstances lactic acid, it follows that the former ohloro-add must be isomeric, and not identical, with the latter. Now, dthough the formula of propionic add does not admit of any isomer, yet that of cbloropropionio add does, os is scon in the following graphic formuhe... 

The primary function of cheese starter cultures is to produce lactic acid at a predictable and dependable rate. The metabolism of lactose is summarized in Figure 10.12. Most cheese starters are homofermentative, i.e. produce only lactic acid, usually the L-isomer Leuconostoc species are heterofermentative. The products of lactic acid bacteria are summarized in Table 10.4. 

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