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FRANK E. YOUNG,FRANCIS 1'.JONES AND HAROLD J. LEWIS
Vol. 56
D-FRUCTOSE DIHYDRATE BY FRANK E. YOUNG,FRANCIS T. JONES AND HAROLD J. LEWIS Western Regional Research Laboratory, Albany, Calafornia Receiued August 22, 1961
The discovery, preparation and properties of a new crystalline dihydrate of 0-D-fructose are described.
P-D-Fructose is most often crystallized from aqueous solutions as anhydrous crystals, although its separation as a metastable phase (possibly the hemihydrate) has also been described. 2 , 3 During an investigation of the phase diagram of the fructose-water system, details of which will be reported later, we observed that fructose can also be crystallized as a dihydrate. The preparation and properties of fructose dihydrate are described here. Preparation .-p-D-Fructose dihydrate may be obtained by seeding a i 5 % aqueous 'fructose solution with anhydrous fructose crystals a t 0" and allowing it to stand a t this temperature for several days. If the spherulites of fructose hemihydrate which ordinarily form first are vigorously stirred, they will usually transform in a few hours to large, well-formed prisms of fructose dihydrate like those shown in Fig. 1. At lower temperatures and concentrations ( -10" and 50 to 60% fructose), seeding with anhydrous fructose sometimes results directly in the foimmation of fructose dihydrate.
gave an average of 83.3 f 0.1% fructose (theory, 83.33%). A large single crystal which had been air-dried a t +lo was found to contain 83.45% fructose. These analyses were made by determining the refractive indices of solutions containing weighed amounts of fructose dihydrate and water. The results were calculated using the tables of Jackson and Mathews . 4 Mutarotation.-The initial rotation and rate of mutaro; tation of fructose dihydrate were determined at 20.0 f 0.1 using a quartz wedge saccharimeter and sodium light. The solution contained 5.18 g. of fructose dihydrate per 100 ml. of solution in 0.001 &I potassium phthalate buffer a t a p H . of 4.59. The initial specific rotation, -124', and the mutarotation coefficient, 52.2 m i n . 3 (using common logarithms), , are in sufficiently good agreement with the accepted values of -132.2' and 59.25 to indicate that fructose is present in t h e p-pyranose forni in this dihydrate. Melting Point.-The melting point of a dry single crystal in a sealed capillary in a constant temperature bath was found to be +21.3". Complete melting a t this temperature required three days, however. Under these conditions it is reasonable to assume that mutarotatory equilibrium was maintained during melting. This is obviously not the case when t8hemelting point is determined by the conventional capillary melting point method. Because of the slowness with which fructose dihydrate melts, this method is very unsatisfactory. The apparent me1 ting point appears t,o depend on the size of the sample and, with a warming rate of about 1 / Z o per minute, ranges from about +22" for specks of powder on the walls of the capillary to about, +29O for a sample in a thin-walle d tube 2 mm. in diameter. On the other hand, samples in 4-mm. tubes melted completely a t 25" in about 2 hours. When the melting of a fructose dihydrate crystal, such as those shown in Fig. 1, is observed a t 25' with a microscope, etch pits with angular outlines usually appear first. A s melting progresses, these etch pits enlarge until they extend conipletely through the crystal and cause it to have a "honeycomb" structure which eventually collapses leaving slowly melting fragments. As a result, a crystal in a capillary tube may appear to be only slightly melted when in
x PI
Fig. 1.-Fructose dihydrate: A, undistorted crystal; B, crystals flattened by growing against cover glass (100 X ). After seed crystals of the dihydrate have been obtained, it is more conveniently crystallized a t 0" from solutions containing about 65% fructose. Additional crops may be taken a t lower temperatures. Fructose dihydrate crystallizes rapidly, even in solutions saturated with glucose. The solubility of fructose dihydrate decreases rapidly and a t -go, which is probably its optimum crystallization tem erabure, its solubility is only about three-fifths the so1u:ility of anhydrous fructose. Thus crysta1lizat)ion as the dihydrate rather than as anhydrous fructose results in a much more fluid mother liquor, and the crystals are therefore easier to filter and wash. These properties suggest that the crystallization of fructose dihydrate may be used advantageously in the purification of fructose. Preliminary experiments indicate that this is the case. Composition.--Analyses of powdered fructose dihydrate dried to constant weight, at + l o over anhydrous fructose (1) Bureau of Agricultural and Industrial Chemistry, Agricultural Research Administration, United Btates Department of .4griculture. Article not copyrighted. ( 2 ) If. Honig and L. Jesser, Afonalsh., 9, 563 (1888). (3) &I. L. Wolfroni and Alva Thoinpson, J . Am. C h e i t ~Soc., . 68, 7 0 1 (1946).
Fig. 2.-Fructose
dihydrate: optic orientation.
(4) R. F. Jackson and J. .2. Mathews, J . Resenrck Nail. Bur. S t a n d ards, 8 , 412 (1932) (RP426). ( 5 ) t1. S. Isbcll and W. W. l'igiiiau. ibid.,20, 773 (l'J38) (R1'1 104)
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D-FRUCTOSE DIHYDRATE
Julie, 1952
739
Fig. 3.-1-1b:iy powder photogixpli of fructose dihydrate. reality, very little solid structure is left to hold the liquid within the framework of the original crystal. The high viscosity of the melt also helps t,o prevent collapse. This behavior was shown by a large single crystal which appeared to be only slightly melted after 48 hours a t +21.4" but) which crumbled to small fragments in a drop of solution when pressed lightly with a spatula. Optical and Crystallographic Properties.-Fructose dihydrate somewhat resenibles anhydrous fructose i n cryst.al habit,6 although its crystals arc! usually two to five times lohger than they are wide when they are not, confined by t,he container walls during growth (Fig. la). The crystals shown in Fig. l b are slightly flattened by growing against t,he cover glass which also has trapped a few air bubbles. The higher birefringence and lower melting point of fructose dihydrate clearly distinguish it from anhydrous fructose. Fructose dihydrate crystals are prismatic with a rhomb shaped cross-section as shown in Fig. l b . The dome is often absent. The hemimorphic character required by the optical activity of fructose was not apparent in any of the crystals examined. Ordinary views such as Fig. l a give an interference figure showing the axial plane tilt,ed about half-way between the acut,e biseetrix and t,he optic normal. Such a view shows the refractive index 01 for lengthwise vibrations and an intermediate value B' for crosswise vibrations. End views show a centered obtuse bisectris interference figure and proCRYSTALLOGRAPHIC PROPERTIES O F 8-DFRUCTOSE DIHYDRATE Crystal systcm: orthorhombic; Class 6: rhombic bisphenoidal. Prismatic habit. Acute an$e on rhomb-shaped end views = 68 . Refractive indices (5893 A.; 14"): CY = 1.5061 f 0.0005 6 = 1.5221 f 0.0005 y = 1.5444 f 0.0005 Optic axial angle: 2 v = 80" Est. from curvature AND
Calculated from sin2 V =
CY2(-r*
- BZ)
0Yr2 Calculated from sin E = p sin V Optical character: ( + ) Dispersion: ( T > v ) faint. Optical orientation: see Fig. 5 .
TABLE I1 X-RAY DIFFRACTION POWDER DATA FOR ~-D-FRUCTOS IC Dr HYURATE d in
A.
Est. I"
din
.i.
Est. Io
2.385 W 8.10 F'W 2.346 M 6.60 VS 2.301 w 5.83 VS 2.258 w 5.33 W 2.219 5.14 MS 2.183 M 4.77 S 2.145 M mi 4.36 S 2.114 VW 4.05 vs 2.077 W 3.96 VW 2.049 , W 3.51 MS 3.42 MS 2.004 MW 1.974 W 3.34 S 3.20 VW 1.947 MW 1 ,889 vw 3.02 RI 1.795 M 2.984 MW 1.756' M 2.866 hl 1.716 A MW 2.776 w 1,640 vw 2.662 M 1.612 17 w 2.583 MW 2.518 W 1 ,534 W 2,422 1.449 W hlS VS, very strong; S, strong; MS, medium strong; 31, medium; MW, medium weak; W, weak; VW, very weak.
w
TABLE I OPTICAL
vide thc refractive indices p and y . A few fragments in posit.ion t.o show an optic asis interference figure have occasionally been obtained. From these, the curvature of the isogyre indicates t,hat t,he optic axial angle (2 V) is about 80"; the dispersion is faint, with ( T > v) and. the optical character is (+). Figure 2 shows t,he opt,ic orientation and crystal morphology. The optical properties (Table I ) were determined in SOdium light a t 14' on crystals mounted on a hollow slide cooled by circulating tap water to avoid melting. The refractive indices of the immersion oils were determined on a n Abbe refractometer a t the same temperature. Tht: X-ray powder pattern of fructose dihydrate is shown in Fig. 3. The spacing and intensity data obtained from this figure are given in Table 11.
,
21.' = 81.5"
CY2)
2E
=
156"
(6) Snhydrous fructose has been described b y Bates' and Winchell.' (7) F. J. Bates and Associates, "Polarimetry. Saccharinietry and the Sugars," National Bureau of Standards Circular C 440, 1942, p. 643. (8) A. N. Winchell, "Optical Properties of Organic Compounds," University of Wisconsin Press, 1943, p. 186.
The authors wish to thank Dr. Fred Stitt for valuable suggestions and criticisms and Dr. Kenneth Palmer and Mr. Dale Black for the X-ray diffraction measurements.
