0 ASSIGNMENT OF D AND L PREFIXES TO THE TARTARIC ACIDS

HUBERT BRADFORD VICKERY The Connecticut Agricultural Experiment Station, New Haven, Connecticut

A RECENT issue of THIS JOURNAL contains two brief articles under the above title. l\Teniteescul concludes, after consideration of the work of Freudenberg 23

and of Woh14 on the correlation of the configuration of the dextrorotatory isomer of tartaric acid (I) found in grapes with D-glyceraldehyde (11), that this substance should be named D(+)-tartartic acid. Abernethya points out that the absolute configuration of (+)- tartaric acid has been determined by Bijvoet and collaborators6~' to be that of the D family. Neverthe- less, since L-threose (111) when oxidized yields the corresponding L-threaric acid (IV), that is to sap (+)-tartaric acid, he maintains that this substance should be called L(+)-tartaric acid. He dismisses as "superficial" the argument based on the work of FischeP that reduction of (+)-tartaric acid yields ~ r n a l i c acid (V), although this observation clearly

COOH CHO CHO

H- A-OH H-A-OH I

H-C-OH

Ho-h-H AHOH Ho-c-H I I I

COOH ~ H , O H f + )-Tartaric acid D-Glvceraldeh~de L-Threose

HCM-H

AOOH

indicates that the asymmetric centers in (+)-tartaric acid both belong to the D family, and he regards as "unsatisfactory" the argument that the use of the D

prefix for (+)-tartaric acid brings the nomenclature of this substance into conformity with the mles of nomen- clature of the amino acids.¶ We thus have before us a clearcut difference of opinion, and there is a t the present time no authoritative rule or statement of preferred custom bv which such a difference mav be resolved

NENI~ESOU, C. D., J. CHEM. EDUC., 34,147 (1957). FREUDENBERG, K. , Be?., 47,2027 (1914).

"UDENBERG, H., AND F. BMUNS, Ber., 55,1339 (1922). ' W o n , A,, AND FR. MOMBER, Ber., 50,455 (1917).

ABERNETAY, J. L., J. CHEM. EDUC., 34,150 (1957). "IJVOET. J. M.. A. F. PEERDEUIX. AND A. J. VAN BOMMEL.

Nature, 168,'271(1951). PEERDEMAN, A. F., A. J. VAN BOYMEL, AND J. M. BIJVOET,

Proe. A d . Sd. Amsterdam, B54, 16 (1951). a FISCHER, E., Ber., 29,1377 (1896).

Chem. Eng N e w , 30,4522 (1952).

This difficult problem is presently under consider- ation by the Committee on Nomenclature, Spelling and Pronunciation of the American Chemical Society. It should be pointed out that the ultimate decision of this committee will affect the nomenclature of all symmetrical substances which have identical asym- metric structures a t both ends of a chain: for example, threitol and higher symmetrical alcohols derived from the sugars, tartaric acid and its higher homologues, cystine and its higher homologues and such substances as qr-diamincpimelic acid among the amino acids, and so forth. The problem is thus an extremely mportant one and is far more complex than would

appear since its solution involves an agreement among authorities in many fields of organic and biological chemistry who often have widely divergent views.

Pending the action of committees on nomenclature, it may help to set forth a few recently ascertained facts with respect to the tartaric acids. As a premise, it is necessary to draw attention to the exact meaning of the capital letter prefixes as used in carbohydrate and amino acid nomenclature. Rule 4 of the official rules of carbohydrate nomenclature of the American Chemical SocietyIo states in part,

Configurational relationships will be denoted by the capital letter prefixes D and L . . .placed immediately before the sugar stem name and be employed only with compounds which have been definitely related to the reference standard glyoeralde- hyde.. . If optical rotational sign under specified conditions is to be indicated, this may be done by adding (deztro) or (levo), which are italicized in print, or by adding (+) or ( - ) . Racemic modifications may be indicated by the prefixes DL or (*)or inactive, the last being italicized in print.

Rule 2 of the official rules for the nomenclature of amino acids,s which have now also been adopted by the International Union of Pure and Applied Chemistry, states in part,

Distinction between the enantiomorphs of the amino acids is made by a prefixed small mpilal letter D or L to denote the con- figurational family to which the a-carbon at,om belongs.

Rule 3 (a) states,

The small capital letter prefixes D and L denote that the substance named is configurationally related to the corresponding enantiomorph of glyeersldehydc. Where confusion is possible between the use of the capital letter prefix for the configuration ot the a-carbon atom in amino acid nomenclature and for that of the highest numbered asymmetrio carbon atom in carbohydrate nomenclature, a subscript in added to the small oilpits1 letter prefix. Where the prefix is wed in the amino acid sense, the subscript B is added; where the prefix is used in the carbohydrate sense, the sublbseript g is added. These subscripb (lower case roman liptters) refer, respectively, to swine, the fundamentd

lo Chem. Eng. News, 31,1776 (1953).

VOLUME 34, NO. 7 , JULY, 1957 339

substance to which amino acids that b a r struotural reresemblance to the carbohydrates can be formally related, and to glycer- aldehyde, the fundamental substance to which the configura- tion of the carbohydrates is formally related.

These complex rules may he summed up by the statement that the capital letter prefixes may properly he used in naming sugars and amino acids or related substances only when the confignrational relationship to one of the enantiomorphs of glyceraldehyde has been established. An even more forthright statement is, if you know what you are talking about you may use the capital letter prefixes, otherwise not. Careless or ignorant use of them merely adds further confusion to an already sufficiently confused subject.

This last consideration has become especially im- portant during the past few years as information accumulates on the configurational relationships to D- or L-glyceraldebyde of an increasing number of substances which are neither carbohydrates nor a-amino acids. Accordingly, the use of the capital letter prefixes has in practice been extended. The relationships of several or-hydroxy acids, such as lactic acid and malic acid, have long been known, and the question of the proper use of the prefixes naturally arises with respect to tartaric acid.

The most important new fact regarding (+)-tar- taric acid is that its absolute configuration has been established by Bijvoet and his col lab~rators .~~~ The configurational relationship to D-glyceraldehyde was made highly probable by Fischer and by Freudenherg, and, as a result of Bijvoet's work, (+)-tartaric acid may now quite properly be regarded as an even more fundamental standard than D-glyceraldehyde. What this means may he visualized if one imagines a single molecule of (+)-tartaric acid enormously magnified and oriented with the bond between the carbon atoms 2 and 3 in the north-south direction. An observer standing a t the middle of this bond looking north would then see the three groups, H, OH, and COOH arranged in a circle in this order when read clockwise. If the observer then looked south, he would see the three groups arranged in a circle also in the order H, OH, and COOH when read clockwise.

What Fischer accomplished was to show that with (+)-tartaric acid the three groups would be seen in the same order as they would be observed if one examined D-glyceric acid or D-glyceraldehyde in the same way. However, it was impossible with the techniques then available to decide whether the order was clockwise or counterclockwise. He was thus led to postulate that the order of H, OH, and CHO in D-glyceraldehyde is clockwise. Bijvoet's observations have now proved that this postulation, which had only an even chance of being right, was in fact the correct statement of the situation.

In addition to the physical evidence offered by Bijvoet and his group, there is today a rapidly in- creasing body of information derived from biochemical investigations. Vickery and Pa mer" allowed excised tobacco leaves to take up (+)-tartaric acid from 0.2 M solutions adjusted to pH 5 or to pH 6 with sodium hydroxide for 48 hours in darkness. They recovered unchanged from the leaves all of the tartaric acid

'' VICKERY, H. B., AND J. K. PALMER, J. Bid. Cham., 207,275 (1954)

taken up and, accordingly, concluded that there are no enzyme systems in this tissue capable of metab- olizing (+)-tartrate. Furthermore, the presence of a relatively large amount of (+)-tartartic acid in the leaves exerted no influence upon the enzyme systems which actively metabolized L-malic acid and increased the concentration of citric acid during the experiment. This observation has recently been c~nfirmed.'~ The complete indifference of (+)-tartaric acid to attack when present in a tissue which contains enzyme systems adapted to the metabolism of L-malic acid as well as the natural L-amino acids was interpreted as support of the view that (+)-tartaric acid has the configuration characteristic of the D family.

This negative evidence has recently been supported by positive evidence advanced by Kun13 who has found that a soluble enzyme system prepared from the mitochondria of beef heart muscle reduces diphos- phopyridine nucleotide at pH 8.3 in the presence of magnesium ion when either (-)-tartrate or meso- tartrate (in which one asymmetric center is certainly L) is added as substrate. The product of the reaction was oxaloglycolate in equilibrium with its more stahle dienol form dihydroxyfumarate. The reaction mas reversible inasmuch as dihydroxyfumarate was reduced to tartrate by reduced diphosphopyridine nucleotide under properly chosen conditions. In an earlier paper,14 it had been shown that (+)-tartrate is not oxidized by any of the enzyme preparations from the same source. These observations clearly indicate that (-)-tartrate has a configuration such that it is attacked by enzyme systems present in an animal tissue. Such systems as a general rnle attack only substances which have the configuration of the L family. This is not proof, to be sure, but it is highly significant contrib- utory evidence, and other observations all of which point in the same direction have been recorded in the literature. For example, Stafford, Magaldi, and Ven- neslandls have obtained evidence of the presence of a tartaric acid oxidase in lupine seedlings which attacks (-)-tartrate and meso-tartrate but not (+)-tartrate. Axelrod and Sewe1l'"ave found that (+)-tartrate behaves as an inhibitor of prostatic acid phosphatase, a property that it shares with D-glyceric acid and a number of other u-hydroxy acids of the D family, but which is not possessed by (-)-tartrate." It is obvious that interest in the biochemical relationships of the isomeric tartaric acids is rapidly increasing and that the problem of clear and unequivocal communi- cation has become acute.

For the present, i t would seem essential for investiga- tors invariably to specify the direction of the specific rotation of the enantiomorph of 'tartaric acid under examination. For this purpose, the names (+)- tartrate, (-)-tartrate and meso-tartrate convey full

' a VICKERY, H. B., J . Bid. Cham., in press. Is KUN, E., J. Bid. Chem., 221, 223 (1956).

KUN, E., AND M. G. HERMANDEZ, J. Biol. Chem., 218, 201 (19561. . .

l6 STAFFORD, H. A,, A. MAOALDI, AND B. VENNESLAND, Ssience, 120, 265 (1954).

l6 AXELROD, B., AND G. E. SEWELL, Fedemlion Proe., 15, 214 (1956).

1' Axelrod and Sewell unfortunately refer to their inhibitory substance as btartrate, but the internal evidence in the pub- lished abstract of their paper makes it almost certain that (+)- tartrate is meant.

JOURNAL OF CHEMICAL EDUCATION

and unequivocal infomation. To add one or other of the capital letter prefixes commits the author to acceptance of one or other of the opposing views set forth by Nenitzescul and by Abernetb~.~ This is, of course, an author's privilege, but until agreement has been reached among the many groups of chemists who are concerned, a "wait and see" attitude would appear t,o he more conservative.

In order to keep the facts of this confusing matter clearly in mind, it may be helpful to employ a device long used to explain the configurational relationships of the isomers of cystine to students of biochemistry. It is the blackboard equivalent of the imaginary magnification process referred to above. The struc- ture of L-cystine may be written as in formula VI. This shows unequivocally that both asymmetric centers belong to the L family. When the formula of (+)-tartaric acid is written in the same style, as in

formula VII, it is equally clear that hotb asymmetric centers belong to the D family; furthermore the fact that carbon atoms 2 and 3 are both a carbons is empha- sized. When the formula is written as in I, there is a tendency to regard carbon 3 as a B carhon and to assume that the three groups attached to i t fulfill the same function as they do in L-threose (111). The fallacy in this sssumption has been pointed out by Gorin.I8

COOH COOH 8"" coon NH-k-H KIT-&-H H --OH H - L O H

I i J p. CH? I S

~Cyst ine D(+ )-Tartaric acid (W (VII)

" GORIN, G., J. CHEM. EDUC., 33, 478 (19561.

VOLUME 34, NO. 7, JULY, 1957