Malic acid, levo

The optical activity of malic acid changes with dilution (8). The naturally occurring, levorotatory acid shows a most peculiar behavior in this respect a 34% solution at 20°C is optically inactive. Dilution results in increasing levo rotation, whereas more concentrated solutions show dextro rotation. The effects of dilution are explained by the postulation that an additional form, the epoxide (3), occurs in solution and that the direction of rotation of the normal (open-chain) and epoxide forms is reversed (8). Synthetic (racemic) R,.9-ma1ic acid can be resolved into the two enantiomers by crystallisation of its cinchonine salts. 

Some interest also attaches, on stereochemical grounds, to the effects of the isomers of tartaric acid and malic acid. At 0.01 M, erythraric acid (meso-tartaric acid) caused slight competitive inhibition,181 183 186 whilst d- or L-threaric acid (levo- or dextro-tartaric acid) had no effect138181 commercial DL-threaric acid ( racemic acid ) had a small effect that disappeared on purification.181 Some authors have noted slight inhibition with L-tartaric acid.147 166 183 L-Malic acid resembled erythraric acid in its action,136 147 1 65 188,185 but n-malic acid appeared to be non-inhibitory.136-183 Unsubstituted w-dicarboxylic acids cause little, if any, inhibition.135 147 166183... 

D-Glucitol, dulcitol, L-perseulose, n-perseulose, Schardinger /3-dextrin, (levo)-malic acid Raffinose Xylitol... 

Stereo Isomerism of Malic Acid.—On examination of the formula of malic acid it will be seen that one of the carbons is asymmetric i.e.j it has united to it four different elements or groups, viz., (—H), (—OH), (—COOH), and (—CH2—COOH). We should, therefore, expect to find that malic acid is optically active and that it exists in the three forms of dextroj levo, and inactive. This is in accordance with the facts. The formulas for the three stereo-isomeric forms of malic acid may be written as follows, corresponding exactly to those for lactic acid. 

Isomerism of Tartaric Acid.—Examination of the formula for tartaric acid, which, by the facts given above, has its constitution fully established as symmetrical di-hydroxy succinic acid, shows the interesting fact that there are present two asymmetric carbon atomsy and that each of these has linked to it the same set of four different groups. We should, therefore, expect to find tartaric acid existing in the dextrOy the levo and the inactive forms. The stereo-chemical formulas similar to those of lactic and malic acids we may write as follows. 

In support of the equilibrium displacement mechanism, a study of the Pfeiffer rotation of the system D, L-[ Ni ( o-phen) 3] 2+ with levo-malic acid as an environment substance in water produces some interesting results (Figure 2). The Pfeiffer Effect reaches its maximum in just over k days, and if, at that time, exactly as much dextro-malic acid is added to the system as the levo enantiomer already present, then this has the effect of removing (deactivating) the environment substance, so that the excess of the complex enatiomer has no alternative but to undergo racemization. It should be noted (Figure 3) that the rate of this racemization is identical to the racemization rate of the optically pure complex which has been resolved by conventional means. 

The Pfeiffer Effect and pH for the System Levo-Malic Acid and D.L-[Ni(o-phen) Cl2... 

The malic acid obtained from fruits is optically active. Its aqueous solutions exhibit a behavior toward polarized light which is unusual. In concentrated solutions the acid is dextro-rotatory as the liquid is diluted the extent of rotation decreases until it becomes zero, when, upon further dilution, the solution becomes levorotatory. A number of theories have been put forward to explain these and similar facts, but none is satisfactory. There are many unsolved problems in stereo-chemistry. Optically active substances are involved in the building up of living matter, and nature in selecting the units for this purpose differentiates between dextro and levo compounds. The importance of this field of investigation is evident, and valuable work is being done in it. 

The term racemic is used as a general term in describing stereo-isomers to indicate that an inactive substance consists either of a mixture of the dextro- and levo-rotatory forms or is produced as the result of the formation of a molecular compound from these forms (315). Thus the, acid which has been called inactive malic acid is said to be the racemic form of malic acid. 

Fig. 6. The Pfeiffer effect and racemization in water of [Ni(o-phen)3]Cl2 with levo malic acid as the environment compound. Optical rotation as a function of time for A levo-ma ic acid B levo-ma ic acid -I- D,L-[Ni(o-phen)3]Cl2 C system B + dextro-malic acid added after 123 hr D racemization of the L-[Ni(o-phen)3]Cl2 produced by the Pfeiffer effect (system C allowed to decay).

German name, Trauben-saure, is derived from the word for grapes. It is probable that it does not exist in grapes as racemic acid but that it is formed from the dextro acid as this transformation can easily be effected by the action of acids or even by water alone. When tartaric acid is prepared synthetically from succinic acid, from glyoxal, or from malic, maleic or fumaric acids either racemic acid or meso-tartaric acid is always formed. That is, synthetic reactions result in the formation of an inactive form. The methods of splitting racemic acid into its optically active components has been fully discussed. The sodium-ammonium racemate is the only salt that is of importance. This has been spoken of in connection with the method of splitting racemic acid into its components.. Like the free acid this salt exists, in dilute solution, as equal molecular parts of the dextro and levo forms. Only in concentrated solution does it exist as the racemate itself. 

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