CELLULOSE, GLYCOGEN A N D STARCH THOMAS JOHN SCHOCH Corn Products Refining Company, Argo, Illinois
T H E HIGHER molecular weight polysaccharides are primarily of interest in the field of plant biochemistry, where they occupy a status somewhat analogous to that of the proteins in the animal organism. Rather belatedly, it has been realized that polysaccharides may also have a definite importance in the functioning of the animal organism ( I ) , a subject which would seem to offer further opportuqty for investigation. In this review the more important carbohydrates will be considered from a functional and mechanistic viewpoint, interpreting their properties astheresult of grossmolecular shapes and forces rather than of specific chemical groups. Thus, glucose is the predominant sugar in nature, providing the common building block for such typical polysaccharides as cellulose, glycogen, and starch. Yet these three substances differ enormously, not only with respect to their individual functions in nature hut likewise in their intrinsic physical and chemical properties. These differences must be attributed primarily to the shape of their respective molecules, the fashion in which the glucose units are built up to give the polysaccharide. The cellulose molecule (2) is perhaps the simplest t "w e of hieh carbohvdrate. an extended linear .> .~olvmeric " chain of hundreds of glucose "units*' connected by normal primary chemical linkages (Figure 1). It has been difficult to establish the number of glucose units in the cellulose molecule, since the methods employed to purify, dissolve, and process the cellulose almost certainly cause extensive degradation, usually by hydrolytic scission of the Pglucosidic bonds between the glucose units. From X-ray, osmotic pressure, and viscosity data, it can only be said that the chain is a t least two hundred glucose units in length and more probably upwards of a thousand units. Celluloses from different botanical sources will have different chain lengths, and there is undoubtedly a wide spectrum of molecular sizes in any one sample. This has been indicated experimentally by fractional precipitation of cellulose acetate, thoueh here aeain there is some auestion whether the
-
observed heterogeneity is truly representative of the original native cellulose or whether it is due to degradation during preparation of the acetate. Nature has one primary function for such linear chain molecules, to lend structural strength to living matter. This is accomplished in part by arranging the linear molecules in side-by-side fashion, to build up bundles or strands having a certain degree of lengthwise strength. Thus a "long-fiber" grease pulled apart between the fingers will form strands or fibers. But mere linearity alone does not provide sufficient strength, since the molecules may still slip past one another much too easily. Some sort of cementing force between adjacent molecules is necessary. In the cellulose molecule, this issupplied by the hundreds of free hydroxyl groups along the h e a r chain, three to each glucose unit. Highly polar groups (such as hydroxyl or amine) exhibit a weak extra-molecular attraction for one another, of the nature of hydrogen bonding or secondary valence. While the attraction between any two hydroxyl groups is far less than that of a single primary chemical linkage, the aggregate associative force of hundreds of hydroxyl groups along the cellulose chain may be very considerable. The result is that the molecules of cellulose are laid down in a sort of three-dimensional brick wall, the attraction between the hydroxyl groups of adjacent molecules serving as the "cement" (Figure 2). Indeed, these forces of attraction are so strong that when two molecules touch, there is something akin to a "zipper" action to bring them into complete alignment. Once aligned, it is perhaps easier to break the molecules in two by hydrolysis than to "un-ripper" them mechani. cally. Thus we may conceive of mechanical hydrolysis, the shearing of the cellulose molecule by cutting a piece of paper with a. scissors. This concept has been suggested by Staudinger to explain the molecular degradation of solid polystyrene by ball milling, and by Cohen in connection with the ball milling of proteins, where
* The term "gluoose unit" refen to the basic a-D-glucopyrsnaside group, CsHLa06.
Figure 1. Polymerio 8-Glnoopgranodde Chain of Celluloss. Aldehydic T.rminn. D..iyml.ted bp i b t e k k . Con-ntiond Numbering of Cprhona in Glucose Residue. Indica,.d by Circ1.d N.2m.r.b.
~ i g ~ r2.a Lineev Alipnment of C d l ~ l o sC~M N . Amoeieti.. C.o~-bondin*.
Ei26
Dots Indic-ta
NOVEMBER. 1948
peptide splitting, deamination, and scission of disulfide bonds are observed. Actually, this degree of orientation represents an ideal situation which is seldomly encountered in n a t u r e f o r very good reasons. A tree whose cellulose molecules were all laid down in this perfectly oriented pattern would have tremendous rigidity hut no flexibility. Consequently, it would shatter in the &st strong wind. Similarly, a fully oriented cellulose fiber would possess great tensile strength but could not be knotted without snapping. To provide flexibility, cellulose is partly organized and partly amorphous (Figure 3). These areas of parallelwise association of chains are termed micelles or crystallites. X-ray evidence indicates that a single cellulose chain may extend through and interconnect a number of micellar areas. The latter represent a truly crystalliidstate, and the cellulose molecules in such a pattern will diffract X-rays to give a spectrum, from which can be calculated the distance between the linear chains and the periodicity of the repeating glucose units. The intensity of the X-ray pattern also indicates, a t least qualitatively, the degree of orientation within a particular sample of cellulose. Thus ramie and tiax exhibit the greatest degree of orientation and possess maximum fiber tensile strength. Cotton has moderate orientation and wood cellulose least. The macrostructure of certain cellulose fibers is such that the micelles are arranged lengthwise in the fiber. More frequently, the structure appears to be more complex. With cotton, the strands of crystallites seem to be organized into fibrils which twist mound the cotton fiber a t a pitch of 30°, alternate layers being laid down in opposite direction (Figure 4). This is in accord with the best building practice for sheathing a house, where maximum strength is realized by nailing the sheathing boards in diagonal fashion on the framing. The shrinkage of cotton might be explained as due to lateral swelling of the fibrils and consequent shortening of the fiber. These intermolecular forces have a further important effect-they render cellulose insoluble in water. For while wat,er can penetrate into theamorphous and disorganized interstices between the micelles and thus swell the cellulose to a limited degree, it cannot pry apart the chains in those areas where theyare arranged in side-by-side fashion. In order to dissolve the micelles, it is necessary to .use an agent which will associate preferentially with the cellulose molecule and satisfy its intermolecular forces. For example, cellulose can be dissolved in cuprammonium hydroxide or in aqueous solutions of the quaternary bases, since these agents have a stronger attraction for the hydroxyl groups than the latter have for each other. The solvent action of the quaternary bases is directly related to the molecular volume of the substituent groups (R', R2, R8, R4), the - N-OH acting as the thin edge of an entering wedge to split apart the cellulose micelle (3). Another device which is employed to minimize the associative tendencies of the cellulose chain is partial derivatization of the hydroxyl groups, as by the intro-
627
C
Fiym.. 3.
Micell.. Org.ni..tion
I
of Natural C.1lulo..
duction of a small proportion of methyl, ethyl, or hydroxyethyl groups. Depending on the degree of substitution, these products can be dissolved in water or other solvent to give clear stable solutions. This effect cannot be ascribed to reduction of the hydroxyl content of the cellulose since in the case of hydroxyethyl cellulose the number of hydroxyl groups is identical with that of the original cellulose. Hence the substituent ether groups must act as irregularities along the cellulose chains, interfering with orderly side-by-side alignment, much in the same fashion as a faulty "zipper." As might he expected, such products do not yield filaments of high tensile strength, but they do exhibit good plasticity and are consequently employed in lacquers and coatings. In the manufacture of rayon fibers, it is more practical to dissolve the cellulose by blocking intermolecular association through formation of the xanthate: Cellulose-O\
C=S
NaS/
This derivative dissolves to give a viscous solution in which the chains arerandomlyarranged. The solutionis then extruded through spinnerettes into an acid bath which splits off the xanthate groups and regenerates the cellulose. During the process of spinning and especially during subsequent stretching of the filament, the cellulose chains are pulled into lengthwise orientation and intermolecular association is again established (Figure 5), though not in the original micelle pattern of the native ccllulose. In describing this orientation by stretching, S. S. Kistler (4) has used the delightful simile of " ...orienting a plate of badly confused spaghetti by pulling it from opposite sides." The substance, glycogen, presents a striking contrast to cellulose. This polysaccharide is obtained from the liver of animals, the muscle tissue of certain hivalves, and from such vegetable sources as green sweet corn (6). Like cellulose, glycogen is composed of hundreds of glucose units, though these are polymerized through or-glucosidic linkages rather than &linkages as
Figure 4. Structure of the Cotton Fibs.. Winding of Alternate Showin. Re-Fibril Lagem
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628
in cellulose. It is readily soluble in water and these solutions can be frozen and thawed without insolubili5 iug the glycogen, a treatment which renders many colloidal substances insoluble. No one has ever succeeded in spinning fibers from glycogen or from its acetate. Nor is it possible to obtain an x-ray pattern from glycogen; it always seems amorphous and dissociated. While glycogen has exactly the same proportion of free hydroxyl groups as cellulose, there is little or no evidence of the sort of associative forces which bind together the linear chains of ~ellulose. The facts of the matter are that nature has designed glycogen as a reserve food depot, not as a structural building material. Consequently, it has to be readily and immediately available to natural enzymes, which are often incapable of digesting associated polysaccharides. Particularly in the animal organism, this feature of immediate availability is important, since the liver is rapidly depleted of its glycogen inresponse to such stimuli as fear. So we 6nd that glycogen has a highly branched or tree-like structure (6), composed of perhaps a hundred short linear branches, each some 9 to 11 glucose units in length (Figure 6). The shape of the molecule is therefore roughly globular or spherical, a form which is as incapable of any sort of orientation as a bushel of apples. Chemicral evidence is in accord with this structural concept of the glycogen molecule. Its reducing value toward copper or ferricyanide is almost negligible, indicating that the content of terminal aldehyde groups is vanishingly small. If glycogen is fully methylated so that all free hydroxyls are converted to methoxyl, subsequent hydrolysis aad identification of the resulting methylated glucoses give a clue to the original structure. Approximately 10 per cent of 2,3,4,64etramethylglucose is formed, derived from the end glucose units of each branch, since these have free hydroxyl groups on Carbon 4. Hence the average branch length must be 9 to 11glucose units. An equivalent amount of the 2,3dimethylglucose is formed from those glucose units which participate in branching, showing that the branching must take place through the Carbon 6 position (Figure 6). Further evidence for a branched molecule is affordedby the action of the enzyme p-amylase, which attacks the nonaldehydic terminus of an or-glucopyranose chain, successively splitting off two glucose units a t a time in the form of maltose. Enzymatic degradation is halted if a branch point is reached. When treated with p-amylase, glycogen yields 47 per cent of maltose and 53 per cent of so-called "limit dextrin," the latter representing that portion of the molecule protected from enzyme attack behind points of branching.
Next we come to starch, which exhibits certain of the properties of both cellulose and glycogen, as well as some special peculiarities of its own. Our concepts of starch have changed so completely during the past ten years that some redefinition of the substance is necessary. For present purposes, starch may be defined as the naturally occurring a-glucopyranoside polymer found throughout the vegetable kingdom in the form of minute spherulites or granules, insoluble in cold water and birefringent under polarized light. As will be shown, it is necessary to exclude any reference to blue coloration with iodine, though this has heretofore constituted a standard criterion of starch. Also, it seems advisable to exclude those obscure starch-like substances which are present in dissolved or amorphous form in the plant (particularly in the leaves) and which may be the forerunners of granular starch. While starch functions as the reserve carbohydrate of the germinating seed or tuber, germination is a slow process and the ready availability of glycogen is unnecessary. Consequently, the starches are organized into discrete granules which are insoluble in cold water. The size and shape of the granule are characteristic for each kind of starch and a qualified observer can usually identify the source of the starch by microscopic examination (7). Thus potato starch granules are large (15 to 100 p in diameter) and oval in shape, with pronounced "oyster-shell" striations. Corn starch is smaller (10 to 25 p) and often polygonal, the result of internal pressures within the kernel during development of the starch. Tapioca starch is oval and frequently truncated or cup-shaped. Many of the starches appear to be lamellated, as if formed by accretion of successive layers of carbohydrate. It has been reported that starches grown in a constant environment (i. e., constant light, temperature, and humidity) do not show these lamellations, which may therefore represent periodic growth rings. In addition, the behavior of starch granules toward mechanical crushing or toward cold concentrated acid suggests some sort of radial or "trichitic" organization. So we might consider the granule as a spherocrystal, a so* of mismated cross between an onion and a sycamore "button-ball." In further confirmation of its spherocrystalline character, the granule shows a brilliant Maltese cross birefringence pattern when viewed under the crossed Nicols of a polarizing microscope. This orientation must extenddown to tbemolecular level, since the granular starches all give crystalline X-ray patterns. While the latter differ from that of cellulose, they reveal that the molecules themselves are organized in side-by-side fashion and must therefore have a certain degree of linearity. The synthesis of starch in the plant proceeds through a series of reactions: CO,
rigurn 5 .
Orientation of Celluloas Chhns i n Rayon Fiber b y Lengthwise Stmtching
+ H20
- Glucose
Glucose 1-phosphate
-
Starch
The preliminary step in this process-the photochemical formation of glucose-is still very obscure, though efforts are being made to trace its mechanism through the use of radioactive C*02. The glucose is enzymatically
velopment of the starch. I t has long been thought that these starches contained two different carbohydrate substances, and numerous attempts have been made to effect their sepa,ration. One of the oldw methods involves aqueous leaching of partially swollen starch granules. While the granule is completely insoluble in cold water, it undergoes progressive swelling when heated in aqueous suspension beyond the so-called gelatinization temperature of about 60%. The Maltese cross polarization pattern disappears and the granule swells five or more times in diameter, giving rise to the familiar viscous consistency of a cooked starch paste. A F i w n 6. Branohed Btructum of Gbcosen, Shoring t h e e - l b Branch point. On M.th,.htion. Gluc- Remidus A. 6,and CYidd Dimethylsmall proportion of soluhle polysaccharide diffuses from ,,l,,co.s. Trim.thylgluco.. and T.t..methylglucoe.. Rwpectirdg. the swollen granule into the aqueous substrate. This Aldahydio To~minusof Brenched Molecule Indicated by Rete~iak. material has a number of characteristics which differDotted Portion Indicat- Terminal Branches Removed by B-Amyl-. entiate it from the whole starch substance; it yields converted to glucose l-phosphate and this is then poly- with iodine a blue color four or five times more intense merized through the agency of phosphorylase enzyme to than the parent starch. and its solutions show a proa linear chain of alpha-linked glucopyranose units nounced tendency to "retrograde," a term applied to (Figure 7). This reaction has been duplicated in uztro the spontaneous formation of insoluble aggregates. by acting on glucose l-phosphate with the phosphoryl- This soluble fraction has sometimes been designated as ase from potato juice in the presence of an activator the 'Lamylose"; the residue of swollen but undissolved such as starch or glycogen (8). While the mechanisms granules has been termed the "amylopectin." We now know that this method of fractionation is of enzyme actione---either synthetic or degradativeare still very obscure, it has been presumed that the en- very imperfect. Nevertheless, K. H. Meyer and his aszyme attaches itself to the activator, then coaxes the sociates have identified the amylose and amylopectin glucose l-phosphate into position and effects a coupling fractions as linear and branched types of glucose polythrough a-1,4 linkage, with the elimination of phos- mers, respectively (10). The evidence for these strucphate. By successive repetitions of this process, there tures miy be summarized as follows: (1) When subjected to methylation analysis for nonis built up a long linear polysaccharide chain, the socalled "synthetic starch" of Hanes. It is soluble when aldehydic terminal glucose units, the linear fraction first prepared and gives a blue color with iodine, quali- gives a yield of 2,3,4,6-tetramethylglucose correspondtatively identical with that produced by most starches. ing to a chain Ihngth of some 300 to 400 glucose units. Solutions are unstable and the material flocculates Similar methods applied to the branched component spontaneously, this insoluble precipitate giving an X- indicate a branch length of 25 to 30 glucose units. The ray spectrum similar to that of granular potato starch. primary difference between glycogen and the branched Like cellulose, these linear molecules tend to associate component of starch is therefore one of branch length. (2) Each molecule, whether linear or branched, has in parallehise manner, giving aggregates which are too large to remain in solution. However, i t should be only one aldehydic terminus. While i t is not possible noted that this material differs from cellulose in possess- to determine the absolute aldehyde content with certainty, an approximation can be had from the reducing ing only alpha linkages. There is a second enzyme system in brain and liver value toward ammoniacal silver oxide. The reducing tissue which assists in the synthesis of an entirely dif- value of the linear component is of the order of 400 gluferent type of polysaccharide. When a linear carbohy- cose units, coinciding with methylation assay for the opdrate has been built up to a certain chain length by posite nonaldehydic terminus and therefore indicating phosphorylase synthesis, this second enzyme appears a linear chain. The reducing value of the branched to provoke a point of branching, presumably by attach- fraction is much smaller, equivalent to a molecular size ing glucose to a Carbon 6 position (9). The phosphoryl- of a t least a thousand glucose units. (3) &4mylase converts the linear fraction to subase then continues to add glucose to both these ends, with occasional branching to produce a tree-like struc- stantially 100 per cent maltose; hence the linear chain ture. This polysaccharide is entirely different from the linear "synthetic starch," since it is relatively stable in solution and yields red or purple colorations with iodine. Its properties and structure seem to be more closely related to those of glycogen. Indeed, the latter substance is probalJy synthesized in the animal organism by a similar joint enzymatic action. The normal starches (corn, wheat, potato, tapioca) contain both types of polysaccharides, linear and branched, seemingly produced concurrently during de-
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presents no structural hindrance to enzyme action. The branched fraction gives only 50 per cent maltose, furt,her conversion being blocked by branch points. (4) Consistent with its linear character, the acetate of the linear component can be fabricated into fibers whose tensile strength compares favorably with those of cellulose acetate. No such fibers can be formed from the acetate of the branched component. (5) When a solution of a linear high polymer is placed between two concentric cylinders, one of which is rotated, the mdecules tend to line up in the direction of flow. This system then behaves as an optical grating, and the extent of molecular alignment can therefore be meaeured by the degree to which the system polarizes light. Under these circumstances, the linear fraction is readily oriented, while the globular branched component exhibits no such alignment. In the past few years, a new technique of starch fractionation ( 1 1 ) has been perfected which yields starch fractions of highest purity, without the hydrolytic degradation or physical changes inherent in earlier methods of separation. Briefly, the starch is gelatinized in hot water and the paste either boiled or autoclaved to dissolve the swollen granules. Rutyl or amyl alcohol is t,hen added and the mixture cooled to room temperature. The linear component separates as a crystalline complex with the alcohol, in the form of microscopic needles or beautifully formed six-petaled rosettes. These are collected by high-speed supercenfrifuging. This product may be readily purified by dissolving in hot water and cooling in the presence of butyl or amyl alcohol. The branched component remains dissolved in the mother liquor and is recovered by Rocculation with excess methyl alcohol. Since the nomenclature of the starch fractions has been sadly twisted and confused by past misuse, it is preferred to designate the linear and branched components merely as the "A-fraction" and "B-fraction," respectivelv. While amyl alcohol is the be arcomplished by any polar organic compound capable of hydrogen-bonding. Other effective agents are the aliphatic and alicyclic alcohols, the higher fatty acids, nitro compounds, esters, ethers, and mercaptans. According to this method of fractionation, corn starch rontains 28 per cent of the linear A-fraction, potato starch contains 22 per cent and tapioca 17 per cent. There is a sharp contrast betveen the physical properties of these two components. The A-fraction is quite soluble in hot water but these solutions are highly unstable. If a 5 per cent solution of the linear corn Afraction is cooled to room temperature, it sets up to a rigid irreversible gel, which might be pictured as a sort of L'brush-pile" of interlocking linear molecules. At lower concentrations, the A-fraction "retrogrades" as an insoluble precipitate, representing an orderly crystalline aggregation of linear chains. No amount of reheating can liquefy the A-fraction gel or redissolve the precipitated aggregates. Only caustic alkali or the quaternary bases can effect solution. In contrast, the
JOURNAL OF CHEMICAL EDUCATION
branched B-fraction yields viscous but stable solutions; it is this component which is responsible for the technologically useful properties of starch as a protective colloid and as a sizing agent. The two fractions are best characterized by their reactions toward iodine. The linear A-fraction binds iodine as a blue complex; indeed, the familiar blue starchiodine coloration must be attributed entirely to this component. The branched B-fraction has little aflinity for iodine, giving only weak red or violet shades. This reaction has been utilized for the quantitative estimation of A-fraction, either by spectrophotometric evaluation of the intensity of blue color or by potentiometric titration with iodine. Values for the various starches are in close agreement with actual yields by preeipitation with amyl alcohol. In aqueous solution, the linear A-fraction molecule normally assumes an extended position, though prohably with a certain degree of random kinking. In the presence of iodine or such specific precipitants as amyl alcohol or fatty acid, the A-fraction presumably coils into helical form, with the iodine or alcohol located on the inside of the spiral (19). This concept was first proposed on purely hypothetical grounds, to explain the blue color of the starch-iodine complex. Since the hydroxyl groups would be located on the exterior of such a helix, the inside surface would be essentially hydrocarbon in character and the iodine might therefore assume a color approaching that of its solutions in nonpolar hydrocarbon solvents. Recently, R. E. Rundle and his associates have shown that the iodine or alcohol complexes of the linear component give a peculiar type of x-ray spectrum, corresponding in molecular spacings to an arrangement of closely packed helices. While this concept is undoubtedly attractive, it should perhaps be regarded with certain reservations, since iodine yields blue colorations with such widely diverse substances as polyvinyl alcohol, basic lanthanum acetate, and benzylidenephtbalide. No one has adequately explained the individual functions of the two fractions in the plant., While most of the common starches contain 15 to 30 per cent of linear material, there is a group of so-called waxy or glutinous starches which consist entirely of branched polysaccharide (IS). Chinese waxy rice first attracted attention almost a century ago by reason of its red coloration with iodine. Subsequently, waxy strains of corn, sorghum, and other cereals have been discovered. Since the waxy characteristic is genetically recessive, it has been suggested that these cereals represent a primitive variety, largely bred out during centuries of cultivation. The waxy starches show much the same gelatinization behavior in hot water as do the normal starches. However, due to the absence of any linear component, these pastes are much clearer and less inclined to retrograde than are the normal starches. Consequently. substantial amounts of waxy maize and sorghum starches are being produced in this country, for those uses where their higher paste stability is technologically advantageous.
NOVEMBER, 1948
At the other end of the scale, it has recently been found t,hat.the starch of oommon wrinkled-seeded garden peas is composed largely of linear material (14). When an aqueous suspension of this pea starch is heated, the granules swell only slightly, then become completely insoluble without ever forming a viscous paste. Given a certain freedom of motion by the slight swelling action, it appears that the linear molecules "zipper" together, becoming insoluble and resistant against further swelling. A sugary mutant of corn has just heen reported containing 50 to 66 per cent of linear material (15). It has been suggested that the ratio of ohos~horvlase and branch-synthesizing enzvmes " r e d a t e s the proportion of linear i n d branched poGsaccharides in the plant. It seems plausible that the starches of wrinkled pea and sugary corn might he produced in the absence of the enzyme responsible for branching. However, this proposal does not explain how both linear and branched polysaccharides can be producgd concurrently in the normal starches. While the waxy starches and the branched B-fraction of the normal starches do not exhibit the exaggerated retrogradation of the linear component, it would be incorrect to say that they have no associative tendencies. In the first place, the granular structure of the waxy starches is indistinguishable from that of the normal starches. They give the same interference cross when viewed under polarized light, and they yield an x-ray pattern indicative of side-by-side crystalline aggregation. The outer branches of the branched molecule have an unbroken linearity corresponding to some 25 to 30 glucose units and these linear segments should exercise a certain degree of attraction for one another, though naturally of a much lower magnitude than the linear fraction. K. H. Meyer has attributed granule structure primarily to regions of .oriented aggregation between the outer branches of adjacent branched molecnles (Figure 8). These so-called "fringe micelles" are believed to provide the coherence which prevents the granules from dissolving in cold water. The swelling and final dissolution of the granule above the gelatin; zation temperature are ascribed to gradual relaxation of the associative bonding within these fringe micelles. As another instance of aggregation, the branched Bfraction is precipitated from solution by a process of freezing and thawing. Under certain circumstances, this precipitated ~ - f ~may~ even ~ exhibit t i ~an ~x-ray pattern similar to that of the native starch. It seems reasonable to believe that an orderly association of the outer branches has been established, through the same sort of "zipper" action which is responsible for the much strongerassociation of linear chains. ~thas been suggeded that the staling of bread is due to a slow crystallization of the branched starch component within the crumb structure of the bread (161. It has already been mentioned that glycogen cannot'be crystallized or aggregated, even when its solutions are repeatedly frozen and thawed. With a branch length of only 9 to 11glucose units, i t appears that glycogen is incapable of any kind of orderly association. A similar situation is A
rimre 8, Schematic Diamam of showingiksoci.tion o ~ ~ d j p c e n Bt
.
~
of the stuch
~
~
-ring.~
T ~ ~O U .~ ~
M'cel"s."
encountered with limit dextrin formed by the action of @-amylaseon the branched starch fraction. Since this material has lost its outer linear branches, it shows no tendency to associate or become insoluble. So there appears to be a close parallel between the polysaccharides of'the plant and animal organisms. The glycogen of green sweet corn disappears as the corn matures, with concurrent development of granular starch in the kernel. Hence the glycogen may represent merely a temporary storage depot of disaggregated and readily available carbohydrate. T o complete the parallelism, it mill be recalled that Claude Bernard in 1877 reported the presence in paralyzed muscle tissue of a "glycogen" which stained blue with iodine (17). It would seem that some abnormality in the enzyme system had synthesized a linear starch-like molecule instead of the highly branched glycogen. LITERATURE CITED (1) STACEY,M., "Advances in Carbohydrate Chemistry," AND M. L. WOLFROM, Academic edited by W. W. PIGHAN plprr N.~-W O. T 1.0.4.,6 .. ~..0. -1,2nn.. . ~fil-201. H F v. .~..w a. - .. . ... - ---- ,. .. V. ..L .,. . .- - .- , T AND H. HIBBERT, ibid., Vol. 2, pp. 203-33. (2) MEYER.K. H., "Natural and Synthetic High Polymers," Intersoience Publishers, Xew York, 1942, pp. 221-345; for an exhaustive treatise, see "Cellulose and Cellulose Derivatives." edited bv E. Om. Interscience Publishers. New York., '1943. (3) LIESER,TH.,AND R. ERERT,Ann., 528,276-95 (1937). (4) KISTLER, S. S., "Advancing Fronts in Chemistry," edited by S. B. Twrss, Reinhold Publishing Corp., New York, 1945, Vol. 1, pp. 15-23. (5) MORRIS,D. L., A m C. T. MORRIS, .I. Bid. Chem., 130,53544 (1939); SUMNER, J. B., AND G. F. SOYERS,Arch. Bioehem., 4,7-9 (1944). (6) METER,K. H., ''Advances in ~nzymolom,"'editedby F. F. NORD,AND C. H. WERKMAN, Interscience Publisher% Yew York, 1943, Vol. 3, pp. 109-35. (7) RE~,,E,, E. T.,
