Sugar Structure and Taste

15

Sugar Structure and

Taste

R. S. SHALLENBERGER

Downloaded by UNIV QUEENSLAND on July 17, 2013 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1971-0117.ch015

New York State Agricultural Experiment Station, Cornell University, Geneva, Ν. Y. 14456 Thesaporousunit for sweet taste in sugars is the α-glycol moiety in the gauche conformation. Sugar sweetness is di minished when the glycol unit has the eclipsed conformation and an intramolecular hydrogen bond. Also if an OH group is disposed to hydrogen bond elsewhere in the molecule (the ring oxygen atom), the ability of an α-glycol moiety to elicit sweet taste is diminished. Whenα-glycolOH groups are in the anti conformation, they are apparently too far apart to cause sweet taste. Evidence for the above conclu sions is derived from studies of various sugars, model com pounds, and the mutarotation reaction and leads to a general concerted hydrogen bond model for the initial chemistry of sweet taste. ' T p h e relation between sugar structure and taste has interested investigators for many years. W i t h the development of a more complete stereochemical description of the sugars, reasons for their varying sweet ness seem to become apparent (1, 2). One saporous unit i n the sugar is the ^-glycol moiety. It seems that varying sugar sweetness is caused by: A

1. Eclipsing of vicinal hydroxyl groups permitting them to participate in an intramolecular hydrogen bond 2. Positioning of an hydroxyl group, which is also part of an α-glycol pair of hydroxyl groups, so that it may hydrogen bond elsewhere i n the molecule (for example to the ring oxygen atom) 3. A pyranose or furanose ring structure and an «-glycol configura tion leading to the anti glycol conformation Among the most interesting examples of varying sugar sweetness is the fact that crystalline or freshly dissolved β-D-fructopyranose is about twice as sweet as sucrose, but after mutarotation or during thermal muta rotation, sweetness diminishes markedly (3). a-D-Glucopyranose is about two-thirds as sweet as sucrose, but the mutarotated solution is even less 256 In Carbohydrates in Solution; Isbell, H.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.


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sweet. Yet crystalline β-D-glucopyranose is sweeter than the a-anomer, and the mutarotated solution is again less sweet. In the first one would conclude that the α-anomer is sweeter than the β-anomer, but i n the second the reverse conclusion could be reached. α-D-Galactopyranose is the 4-epimer of α-D-glucopyranose, but it is only one-half as sweet as glucose. α-D-Mannopyranose, the 2-epimer of glucose is also only onehalf as sweet, but the β-D-anomer is bitter. Finally, because of their varying biological activity, it is believed that optical isomers have differ ent sweetness. W h i l e this is true of the amino acids, the D - and L-series of sugars are equally sweet (2). Several examples are given to indicate that the eclipsing of O H groups results in reduced sweetness, but the best evidence comes from the nature of the fructose mutarotation reaction. W h e n crystalline β-D-fructopyranose is newly dissolved i n 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 i n the sucrose molecule or β-D-fructofuranose. At equilibrium in water at 20°, gas-liquid chromatography indicates that there is 76% β-D-fructopyranose, 2 0 % β-D-fructofuranose, and 4% of an unknown compound, which has a specific rotation of about + 1 2 2 ° (if the value of + 1 7 ° assigned by Hudson (7) to β-D-furariose is cor rect). W e deduced that the furanose form is void of sweetness for at least two reasons. As an example of hydrogen bonded hydroxyl groups, both hydroxy-methyl substituents are so dispersed as to be (perhaps) completely bonded to the ring oxygen atom (8).

H C 2

This is one example of the second criterion mentioned previously. H o w ever, the other O H substituents, depending upon the furanose ring con formation, are either eclipsed or i n the anti conformation. In the former they are disposed to form a strong intramolecular hydrogen bond; i n the latter they are incapable of such bonding. Further evidence to support the contention that free β-D-fructofuranose is nearly tasteless is seen i n the thermal mutarotation (3) of D-fructose. As the temperature of D-fruc-

In Carbohydrates in Solution; Isbell, H.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.


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tose solutions is increased, the relative sweetness of the solution, measured under comparable conditions, drops markedly. The optical rotation i n creases, suggesting the formation of a more dextrarotatory isomer, at the expense of the sweet β-D-pyranose. These phenomena are shown i n Figure 1. (The sweetness data are those of Tsuzuld and Yamazaki (3).) A t 60° we find 5 8 % β-D-fructopyranose, 3 0 % β-D-fructofuranose, and 12% of the unknown compound, tentatively assigned to a-D-fructopyranose based on furanose conformation principles. T w o cis bulky hydroxy methylene substituents on a furanose ring are unlikely. Based upon this, we predicted that if the configuration and con formation of di-D-fructose anhydride I established by Lemieux and Najarajan (9) was correct, the compound should be nearly tasteless (the dihedral angle between α-glycol groups is 150° and 75° in the a- and β-furanoside rings, respectively). This was true. CH OH 2

Thus, it is assumed that foods containing free fructose which have, or are i n , an environment for anhydride formation w i l l lose sweetness. Another example of where reduced sweetness can be associated with intramolecular hydrogen bonding is the sugar galactose. Only one-half as sweet as glucose, the difference must be i n the fact that the O H group on position-4 in galactose is axially disposed and capable of bonding to the ring oxygen atom. The same situation is found i n α-D-mannose. W h y β-D-mannose tastes bitter is not known, but it posssesses within its struc ture the key to structural requirements for bitter taste. The glucose anomers have interesting taste characteristics. As men tioned before, by their behavior in solution, one could conclude that either the «-D or the β-D-anomer is sweeter. W h a t this information


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Structure and Taste

Suggests is that perhaps the sugars have gone into equilibrium with an unknown, but a significant proportion of conformers whose vicinal O H groups are eclipsed, anti, or disposed to bond the ring oxygen atom, CH OH 2

CH OH 2


such as

Figure 1.

Change in the sweetness and concentration of O-fructopyranose during thermal mutarotation



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C A R B O H Y D R A T E S IN S O L U T I O N

There is little evidence for such phenomena. Energetically, boat forms of the sugars at present are, highly unfavored, but the B3 conformer shown for α-D-glucopyranose may explain the reaction of that sugar with borate. The 1C conformer shown for β-D-glucopyranose is equally un favorable but a possible candidate for marked decrease i n sweetness of this compound i n solution. The stabilizing Η-bonds shown should be severed at elevated temperatures, and if sugar sweetness does vary inversely with the degree of intramolecular Η-bonding, the sweetness of glucose i n solution should increase with temperature. This is an estab lished fact with important practical application. The strongest evidence for the notion that anti α-glycol conforma tions, such as persist in β-D-fructofuranose, are devoid of sweet taste is found in levoglucosan.

OH

OH

W h e n crystals of this material are placed upon the tongue, something happens, but a taste panel that was established cannot say just what. They agreed, however, that the compound is tasteless. Possibly the panel responds to a sense of coldness because of the heat of dissolution of the compound. From studies such as these, it was deduced that the ideal sugar moiety eliciting sweet taste was the α-glycol unit i n the gauche confor mation, regardless of whether or not the O H groups were cis or trans. A diagrammatic way of representing the relation between glycol con formation and sweetness is shown in Figure 2. W i t h an Ο · · Ο dis tance of near 2.5 A , the eclipsed O H groups are so strongly H-bonded, they cannot elicit sweet taste, and when they are anti with an Ο · · Ο distance of 3.71 A , they are too far apart. The maximum for sweet taste seems to be about 3 A . Many more examples of the relation between sugar structure and taste could be given. Birch et al. (10) have studied a number of free sugars, sugar derivatives, and substituted sugars. As these studies developed, it became apparent that if the sugar sweetness varied inversely with the degree of intramolecular hydrogen bonding, then perhaps the initial chemistry of sweet taste resulted from



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intermolecular H-bonding (11). Thus, we began to view the α-glycol unit as an ΑΗ,Β system conventionally used (12) to describe and define the hydrogen bond. Based upon our conclusion that the α-glycol unit when i n the gauche conformation was the most favorable for causing sweet taste, we decided that the receptor site could also be an ΑΗ,Β unit and the A H proton distance to Β must also be about 3 A although the identity of the receptor ΑΗ,Β unit remained uncertain. Viewing the sweet unit of a sugar as an ΑΗ,Β unit led us to look for such a unit in the various sweet-tasting compounds, particularly with an A H , proton to Β distance of about 3 A . W i t h varying chemical identity, the unit is present in all compounds with sweet taste ( 1 ) and probably is a prerequisite for sweet taste. Thus, the initial chemistry of the sweet taste response seemed to be a concerted intermolecular hydrogen bond interaction be tween the saporous unit of a sweet substance and a commensurate chemi cal site on the receptor. Sweet Compound

— A — Η ... — Β

Β HA

— —

Receptor Site


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It was with the sweet taste of enantiomorphs that we encountered the strongest criticism to our explanation of why sugars vary in their sweetness and also as to the identification of the saporous unit as an ΑΗ,Β system in all sweet compounds. Amino acids which belong to the D-series are generally sweet tasting, but those i n the L-series are tasteless or bitter when R i n R C N C N H C 0 H is larger than the ethyl radical. Equally, the naturally occurring D-series of sugars usually taste sweet, while it has generally and tacitly been assumed that the synthetic L-series is tasteless. To support this assumption, it has been reported that L glucose is tasteless, and D-mannose is sweeter than L-mannose (13). The D and L-sugars and amino acids are enantiomorphs and differ in absolute configuration about every asymmetric carbon atom. The pentose L-arabinose is structurally related to the sweet-tasting hexose, D-galactose, and, to recognize this, we predicted (2) that L-arabinose would probably taste sweet. L-Arabinose d i d taste just about as sweet as D-galactose, and L-sorbose, the 5-epimer of D-fructose, also tasted sweet although it was only about one-fifth as sweet as D-fructose. To extend these findings, a series of seven pair of D - and L-sugars was submitted to the taste panel to compare the sweet taste character istics of each enantiomorphic pair. The method used was that of a paired comparison technique where sweetness scores are assigned to the taste of each sugar. To prevent the need to identify the anomeric and crystalline form of each sugar ( the supply of certain rare enantiomorphs was small ), 10% solutions of the sugars were allowed to come to mutarotational equilibrium before they were tasted. Thus, the tautomeric or conforma tional composition of a reducing sugar solution would not be important so long as only enantiomorphic sugars are compared. W e found (2) that there was no statistically significant difference between the sweet taste of the enantiomorphic sugars. D-glucose was just about as sweet as L-glucose. The critical enantiomorphic pair to be compared were D - and L-fructose and M . L . Wolfrom confirmed that L-fructose tastes very sweet (14). W e conclude that the strange stereochemical features, which stu dents have been assigning to the taste bud for years, many be more myth than fact. A l l that really needs to be accounted for is the fact that the one amino acid (alanine) which tastes sweet in its D - and L-forms has a side chain smaller than the ethyl radical. If one constructs a spatial bar rier about 3 A from the postulated ΑΗ,Β site, it becomes a simple matter of whether or not an amino acid in its L-form can be positioned over the site. A sugar with vicinal O H groups as ΑΗ,Β can make any approach to the receptor site to elicit sweet taste, and, therefore, there should be


2


2


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Figure 3. Positioning of β-Ό-glucose over the proposed taste receptor site to initiate sweet taste response no difference in the ability of D - and L-forms to elicit sweet taste. The model with β-D-glucose as the tastant is shown i n Figure 3. T o demon strate the ability of β-L-glucose to be positioned over the site, the anomeric hydroxyl group and the - C H O H group need to be interchanged and the molecule turned over. 2

Literature Cited 1. Shallenberger, R. S., Acree, T. E., Nature (London) (1967) 216, 480. 2. Nature (London) (1969) 221, 555. 3. Tsuzuki, Y., Yamazaki, J., Biochem. Z. (1953) 323, 525. 4. Isbell, H., Pigman, W. W., J. Res. Natl. Bur. Std. (1938) 20, 773. 5. Shallenberger, R. S., Acree, T. E., Carbohydrate Res. (1966) 1, 495. 6. Shallenberger, R. S.,"Frontiersin FoodResearch,"p. 40, Cornell Univer sity, 1968. 7. Hudson, C. S., J. Amer. Chem. Soc. (1908) 30, 1564; (1909) 31, 655. 8. Barker, S. Α., Brimacombe, J. S., Foster, A. B., Wiffen, D. H., Zweifel, G., Tetrahedron (1959) 7, 10. 9. Lemieux, R. U., Nagarajan, R., Can. J. Chem. (1964) 42, 1270. 10. Birch, G. G., Lee, C. K., Rolfe, E. J., J. Sci. Food Agr. (1970) 21, 650. 11. Shallenberger, R. S., New Scientist (1964) 23, 569. 12. Pimentel, G. C., McClellan, A. L.,"TheHydrogen Bond," Freeman, 1960. 13. Boyd, W. C., Matsubara, S., Science (1962) 137, 669. 14. Wolfrom, M. L., personal communication. RECEIVED December 2, 1971


Sugar Structure and Taste

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