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(16) MEYER,J . , AND NERLICH, R . : 11. anorg. Chem. 116, 117 (1921). (17) MORSE,H. N.: Exercises i n Quantitative Chemistiy, p. 468. Ginn and Company, New York (1905). (18) MORSE,H . N . : Am. Chem. J. 18, 401 (1896). AND KELLEY, K. K . : J. Phys. Chem. SO, 47 (1926). (19) PARKS,G . S., M.:Rev. universelle mines 11,367 (1935). (20) POURBAIX, (21) ROGERS, T.H., PIGGOT, C. S.,BAHLKE, W. H., AND JENNINGS, J. bl.: J. Am. Chem. Soc. 43, 1973 (1921). (22) SARKAR, P. B . , AND DHAR,K. R . : Z . anorg. Chem. 121, 135 (1922). .4.,AND EYRING, H.: J. Am. Chem. SOC.64, 2661 (1932). (23) SHERMAN, (24) VOLHARD, M. J . : Bull. SOC. chim. [2] 34,714 (1880). (25) WHCHTER, A.: J. prakt. Chem. 30, 326 (1843). (26) WHITESELL,W. .4., AND FRAZER, J. C. W . : J. Am. Chem. SOC.46, 2&11 (1923).
THE STARCH MOLECULE G . V. CAESAR
AXD
M. L. CUSHING
Research Laboratory, Stein Hall & Co., New York, New York Receaued September 6 , 1940 INTRODUCTION
The structure, size, and spatial configuration of the starch molecule have long been bones of contention. Polysaccharides such as starch, cellulose, glycogen, all yield d-glucose on complete hydrolysis. It is generally agreed that d-glucose forms the principal building block or basic unit of these higher complexes. But how d-glucose is combined in the amyloses, whether or not there may in part be present derivatives of dglucose, the number of units in a single amylose chain (molecular weight), the question of a “second” or associative dimension, the spatial configuration of the molecule,-these and other problems have proved exceedmgly difficult and controversial. One of the purposes of this paper has been to review these questions critically, and another to submit original work of our own, bearing upon the rather understressed but exceedingly important problem of molecular spatial configuration. STRUCTURE OF THE AMYLOSE MOLECULE
h structure of the maltose type is a t present accepted, although Hudson and coworkers have recently demurred (31). The contention of the British school of chemists,-Haworth, Hirst, and coworkers (1, 24, 21, 13, 25, 23, 26, 15, 18, 14, 22, 19, 20, 16, 17, 29, 30),-that amylose is a polymer of glucopyranose units in the maltose (a)linkage, appears to be well founded
777
THE STARCH MOLECULE
experimentally. These workers have obtained, under what they claim t o be gentle conditions of treatment, a yield of triacetylstarch amounting to 96 per cent of the original substance, from which upon removal of the acetyl groups “the original starch could be obtained unaltered.” Their yield of 2,3,6-trimethylglucose was a t least 85 per cent, plus small amounts of 2,3,4,6-tetramethylglucose.These results, however, are directly opposed to those obtained by treating starch with a filtrate from cultures of Aerobacillus macerans (31),which yielded 40 per cent of a curious crystalline “dextrin” (“Schardinger’s” (41)). At the meeting of the American Association of Cereal Chemists held on May 20, 1940, Hudson stated (if the authors understood him correctly) that approximately one-half of this crystalline product appeared to be a cyclic acetal. This is difficult to reconcile with the findings of Haworth, Hirst, and coworkers. The products of the action of Aerobacillus macerans and those obtained by the British chemists should certainly be rationalized. If the “dextrins” are actually present in this proportion in the raw starch molecule, then either very serious errors must have occurred in the technique of the British school or perhaps acetal types may suffer hydrolysis during esterification, etc. I t might also be suspected that, in spite of precautions, these curious products may actually be synthesized by Aerobacillus macerans. Hudson does not believe so (31). For the present a t least, the weight of evidence favors a structure which is of the maltose type. CH-4IN LENGTH O F AMYLOSE (MOLECULAR
WEIGHT)
Farrow and his collaborators (6) have devised a method whereby they claim to determine the copper-reducing power of starch and of modified starches. Searly all workers have stated that starch has no reducing power, although in many cases small iodine numbers are recorded.’ Farrow has modified the usual Fehling’s type of procedure by adding a known excess of glucose to the starch to be determined, in order to saturate the starchy product with cuprous oxide and prevent any errors due to small amounts of cuprous oxide being lost during manipulation, owing either to oxidation or to occlusion with the starch. By this procedure he finds that all unmodified starches possess a small but measurable reducing power. By comparing this reducing power with a standard such as maltose, calculations can be made of the chain length. The values obtained range from 460 to 1470 glucose units, giving a molecular weight of 74,500 to 238,000. These figures compare well with the physical determinations of molecular weight by other workers. Staudinger (45,46) by viscometric methods has found molecular weights On the basis of Farrow’s method, these small iodine numbers actually agree rather closely with Farrow’s results.
\
778
G. V. CAESAR AND M. L. CUSHING
from 100,000 to 286,500, and Beckman and Landis (2) by ultracentrifuge methods have found molecular weights from 25,200 to 390,000. Hess and Lung (27) calculated a molecular weight of 500,000 from the Staudinger viscosity results on methyl starch. Caldwell and Hixon (5), by a chemical method of periodic acid oxidation, found molecular weights to be greater than 22,000 (136 glucose units). All of these results are in serious disagreement with the “end-group” molecular weights of Haworth (23). The number of glucose units by the end-group method average 25 to 30. (Hess and Lung (27) found 52 to 54 for one preparation.) However, Farrow points out that even if one were to start with a single chain of 10,000 units, a 3 per cent hydrolysis (in a random fashion, as seems correct) wocld produce 300 chains with an average chain length of 33.2 glucose units. Thus any slight degradation during end-group determination might lead to a result which is uniformly attained (25 to 30 units) by Haworth’s method. The behavior of aqueous pastes of starches (4)seems difficult to reconcile with the picture of a relatively short chain (25 to 30 glucose units). Haworth’s technique, although undoubtedly careful, nevertheless is drastic, and the glucosidic linkages in both amylose and cellulose are well known to be extremely sensitive to acid treatments. Application of Taylor’s alkali-labile method (50, 57) of analysis of degeneration should be very informative. There is still insufficient proof that the primary chains remain substantially unaffected by acetylation and methylation treatments. A criticism, however, that could be leveled against all methods of chain length measurement lies in the possibility of end groups being occluded within the micelles and therefore unavailable to reagents. This is a very definite possibility. Taylor was inclined to lay considerable stress on such occlusion (52, 47). If certain chain ends are covered up within the micelles, as postulated by Mark for cellulose (33), the values obtained in determinations of the length of the exposed or available chains would be fictitiously increased. We have, on the one hand, the very high probability of degeneration from various treatments, which produce too low molecular weights; on the other hand, we have possible concealment or occlusion owing to associative forces (0-H. . .O)* which would produce too high molecular weights. It is true that association should be less in dilute dispersions than a t relatively high concentrations, and indeed on occasion might be low. But in our opinion and on the basis of our technological experience with starch, strong associative forces are indicated even a t very dilute aqueous concentrations. Since water is more or less associated up to the vapor state, it is inconceivable that polyhydroxylated compounds are ever substantially unassociated. For all these reasons we conclude that the values for the chain length as 2
The symbol for hydrogen bonding employed by Pauling (37).
THE STARCH MOLECULE
779
determined by Haworth and coworkers are probably too low, and that the results indicated by Farrow, Beckman and Landis, Staudinger, and others, may be too high. On the whole we are inclined to favor Farrow’s technique. Investigations of this interesting adaptation are already under way in this laboratory. ((SECO?iD” OR .%SSOCIATIVE DIMENSIOK
In recent years the conception of a covalent coordinate valence linkage through hydrogen (the “hydrogen bond”)3 has been exhaustively investigated and experimentally established (3, 10, 11, 37, 38, 42, 43, 44). Hydroxylated compounds, in particular, have been found to yield this form of linkage, and expression has been given to the thought that it or some other expression of interatomic forces might play a large r81e in starch and cellulose, causing the primary valence chains to associate into bundles or micelles (30, 33, 35, 38, 42, 52). The late T. C. Taylor particularly stressed this conception of a “second” or associative dimension. He supported it publicly in only one of his contributions (52), but before illness incapacitated him he had been accumulating experimental evidence in its favor. We were privileged to be closely associated with Taylor, who was an outstanding investigator of the physical chemistry of starch (56, 51, 53, 61, 48, 60, 54, 59, 57, 58, 50. 55, 52, 49, 47). He was one of the few to appreciate that the technological properties of starch are determined more by physical make-up than by chemical constitution along classical linrs. He challenged the still widely held misconceptions concerning the loosely packed hydrophilic interior and the relatively hydrophobic envelope in the starch granular package (36, 51, 48. 60, 55), the preoccupation with phosphoric acid (34, 39). etc. He shon-ed by electrophoresis separations on the disorganized “shells” of granules that these shells were composed of the same proportions of aamylose4and P-amylose as the whole granule (47). Owing to his increasing ill health, much of the most interesting and valuable portions of Taylor’s researches unfortunately were never put in final form for publication. In justice to his memory, and on account of their very great intere.st and importance, particularly as regards the associative dimension, we takc the liberty of quoting a t length from some of his contributions (47) : “For purposes of classification it might be well to use the term starch solely for the microscopic package or granule and the term amylose with a qualifying prefix for thc material or materials which go t o make the organized system. “The gross structural make-up of this organized system, even after partial gelatinization, may be discerned under the microscope as long as there is sufficient difference in the indices of refraction between the medium and the sample. .i laycr structure ~ _ _ _ _ 3 .it present regarded as largely ionic in character. 4 Or “amylopectin.”
780
G. V. CAESAR AND
M. L. CUSHING
is observed repeatedly in many common starches and crystal-like masses have been described often as existing within these layers. So far, however, there is no convincing data that will allow the correlation of these physical forms with specific chemical entities. “As gelatinization proceeds and a water paste is made, the gross structural chararteristics begin to disappear, the viscosity of the paste which rises rapidly a t first begins to fall and if helped by agitation will fall even more rapidly (4). There is every evidence that a profound physical change is taking place in the microscopically measurable dimensions of the granule. Indeed cursory examination leads one to believe that all structure is gone. While there always is a certain amount of material -well dispersed, still physical and chemical data indicate that even this dispersed amylose as well as the great bulk of material is still highly organized. Viscometric measurements which often reflect molecular magnitude indicate when taken on clear amylose dispersions after repeated dispersing treatments that simplification may come about without there being large enough concurrent chemical changes to warrant concluding that primary chemical linkages are involved in the action a t all. Also, sharp X-ray diffraction patterns that are characteristic of a regular repeated and persistent organization such as in crystal lattices, may be found in the solids precipitated from apparently clear amylose dispersions. “In gelatinization of raw starch there is first swelling of the larger granules without loss in general form, then rupture into fragments with remnants of the original organization and gradual dispersion of some of the fragments into more or less clear pastes. Grinding dry starch in a ball mill before making the paste or roasting an acidulated essentially dry starch in an oven, all in their way alter the course of the physical changes that are discernible in the granules under the microscope during gelatinization of these modified products. Complicating the matter is the fact that the smallest granules in general do not undergo swelling and rupturing as readily as the large, so the paste after gelatinization is very inhomogeneous. Further, since both the swollen granules and to a greater extent the fragmented material have very large surfaces and are jelly-like materials, all manner of colloidal phenomena are in evidence. However interpreted, these phenomena are capable of experimental demonstration and enormous masses of data, much of it conflicting and contradictory, have accumulated in the literature of starch along these lines. “Methylation studies carried out by the British school of chemists and others in the same field have given data from which estimates of the length of the glucose chain formation . . . can be made. “Large as these molecules are linearly, they are not large enough to allow us to explain the obvious complexity of the amyloses nor to account for the many transformations they can be made to undergo. (‘TO achieve the necessary complexity that is possessed obviously by the amyloses, one must turn to phenomena that are recognized in other directions and accepted as plausible in explaining certain experimental experiences, viz. associative forces or hydrogen bonding. These forces are responsible for the ‘second’ or associative dimension of amyloses and cellulose. The combination is symbolized:
R
R
0
0
I
I
I
R
THE STARCH MOLECULE
581
“If this combination happens enough times between parts of two or more molcculcs of the same species, the accumulated effect becomes a large factor in making the associated aggregate act like a chemical entity.5 “This is the condition of affairs in the longer glucose chains with their multiplicity of hydroxyl groups, while glycogen exhibits less of this tendency. . . . Amylose and cellulose have enough more of the functioning groups to show more spectacularly the effects of coordination. “Undoubtedly the stereo configuration of the groups also plays a large part in fixing the effectiveness and extent of coordinate link formation, even if there are many donor and acceptor groups present. . . . “While still highly speculative, the difference between the cellulose with its apparent long chemical dimension and its rather small associative dimension, on the one hand, and the amyloses with their shorter chemical dimension and larger associative dimension, on the other, lies probably in these spatial relations. “Because the completely organized raw starch granule resists change it is no easy matter to obtain much soluble matter by mere heat gelatinization. Indeed it is difficult to disorganize the granule a t all. . . . By and large it is difficult to do any useful work on the almost perfectly elastic particles to cause disassociation of the chains in the sheafs even when the sheafs, so to speak, have been pulled from the original organized granule bundles. The swollen but deformed and still organized granules, being insoluble, are often regarded as an amylose fraction and called ‘amylopectin’. . . . The term is certainly not a precise and easily definable entity despite its age in the literature “ I t is highly probable t h a t many of the differences in viscosity, gelatinization temperature, alkali lability (57, 50), speed of acetylation and methylation, as ivell as rate of enzyme attack . , . are tied up with the availability of groups with whirti to function on single amylose chains. . . . I n other words, n e need not of necessity look for explanations of the many difficulties in the properties of starches to variations of classical chemical configuration that serve N - e l l in the correlation of smaller organic molecules. , . .”
Taylor’s stress upon the associative dimension is well founded. All experimental and technological experience of this laboratory confirm the truth of this conception. SPATIAL CONFIGURATlON
That the amylose molecule is anything but straight has been frequently suspected (7, 8, 9, 12, 13, 39). From a consideration of the nature of the alpha and beta linkages we had long concurred that the amylose molecule was curved in some manner. Very interesting speculations have been advanced by Hanes (12), Freudenberg ( 5 , 8, 9), and no doubt othcrh. Indeed, the conclusion regarding curvature is inescapable from a comparison of the a- and P-d-glucopyranoses, as we shall show. Furthermore, the physical properties of amylose and of the highly organized starch complexes support a molecule that is curved in shape. We proposed to test these hypotheses through construction of molecular 6 It should be pointed out, in respect t o the above associated structure, t h a t in starrh, particularly, a considerable percentage of water is always present, hydrogenbonded to and between the chains and t o a certain extent forcing them apart.
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G. V. CAESAR AND M. L. CUSHING
models of the Fisher-Hirschfelder type, as employed and described by Scattergood and Pacsu (40). Their remarkable conclusions on the effect of the collision diameter of the atoms on the construction of the carbon skeletons of d-glucopyranose suggested that the use of these little scaled spheres and segments of spheres-black for carbon, blue for oxygen, orange for hydrogen-might prove equally informative when assembled into polysaccharide complexes.
A . Construction of models Models were assembled from the Fisher-Hirschfelder atoms of a-dglucopyranose, @-d-glucopyranose, aldehydo-d-glucose, cellulose (three links), amylose (twelve links). In the d-glucopyranose models, and also in the cellulose and amylose models, the carbon skeletons were assembled in a trans-configuration (40). Great difficulty was experienced in holding the amylose chain together at the glucosidic oxygen linkages. Mechanical improvement and strengthening of the valence “plugs” between atoms, while permitting rotation, would be very welcome. The assembled models were photographed. Drawings were made to illustrate the relationships of primary alcohol groups on the cellulose and amylose models, In the photographs, each hydroxyl hydrogen atom is distinguished by a spot.
B. Discussion of models d-Glucose. d-Glucose, the end-product of the hydrolysis of the polysaccharides, was obviously the first model to assemble and study. Figures 1, 2, 4, and 5, are photographs of the a- and @-pyranoseforms in a transatoms (40). Figure 3 illustrates the aldehydo configuration of -C-Cform, included for the sake of comparison. The very important and thought-provoking distinction between the a- and /%forms of d-glucopyranose is beautifully apparent from a study of figures 4 and 5 , which show side views of these little rings of carbon, hydrogen, and oxygen. The secondary alcohol group a t carbon 1 of ad-glucopyranose (figure 4) is located on the side of the ring and is free to rotate only through about 180”. If rotated more than about B O ” , the hydrogen atoms attached to carbons 3 and 5 interfere or “collide.” In @-d-glucopyranose the secondary alcohol a t carbon 1 is located on the top of the ring, so that it is free to rotate 360” (figure 5 ) . Also, in the @-formthe planes of rotation a t 1 and 4 are parallel; in the a-form these planes are a t 90” to each other. The photographs (figures 4 and 5 ) show that when glucose units are combined in a 1,4-glucosidic linkage, the chain of the a-linkage will be curved (amylose); in the @-linkagesit will zigzag back and forth to form an essentially linear molecule (cellulose). The rotational positions of the hydroxyl groups a t 1 and 4 govern the glucosidic attachment of other glucopyranose units. It is possible for additional
'I
784
G. V. CAESAR AND M. L. CUSHING
units to become attached only when tile hydroxyl groups at 1 and 4 are in approximately the rotational positions shown in figures 4 and 5. Interference occurs in other rotational positions from the collision diametem (37) of the atoms. This matter of interference in the polyoses is 8s important as it w8s shown to he in the monoses (40). The positions of the hydroxyl group? at 1 and 4 permitting polymerization to amylose and cellulose might he termed the “critical” rotational positions.
?
FIG.4. e-d-Glueopyimoso (side view) Fm. 5. 8-d-Glueopyrenose (side view)
Amylose and cellulose. Figures 6 and 7 represent photographs of t.welve rings of an amylose and three rings of a cellulose model assembled from &glucopyranose units which are linked 1,4, and CL and p, respectively. These models are interesting and are suggestive of the great differences in the physical properties of the two principal polysaccharides. The cellulose chain is essentially linear; the amylose chain resembles a form of helical spring. Owing to the mechanical difficulties in getting close contacts a t the glucosidir linkages and elsewhere in the chains, precise comparisons of the number of glucose links or rings in a given linear distance
THE BTARCH MOLECULE
785
are untenable; there is also the fact that through slight variations in rot,atioa at the glucosidic oxygen linkages the amylose helix ran he definitely
extended or cornpress~:d. Bot. roughly spmking, it may hc snid that the unstrained amylosr inolt’culc appears to rwntnin ncnrly twice as many
786
G. V. CAESAR A N D M. L. CUSHING
glucosc units in a given distance as the cellulose molecule.6 The effect of this more concentrated grouping of the reactive groups (the hydroxyls) in amylose helps to explain the greater reactivity of cellulose: the hydroxyl groups of cellulose should be more available from a less associated structure. Taylor’s speculations on the effect of spatial relations “in fixing the effectiveness and extent of coordinate link formation’’ (0-H . . ’ 0 )thus appear to be well founded. One of the most interesting facts to be observed from the study of these models of amylose and cellulose is the stereo relationship of the alcohol groups, particularly of the primary alcohol groups at carbon atoms 6. The spatial relationships of these groups is shown in the drawings (figures 8 and 9) for cellulose and amylose, respectively. I n the amylose model (figure 9) the primary alcohol groups twist spirally around the chain a t a uniform spacing of about 4.5 cm.7 (measured from center to center of carbon atoms 6). I n the cellulose model this spacing is about 7.5 cm. The strength and effectiveness of the associative forces:
R-OH..
H .O-R
P R I M R Y ALCOHOL GROUP
FIG.8. Relationship of primary alcohol groups in the cellulose chain P R I M R Y ALCOHOL GROUP
.
r-if \@..
” \
\
FIG.9. Relationship of primary alcohol groups in the amylose chain
Amylose: about 3.3 A. per glucopyranose unit. Cellulose: about 5.3 A per glucopyranose unit. 7 Equivalent t o about 4.5 A. The estimated distances for the 0-He bonds are 2.55 to 2.76 & . (37). 6
. O hydrogm
THE STARCH MOLECULE
787
should accordingly be much greater between chains of amylose than between chains of cellulose, Le., the former would be expected to show the larger “second” or associative dimension, a supposition which accords with the known facts of chemical and physical behavior. From the uniform spiral spacing of one-third of the total hydrogen-bonding groups, the starch micelle might be expected to exist in a twisted rope-like structure, the “rope” being coiled in the form of a helix. Another interesting manifestation is that all of the CHZOHgroups on the amylose model are free to rotate 360°, and appear equally available. This rotational freedom and availability-the primary groups jutting along the outside of the micelle in the form of a spiral fringe-implies that in starch the primary alcohol groups may play a relatieely more important r6le than in cellulose, Also, in the cellulose model (figure 6) the CH20H group marked p’ is wedged tightly and cannot rotate.8 The spacing between the hydrogen of the primary hydroxyl and the oxygen of the secondary hydroxyl on carbon atom 2 is so close that chelation might be expected. The reactivity of one-third of the primary alcohol groups (oneninth of the total groups) in a cellulose chain might thus be affected. Further interesting speculations, susceptible to experimental confirmation, might be made. These models certainly seem to constitute an extraordinary confirmation of the essential accuracy of modern conceptions of physical chemistry.
C. Correlation of structural conjiguration and physical properties Assuming that amylose chains exist in some form of helical “spring,” and that such ‘[springs” are twisted around one another to form a sort of ‘‘rope” micelle-which in turn may be the building block of the granular package which is starch-what chemical and physical characteristics should accord with such a structure? Chemical reactioity of starch. It is well known that in its natural state starch is considerably less reactive chemically than cellulose. In order, for example, to get efficient esterification it is necessary first to disorganize starch in aqueous dispersion as thoroughly as possible and with minimum degeneration, and then to precipitate the disorganized amyloses prior to esterification. The hydroxyl groups in starch do not behave as available as they do in cellulose. This in itself points to a higher degree of organization or association in starch and supports the concept of a closely packed helical configuration. Physical properties of starch films. Dried films or threads of starches and starch esters are more brittle, less tough, and less ductile than similar products obtained from cellulose. The latter are definitely superior in
* I n certain rotational positions a small “play” of about 45” is allowed.
788
G. V. CAESAR AND M. L. CUSHINO
desirable physical properties. It has been generally assumed that this physical superiority of cellulose is due primarily to greater chain length, While we are not prepared to deny that the cellulose chain is longer than the starch chain, we hold that these great differences in physical properties are more likely to arise from chain configuration than from length, particularly in the light of recent estimations of the chain lengths of amylose (6, 32, 33, 45,46).9 One would certainly not anticipate as much toughness and ductility from our configuration of an amylose molecule as from our cellulose model. Parenthetically, this laboratory has recently developed a most interesting and suggestive confirmation of the effect of configuration on the physical properties of films of starch. By special means of disorganization we have produced substantially undegenerated starch, the dried films of which show an extraordinary toughness. There is reason to believe that in this material the starch helix may have been strained into a more nearly linear form. Properties of aqueous starch dispersions. These have been rather extensively treated by one of us (4). The most outstanding property of aqueous starch pastes, and of pastes of starch derivatives generally, is their tendency to change their viscosity enormously as a function of temperature and agitation. It is difficult to rationalize such tremendous viscosity fluctuations except by 0-H. . .O bonding. As previously pointed out, associative forces should be particularly active in a helical structure of chains twisted together like a rope, the strands of which become more or less unraveled from the swelling due to hydration under heat, and contract or rebind upon cooling and standing. If the chains were straighter, the sum of the associative forces should be less effective, and viscosity fluctuations should be less pronounced.10 The special starch referred to above shows more viscosity stability, in aqueous paste form, than we have ever observed experimentally in any form of starch having a remotely similar viscosity range. The "stability coefficient,"-namely, the ratio of the mean ordinate of kinematic viscosity between 10.0"C. and 8O.O0C,to viscosity at 80.0°C.,-of a 10 per cent dispersion of this starch was 1.21. The coefficient for pure water, at these temperatures, is about 1.00! By every test which this laboratory could make, degeneration was at a minimum. Retrogradatzon of starch. The well-known phenomenon of starch retrogradation from aqueous dispersions (58, 5 5 ) has been difficult to rationalize. The resistance of retrograded starch to boiling water (55) is a very extraordinary property. What is retrograded starch? Why should it be SO abnormally resistant to hydration? Retrograded starch is formed fairly There is probably a wide chain length distrlbution. Between helical molecules a mechanical interlocking effect mlght also be ex pected. Q
10
THE STARCH MOLECULE
789
rapidly a t low temperatures, and very much more slowly and to a less extent a t room temperatures. In effect it is a dehydration process: water is squeezed out, so to speak, and is not readily taken back except through the action of caustic soda (55). We may assume that the retrograded starch formed by long freezing consists of exceptionally close-packed or associated amylose chains of which the hydrogen bonds are so powerful as to inhibit normal redispersion in water. Such a highly organized state of chain-to-chain organization would obviously be promoted by a configuration of the type indicated by our models. The water molecules normally present between the “springs” and forcing them apart would be eliminated by freezing: this intramicellar water would a t low temperatures tend to associate more with itself than with the relatively fewer amylose hydroxyl groups. Thus a dehydration would take place and the forces
H
R - O H . . . 0-R would have free play. There may even conceivably occur a certain degree of chelation between the reactive groups on adjacent loops of the helix. It is an interesting fact, in this connection, that starch which has been dried by heating to a low moisture content does not subsequently hydrate and swell to the same extent as before. SUMMARY
The literature bearing upon the structure, length, and spatial configuration of the amylose molecule, inclusive of the question of the “second” or associative dimension, has been critically reviewed. Molecular models have been assembled of d-glucose, amylose, and cellulose, employing the Fisher-Hirschfelder atomic models. From these constructions the cellulose molecule (6-1 ,4-glucopyranose linkage) yields an essentially straight-chain or linear configuration in accordance with accepted tenets; the amylose molecule (a-1,4-glucopyranose linkage) yields a helical spring configuration which tends to rationalize the distinctive properties of the chemical and physical behavior of starch. (1) (2) (3) (4) (5) (6) (7) (8) (9)
(10) (11) (12)
REFERENCES BAIRD,HAWORTH, ASD HIRST:J. Chem. SOC.1936, 1231. AND LANDIS:J. Am. Chem. SOC.61, 1495 (1939). BECKMAN BUSWELL, RODEBUSH, AND ROY:J. Am. Chem. SOC.60, 2239, 2144 (1938). Ind. Eng. Chem. 27, 1447 (1935). CAESARAND MOORE: AND HIXOX:J. Biol. Chem. 123, 595 (1938). CALDWELL FARROW, RICHARDSON, AND HIGGINBOTHAM: J. Textile Inst. 27, 131T (1936). FREUDENBERG: J. SOC. Chem. Ind. 66, 218 (1931). FREUDENBERG: Angew. Chem. 61, 675 (1934). FREUDENBERG, SCHAAF,DUMPERT,AND PLOETZ : Naturwissenschaften 27, 850 (1939). GILLETTEAND SHERMAN: J. Am. Chem. SOC.68,1135 (1936). GORDY AND ST.4NFORD: J. Am. Chem. SOC. 62, 497 (1940). HANES: New Phytologist 36, 231 (1937).
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