Stereoisomers of Lactic Acid

L-lactic acid D-lactic acid DL-lactic acid 

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Stereoisomers of lactic acid produced by lactic acid bacteria are useful for species identification. The optical configuration of lactic acid (Table 13.1) depends on the stereospecificity of the LDH. Some microorga-... 

Table 13.1. Stereoisomers of Lactic Acid Produced by Various Microorganisms.

Fig. 1 Two stereoisomers of lactic acid. The dotted lines from the asymmetric center carbon atom project behind the plane of the paper...

Compared with the chemical synthesis, microbial fermentation for lactic acid production is more ecofriendly, comparatively fast, has superior yields, and can produce one of the two stereoisomers of lactic acid as well as their racemic mixture. It is crucial to select the suitable microbes with high productivity, the low-cost raw materials, and most favorable fermentation conditions, for example, temperature, pH, aeration, agitation, and so on. For development of competitive processes, the search for low-cost raw materials... 

Configurationally muscle lactic acid belongs to the L-series, that is, it has the same configuration on carbon 2 as the reference substance L-glyceraldehyde. However, free L-lactic acid is dextrorotatory, whereas its Zn salt is levorotatory, and this has caused considerable confusion in the literature. To bring matters up to date, the reason why only one of the two stereoisomers of lactic acid is formed enzymatically in animal tissues is explained by the demonstration of Westheimer and Vennesland in 1953 that the enzymatic hydrogen transfer from DPNH to the double bond of pyruvate and the reverse reaction is stereospecific. 

During the same test, it is interesting to determine the optical nature of lactic acid formed from glucose. This analysis makes use of an enzymatic process. The two stereoisomers of lactic acid (l and D) are analyzed separately. This form of analysis is particularly adapted to the identification of heterofermentative cocci Oenococcus oeni, Ixu-conostoc mesenteroid.es), which only form the D isomer, and of Lactobacillus casei, which only forms L-lactic acid. 

Daniels Midland Co., with a capacity of 10-20,000 t/year (9.1-18.2 X 10 kg/year Anonymous 1993). This proprietary fermentation process is presently nonbacterial. In contrast. Sterling Chemicals, a major producer of synthetic lactic acid in the United States, had an annual capacity of 9.5-10,000 t (8.6-9.1 X 10 kg Bahner 1994). The competitive position of fermentation lactic acid over synthetic lactic acid depends upon the ability to selectively produce desired stereoisomers of lactic acid (D— or L+) instead of a racemic mixture produced by the synthetic route. Use of inexpensive carbohydrate feedstocks and advances in separation technologies keep production costs low. 

Figure 11.11 Stereoisomers of lactic acid (a) d(—) lactic acid (DLA) and (b) L(-t-) lactic acid (LLA).

As already mentioned the two different stereoisomeric forms of lactic acid (D and l) give rise to four morphologically distinct polymers. Generally speaking l-PLA is more frequently employed than d-PLA since the hydrolysis of L-PLA yields the naturally occurring stereoisomer of lactic acid, L(+) lactic acid. 

Draw skeletal structures of both configurational stereoisomers of lactic acid (shown in Model 2). 

DUactide (5) exists as three stereoisomers, depending on the configurations of the lactic acid monomer used. The enantiomeric forms whereia the methyl groups are cis are formed from two identical lactic acid molecules, D- or L-, whereas the dilactide formed from a racemic mixture of lactic acid is the opticaUy iaactive meso form, with methyl groups trans. The physical properties of the enantiomeric dilactide differ from those of the meso form (6), as do the properties of the polymers and copolymers produced from the respective dilactide (23,24). 

Lactide (LA), the cyclic diester of lactic acid, has two stereogenic centers and hence exists as three stereoisomers L-lactide (S,S), D-lactide (R,R), and meso-lactide (R,S). In addition, rac-lactide, a commercially available racemic mixture of the (R,R) and (S,S) forms, is also frequently studied. PLA may exhibit several stereoregular architectures (in addition to the non-stereoregular atactic form), namely isotactic, syndiotactic, and heterotactic (Scheme 15). The purely isotactic form may be readily prepared from the ROP of L-LA (or D-LA), assuming that epimerization does not occur during ring opening. The physical properties, and hence medical uses, of the different stereoisomers of PLA and their copolymers vary widely and the reader is directed to several recent reviews for more information.736 740-743... 

Problem 9,19 What stereoisomers would result from reaction of (=)-lactic acid with (S)-l-phenyl-... 

Number of Stereo-isomers.—Plainly, with more than one asymmetric carbon atom in the molecule, we can have more than two stereoisomers, as we had in the case of lactic acid. When we speak of the number of stereo-isomers we do not include the racemic form as it... 

Over the past several decades, polylactide - i.e. poly(lactic acid) (PLA) - and its copolymers have attracted significant attention in environmental, biomedical, and pharmaceutical applications as well as alternatives to petro-based polymers [1-18], Plant-derived carbohydrates such as glucose, which is derived from corn, are most frequently used as raw materials of PLA. Among their applications as alternatives to petro-based polymers, packaging applications are the primary ones. Poly(lactic acid)s can be synthesized either by direct polycondensation of lactic acid (lUPAC name 2-hydroxypropanoic acid) or by ring-opening polymerization (ROP) of lactide (LA) (lUPAC name 3,6-dimethyl-l,4-dioxane-2,5-dione). Lactic acid is optically active and has two enantiomeric forms, that is, L- and D- (S- and R-). Lactide is a cyclic dimer of lactic acid that has three possible stereoisomers (i) L-lactide (LLA), which is composed of two L-lactic acids, (ii) D-lactide (DLA), which is composed of two D-lactic acids, and (iii) meso-lactide (MLA), which is composed of an L-lactic acid and a D-lactic acid. Due to the two enantiomeric forms of lactic acids, their homopolymers are stereoisomeric and their crystallizability, physical properties, and processability depend on their tacticity, optical purity, and molecular weight the latter two are dominant factors. 

Polymers based on lactic acid (PLA) are a most promising category of polymers made from renewable resources. They are not only compostable and biocompatible, but also pro-cessable with most standard processing equipment. The properties of lactic acid based polymers vary to a large extent depending on the ratio between, and the distribution of, the two stereoisomers or other comonomers [ 1-3]. The polymers can be manufactured by different polymerization routes, which are schematically described below (Figure 3.1). 

The stereocopolymers of lactic acid, prepared by the polymerization of various stereoisomers, are discussed in a subsequent section in this book and will not be discussed here. Typical comonomers that have been used for lactic acid or lactide copolymerization are glycolic acid or glycolide (GA) [11-17], poly (ethylene glycol) (PEG) or poly(ethylene oxide) (PEG) [15 3], poly(propylene oxide) (PPO) [16-18], (7 )- 3-butyrolactone (BL), 6-valerolactone (VL) [44-46], E-caprolactone (CL) [47-54], 1,5-dioxepan-2-one (DXO) [55-60], trimethylene carbonate (TMC) [61],... 

Caimcross et al. [16] studied moisture sorption and transport in three different PLA films (high percentage l-lactide, a mixture of lactic acid stereoisomers, and a 50 50 blend of PLLA and PDLA) exposed to RH changes from 0 to approximately 25% at 40°C using a quartz crystal microbal-ance/heat conduction calorimeter (QCM/HCC). In addition, moisture transport, crystallization, and degradation in PLA... 

Lactic acid is produced by the fermentation of carbohydrate material, usually glucose derived by hydrolysis from starch. The fermentation route can provide either enantiomer of lactic acid in high purity and dominates over chemical routes. The structure of lactic acid contains one asymmetric carbon, and can therefore exist as two stereoisomers. L-lactic acid is present naturally in numerous organisms, whilst the mirror image D-lactic acid is very rare in nature. 

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