Fructose furanose form

D-fructose, C HijOo. Crystallizes in large needles m.p. 102-104 C. The most eommon ketose sugar. Combined with glucose it occurs as sucrose and rafftnose mixed with glucose it is present in fruit juices, honey and other products inulin and levan are built of fructose residues only. In natural products it is always in the furanose form, but it crystallizes in the pyranose form. It is very soluble in... 

Some monosaccharides also exist in a five-mem be red cyclic hemiacetal form called a furanose form. D-Fructose, for instance, exists in water solution as 70% /Tpvranose, 2% a-pyranose, 0.7% open-chain, 23% /3-furanose, and 5% a-furanose. The pyranose form results from addition of the -OH at C6 to the carbonyl group, while the furanose form results from addition of the —OH at C5 to the carbonyl group (Figure 25.5). 

Figure 25.5 Pyranose and furanose forms of fructose in aqueous solution. The two pyra-nose anomers result from addition of the C6 -OH group to the C2 carbonyl the two furanose anomers result from addition of the C5 -OH group to the C2 carbonyl.

Each of these compounds, 53-56, was shown to be a very effective competitive inhibitor of the enzyme with respect to the fructose 1,6-diphosphate, whereas several other analogs, including acyclic structures, had no effect. These and other results suggest that the furanose form of the sugar diphosphate is the active form in the enzymatic reaction (105). More recent studies using rapid quenching techniques and C-nmr measurements have confirmed this hypothesis and indicate that the enzyme uses the a anomer 52 much more rapidly than the 3 anomer 50 and probably uses the a anomer exclusively (106). 

By application of first-order, kinetic equations, B. Anderson and Degn claimed that an equilibrated (25°) aqueous solution of D-fructose contains 31.56% of jS-D-fructofuranose and 68.44% of -D-fructopyranose. N.m.r. studies, however, showed that, at equilibrium, a solution of D-fructose contains /3-D-fructopyranose, -D-fructofuranose, a-D-fructofuranose, and a trace of a-D-fructopyranose the distribution of these isomers was shown by gas-liquid chromatography to be 76,19.5, and 4%, respectively. Based on Anderson and Degn s result, Shallenberger reasoned that, as 0.68 X 1.8 = 1.22 (which approximates the reported sweetness of mutarotated D-fructose ), the furanose form(s) must possess very little sweetness. 

The enzymatic synthesis of sucrose also throws light on the formation of the furanose form of fructose in the sucrose molecule. The fact that sucrose is directly formed from D-glucose-l-phosphate and D-fructose supports Isbell and Pigman s34 and Gottschalk s85 evidence that the latter monosaccharide occurs in solution in an equilibrium mixture of furanose and pyranose forms. This makes it unnecessary to postulate a special mechanism of stabilization of a five membered (furanose) ring before the formation of compound sugars containing the D-fructose molecule.86... 

In another recent study it was concluded that both the py-ranose and the furanose forms of the alternate substrate 5-keto-D-fructose act as substrates. The pyranose form can be compared to glucose lacking hydro l at C-1 and having a second hydroxyl at C-2 (ketohydrate form). The furanose form is comparable to fructose but bears an extra hydroxyl moiety at C-5. Correcting for the amount of 5-keto-D-fructose in the pyranose form (98%), a K value for the furanose form could be calculated that is more tfian an order of magnitude smaller than that for fructose. This suggests that there is a favorable interaction between the enzyme and the additional C-5 hydroxyl. However, since no conclusive data for C-... 

Known to occur in both pyranose and furanose forms all other sugars (except apiose, fructose, and allulose) are normally in the pyranose form. 

To accommodate these facts, the earliest mechanisms proposed for degradation of D-fructose assumed that it was present in the furanose form, and that the ring remained intact. It was assumed that the initial reaction was the elimination of water, to form the 1,2-enolic form of 2,5-anhydro-D-mannose, and that further dehydration resulted in 2-furaldehyde. The necessity for D-glucose to isomerize to D-fructose was assumed to account for the much lower reaction-rate of D-glucose. This mechanism does not account for the observation that 2,5-anhydro-D-mannose is less reactive than D-fructose, nor is there any evidence that 2,5-anhydro-D-mannose is present in reacting D-fructose solutions. Nevertheless, similar mechanisms have since been proposed.13-16 Because of the ease of mutarotation of D-fructose... 

FIGURE 7-7 Pyranoses and furanoses. The pyranose forms of o-glucose and the furanose forms of o-fructose are shown here as Haworth perspective formulas. The edges of the ring nearest the reader are represented by bold lines. Hydroxyl groups below the plane of the ring in these Haworth perspectives would appear at the right side of a Fischer projection (compare with Fig. 7-6). Pyran and furan are shown for comparison. 

A Taste of Honey The fructose in honey is mainly in the jS-D-pyranose form. This is one of the sweetest carbohydrates known, about twice as sweet as glucose. The jS-D-furanose form of fructose is much less sweet. The sweetness of honey gradually decreases at a high temperature. Also, high-fructose corn syrup (a commercial product in which much of the glucose in corn syrup is converted to fructose) is used for sweetening cold but not hot drinks. What chemical property of fructose could account for both these observations ... 

D-fhreo-2,5-Hexodiulose ( 5-keto-D-fructose ) is mainly (>95%) in the / -pyranose form (7) in aqueous solution98 furanose forms involving... 

The extent to which the 4-thioaldoses are in the furanose forms is not certain. It has been claimed that 4-thio-L-ribose143 and 4-thio-D-glu-cose144 are completely in furanose forms, because they show no thiol absorption at 2550 cm-1 in their i.r. spectrum. This evidence is not convincing in the syrupy state, the composition may be different from that in dilute solution. The 13C-n.m.r. spectrum of a solution of 5-thio-D-fructose, however, shows141 only the signals of the two furanose forms, in the ratio of 11 89, at 25°. The -n.m.r. spectrum of a solution of... 

Kuhn and Grassner168 were the first to realize that the solution composition of sugars may vary considerably with a change of solvent. They stated that D-fructose in N,N-dimethylformamide exists in furanose forms to the extent of 80%. (This value is probably too high compare with Table VII.)... 

Why do you suppose that the major form of D-fructose in solution is the pyranose form but D-fructose in sucrose is in the furanose form ... 

Four sugars quite frequently occur as furanose residues in Nature. These sugars are D-fructose, n-arabinose, D-ribose, and 2-deoxy-n-erytAro-pentose D-Galactose, L-fucose, and D-psicose have also been found naturally in the furanose form. 

One further aspect of the 13C NMR data merits comment. In the spectra of D-fructose and D-allulose, the anomeric 13C signals of furanose forms appear downfield from those of pyranose forms. As already noted by Hall and Johnson (21), this same kind of distinction is observed with D-ribose. Broader examination of these spectra and also those of various other aldoses shows that stronger deshielding of furanose 13C nuclei is... 

When crystalline / -D-fructopyranose is newly dissolved in water, it is twice as sweet as sucrose, but shortly thereafter it is only slightly sweeter. Fructose mutarotates rapidly, and such phenomena have been associated by Isbell (4) with the formation of furanose forms of the sugars. Using a gas chromatographic procedure (5), we have shown (6) that the mutarotation primarily results from the formation of that isomer present in the sucrose molecule or -D-fructofuranose. 

Figure 20-2 Structure and configuration of the o-ketoses from C3 to C6. As with the aldoses (Figure 20-1), the cyclic form is predominantly an oxacyclohexane (pyranose) ring in the free sugar, but the oxacyclopen-tane (furanose) form is shown for fructose because it occurs widely in this form as the disaccharide, sucrose. Only the a anomers are shown (see Section 20-2B).

Another example of the observation that the side chain attached to the anomeric carbon atom in ketoses affects the proportion of one furanose form [p. 29] is afforded by the series 1-deoxy-D-fructose JV-substituted 1-amino-... 

The results of a detailed study of D-fructose and five O-glucosyl-substi-tuted D-fructoses17 confirmed that increasing the temperature increases the proportion of the furanose forms. This effect is much more noticeable in water and in pyridine than in dimethyl sulfoxide. In other ketoses, the proportion of the keto form also increases with increasing temperature.12... 

Four 1-amino- 1-deoxy-D-fructoses, each having a different substituent on the nitrogen atom, were studied.31 The nature of the substituent makes little difference to the composition, but, in each case, in contrast to D-fructose, there was no great preponderance of the / - over the a-furanose form in some cases, the a-furanose was preponderant. Changing the solvent from D20 to... 

---