Henry Klostergaard California State University Northridge. 91324
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Textbook Errors, 124
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Inversion of Sucrose and Fructose Structure
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The molecular structure of crystalline sucrose, ordinary cane sugar, is well established on the basis of very precise neutron-diffraction analysis ( I ) in 1963. Previously in 1947 and 1952 X-ray diffraction analysis with less satisfactory data had suggested that the structure was that now accepted with resnect to the assienments of asvmmetric carbons (see references in (I)). 1n"carbohydrate terminology the formula assigned is a-D-glucopyranosyl-P-D-fructofuranoside. The total laboratory synthesis of sucrose in 1953 by Lemieux ( 2 ) is consistent with this assienment. It is a peculiar twist of the history of chemistry that the elucidation of the structure of sucrose, which is produced annually in larger amounts than any other substance of similar purity, should be of such recent origin. As a disaccharide, sucrose is somewhat peculiar in that it is non-reducing, forms no osazone and does not mutarotate. These properties are predictable from its formula. Prohably the most frequently mentioned reaction of sucrose in chemistry textbooks is its hydrolysis in which the net reaction is the disappearance of one molecule of sucrose and one molecule of water and the appearance of one molecule of D-glucose and one molecule of D-fructose. When this reaction is carried out in the laboratory, a catalyst such as hydrogen ions or an enzyme is generally used. The new compounds are both reducing sugars, hoth form osazones (the same) and hoth mutarotate. The acid catalyzed rate of hvdrolvsis is lareer hv three orders of maenitude than the rate observed wyth &st other di?acchariies and it is further peculiar that, although sucrose shows (+)-rotation, the equilibrium mixture of reaction compounds shows (-)rotation. The rotation is said to invert: the enzyme cavahle of catalyzing the reaction is called invertase; the reaction mixture is called invert sugar and the reaction itself is called inversion of sucrose. I t has an honored place in the history of chemistry because Wilhelmy's study 125 years ago of the rate at which this reaction proceeds (according to his measurements) can he said to mark the beginning of modern chemical kinetics. Upon further investigation, it has become clear that the reaction is much more complicated than first envisioned. Nevertheless, many textbpoks in organic as well as physical chemistry still treat the reaction as if no further light has been thrown on it, and some have even introduced new errors. Peroetuation of these errors is esveciallv- remettable because tends to hide some elegant and interesting chemistry from students and instructors and because it suppresses speculation ahout some puzzling problems of mutarotation in carbohydrate chemistry and about biosvnthetic pathways. The evidence which has forced a change in our views of the reaction has been built up over a long span of time; only a small part of the more interesting results will be touched upon here. In 1916 Haworth and Law (3)reported the specific rotation of octamethylsucrose to be +66.7O and of itshydrolysis (mild) products, two tetramethylhexoses, +57.0°; these values are in contrast to +66.5O for sucrose and -20.0° for its hydrolysis mixture. They found further that the tetramethvlhexoses could he fairlv well seoarated bv fractional distiliation in vacuum, and that one-was identical to the 298 / Journal of Chemical Education
product ohtained by tetramethylation of D-glucopyranose whereas the oilv tetramethvlfructose showed (+)-rotation as contrasted io the tetramethylfructose obtained from crystalline D-fructose which shows (-)-rotation, Certainly something else takes place in the hydrolysis than just scission of the 0-linkage under their experimental conditions and this change appears especially in the fructose moiety. Both a - and 8-D-glucopyranose can be ohtained in pure crystalline form (specific rotation +112O and +lgO,respectivelv). Their mutarotations to e~uilibriuma t +52.5O result in a:> ratio of approximately 36:64 and can be investigated under anv chosen condition: that aspect of sucrose inversion can be considered clarified. During the three next decades following Haworth's work, extensive experimentation was conducted on D-fructose under different conditions of concentration, temperature, preheat treatment, etc. to study factors influencing rotation, mutarotation, fermentahility, etc. Typical references are (4) and (5)which contain further references. Although several of the quantitative results derived in these works are grossly in error for various reasons, the following facts were established: (1) If the fermentation of a rapidly fermenting D-fructose solution a t room temperature is suddenly terminated, the residual solution will continue to mntarotate for a while towards less (-)-rotation. This result suggests an equilibrium in which the highest (-)-rotation form is less or not at all fermentable, and (2) When crystalline D-fructose known from X-ray diffraction analysis to be in the pyranose form is added at 0°C to a yeast suspension, the fermentation is extremely slow. When a fructose solution prepared at room temperature is treated with yeast a t O°C, fermentation starts immediately a t a respectable rate, but comes rather suddenly to a virtual standstill with the major part of the fructose still unreacted. This experiment suggests that only the furanose form is fermentable, that this form is a minor fraction of the startine material. that a t lower temoerature the mutarotation pyranose-furanose is limiting the fermentation rate, and that the mutarotation has a hieh temoerature deoendence. (3) Preheated fructose when dissoived and treated with yeast shows-initially a higher rate of fermentation, which gradually levels off, suggesting that the furanose form is favored by higher temperature. In 1962 Andersen and Degn (6) obtained substantial clarification of the fructose dilemma. Using a highly active 8-fructofuranosidase, they hydrolyzed sucrose at room temperature so rapidly that in 3 minutes 99% was split. The subsequent mutarotation of a-D-Glucopyranose was . --
Suyg~stiunsuf material suitnhle fur this column and guest columns suitable for puhlicnrion directly should be sent with as man" details as possible, and particularlv u i i h reference to modern t e x t buoks. to \ V H. Ehcrhnrdt, School uf Chemirtr). Georgia lnatiture of Technoion. A t l n n u . Georgia . I I I W ' Since the-purpose of this column is to prevent the spread and continuation of errors and not the evaluation of individual texts, the sources of errors discussed will not be cited. In order to be presented, an error must occur in at least two independent recent standard books.
er the rotation has been adjusted for increase in concentrapreviously clarified and therefore the mutarotation of 0tion and volume contraction during the hydrolysis in the D-fructosefuranose could he calculated by difference. Altube. The data represent readings which would he obtained though their data may he too optimistic with respect to ahif i t were possible to carry out the three individual steps insolute accuracy since apparently volume contraction was dependent of each other. disregarded, the inversion reaction is now substantially Furthermore organic textbooks giving structures for Dclarified. The reaction has three phases: hydrolysis, a-8fructose should show 3-fructouvranose comurisine 68% and mutarotation of a-D-glucopyranose and mutarotation of 8-fructofuranose most of thk--remainder;-if a-iorms are 8-fructofuranose to 8-fructopyranose, each with its own shown at all, it should he stated that they are present only rate constant and temperature dependence. Thus. unless in very small concentrations. simplifying experimental conditions are used, thekinetic Textbooks also make statements that five-membered picture can get very complicated. In Wilhelmy's case the rings (furanwe) are relatively rare. Such statements serve hydrolysis step was rate determining. no useful purpose because they are very ambiguous. On The final invert sugar solution contains 4 maior compobasis sucrose is not rare; neither are nucleic acids. nents: 34% 8-D-fructopyranose, 32% ~ - ~ - ~ l u ~ o p ~ r a n o stonnage e, And on a species basis nucleic acids refute the statement. It 18% a-glucopyrauose, and 16%8-fructofuranose. The other would he more useful to raise the ouestion with the student species, namely, the open chain, a- and 8-glucofuranose why furanose forms occur a t all alihough they are discrimiand n-fructofuranose and a-fructopyranose, can be ignored nated against thermodynamically a t physiological temperain this context, the last two because the data of Andersen tures. I t may he appropriate here to note that if the hiomeand Degn allow only one reaction to take place for 8-frucchanism involves the phosphate ester of the primary alcotofuranose (unless the 3-n-mutarotation is verv fast). hol group most remote from carhonyl group, aldopentoses To il1ust;ate the ovdra~lprocess suppose we atart kith a pvraand ketohexoses are limited to the furanose form:. the .sucrose solution containing 10.0 gI100 ml in a polarimeter nose formation is blocked., tube long enough to give a rotation of +66.4O a t 589 nm and Finally, in view of the experience with D-fructose, it 20-25'C. The hvdrolvsis . reaction alone (nothim else hapwould seem warranted to reexamine the strange mutarotapening) reduces the rotation to +56.1°, nameiy, +59.io tions of some of the aldopentoses and disaccharides. from a-glucopyranose and -2.4' from 6-fructofuranose. Mutarotation of the glucose to equilibrium results in a deLiterature Cited crease from +59.1 to +27.S0 or a drop of 31.2", mutarota(1) Bmwn,G.M.,and Levy.H.A.,Srienre, 141,921 (19631. tion of fructofuranose to its equilibrium with 8-D-fructopy(21 Len3ieux.R. U..sndHuber.G.,u'. Amer Chsm. S a c . 75.4118(19531 (31 Haworth. W. N.,and Law.J. J. Chem S o c , LW. 1314 (19161. ranose results in a change from -2.4O to -48.g0 or a drop (4) Gott*chslk.A..Biochem J., 41,476 (L947l. of 46.4'. It is this reaction which leads to "inversion". Note (5) Hopkins,R. H..and Horwoal. M.,Biochem. J . 47.65 (19501. (6) Andersen,B.,andDegn. H.,Acto C h e m Scond, 16,215 11962) that the rotations given are not the specific rotations; rath-
Volume 53,Number 5, May 1976
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