Chem. Senses 24: 271-279,
1999
© Oxford University Press
Some Taste Molecules and their Solution Properties
Department of Food Science & Technology, University of Reading, Whiteknights, Reading RG6 6AP, UK
Correspondence to be sent to: Professor G.G. Birch, Department of Food Science & Technology, University of Reading, Whiteknights, PO Box 226, Reading RG6 6AP, UK
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Abstract |
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The solution properties of a variety of different sapid substances from all four basic taste modalities, namely, sweet (n = 24), salty (n = 7), sour (n = 11) and bitter (n = 2), have been investigated. Some multisapophoric molecules, i.e. molecules exhibiting more than one taste, have also been included in the study in an attempt to define their properties in relation to the tastes they exhibit; eight sweet–bitter and three salty–bitter molecules were used. The density and sound velocity of their solutions in water have been measured and their apparent volumes, apparent compressibilities and compressibility hydration numbers calculated and compared. Apparent molar volumes (
v) and apparent specific volumes (ASV) reflect the state of hydration of the
molecules, and thus their extent of interaction with water structure. The range of ASVs reported
are 0.13–0.49 cm3/g for salty molecules, 0.55–0.68 cm3/g for sweet molecules, 0.53–0.88 cm3/g for sweet–bitter molecules
and a much wider range (0.16–0.85 cm3/g) for sour molecules. Isentropic
apparent specific compressibilities range from –2.33 x 10–5 to
–8.06 x 10–5 cm3/g.bar for salty molecules,
–3.38 x 10–7 to –2.34 x 10–5 cm3/g.bar for sweet molecules, +6.35 x 10–6 to
–2.22 x 10–5 cm3/g.bar for sweet–bitter
molecules and +6.131 x 10–6 to –2.99 x 10–5 cm3/g.bar for sour molecules. Compressibility hydration numbers
are also determinable from the measurements of isentropic compressibilities and these reflect the
number of water molecules that are disturbed by the presence of the solutes in solution. This
study also shows that it is possible to group isentropic apparent molar compressibility values by
the taste quality exhibited by the molecules in the same order as for ASV. |
Introduction |
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The mechanism of taste is thought to consist of the following major steps: firstly, accession of the stimulus to the receptor site through the saliva followed by the correct orientation of the molecule on the receptor site; then the transmission of the taste through nerve impulses, a process known as transduction (Kinnamon, 1988
); and finally, recognition of the
taste perceived in terms of its quality and intensity (Margolskee, 1995).
The first two steps suggest that, for any molecule to accede and fit on the receptor site, it must be
of an optimum molecular volume and of the right shape (Birch, 1991;
Birch et al., 1993, 1994) to pack within
the structure of water. This has led to a lot of work on the molar volumes of substances (Birch et al., 1993, 1994). Molecules
must also have the respective sapophores to be able to evoke a taste sensation, e.g. an AH, B,
glucophore for sweetness (Shallenberger and Acree, 1967;
Shallenberger, 1993), protons for sourness (Ganzevles and
Kroeze, 1987). Also important is the stereochemistry of the molecule, particularly
the hydroxyl groups at positions 2 and 4 in carbohydrate molecules (Galema and
Hoiland, 1991) which affect their fit within water structure. The hydrophilic and
hydrophobic balance in the molecule is thought to affect the mobility of water in the vicinity of
the solute (Mathlouthi et al., 1993).
Recently the emphasis has been on the role of water in the mechanism of taste (Kemp ,. 1992; Mathlouthi et al., 1993),
more specifically to the changes in the hydration layer and the centre of hydration of the solute
in the solvent which affects the transport of the molecules to the appropriate receptors and/or
their positioning on the receptors. The collapse of water structure, and hence enhanced hydration,
effectively allows the molecule to be transported to different layers of the taste epithelium, where
it is thought the different receptor sites for bitter, sweet, sour and salty lie. The changes in
hydration layer can be studied using isentropic molar compressibilities and hydration numbers
(Hoiland and Holvik, 1978; Hoiland, 1986a,b; Galema and Hoiland, 1991).
The hydration of a solute molecule in water is based on the Frank and Wen
(1957) model of solute–solvent interaction, which pictures three different
solvent-structure regions in the neighbourhood of the solute. Just outside the molecule is a layer
of immobilized, compressed water as a result of electrostrictive and other attractive forces
exerted by the solute. This layer is surrounded by a slightly less compressed or
`structure-broken' region of water molecules, distantly affected by those forces.
The outermost layer is bulk water, which possesses the typical tetracoordinated hydrogen-bonded
structure and is not affected by any of the above forces. Compressibility measurements measure
the changes in the first two layers of solvent around the molecule.
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Materials and methods |
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Chemicals used in this experiment were reagent grade and were obtained from BDH (Lutterworth, Leicestershire, UK), Sigma Chemical Co. (Poole, Dorset, UK), Hoechst (Switzerland) and ICN Biochemicals Ltd (Thame, Oxfordshire, UK). Water used for solution studies was HPLC grade. All measurements were carried out at 20 °C and in duplicate to minimize errors. Since all the parameters measured vary with the concentration tested, implying changes in the nature and extent of solute–solvent interaction and the hydration layer, all measurements were made at 3% w/w for comparison purposes.
Density and sound velocity measurements were determined using an Anton Paar Density Sound Analyser (DSA 48) from Paar Scientific Ltd (Raynes Park, London, UK). Temperature was maintained at 20 ± 0.1 °C. The density of the sample was measured from the period of oscillation of an oscillating U-tube. The sound velocity was calculated from the propagation speed of ultrasonic pulses in a known distance within the sample in the measuring cell. The instrument was calibrated once using air and distilled water. Density and sound velocity measurements were accurate to ±1 x 10–4 g/cm3 and ±1 m/s respectively.
Apparent molar volumes, v (cm3/mol), and apparent specific
volumes, ASV (cm3/g), were calculated from density values using equations (1)
and
(2) respectively.
 | (1) |
where d0 = density of water at one temperature (g/cm3), d = density of solution at the same temperature (g/cm3), m = molality of the solution (mol/kg of water) and M2 = molecular weight of solute (g/mol).
 | (2) |
The isentropic apparent molar compressibilities, K(s) (cm3/mol.bar), were calculated from both density and sound velocity values using
equation (3):
 | (3) |
where ßs = isentropic compressibility coefficient of solution (bar–1) and ßso = isentropic compressibility coefficient of water (bar–1). The isentropic compressibility coefficients are calculated from equation (4):
 | (4) |
where u = sound velocity of solution (m/s). The isentropic apparent specific compressibilities, K2(s) (cm3/g.bar), were obtained from equation (5) below:
 | (5) |
Compressibility hydration numbers, nh, were calculated using the following equation:
 | (6) |
where nw = number of moles of water (mol/kg solution) and ns = number of moles of solute (mol/kg solution).
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Results |
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Table 1 presents the solution measurements of 65 taste substances examined. The dominant taste(s) of each are reported alongside their respective measurements in water.
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Figure 1, Figure 2, Figure 3 , and Figure 4 show the relationships between the molecular weight of the solutes, their apparent molar volumes, their isentropic apparent molar compressibilities and their compressibility hydration numbers respectively, divided into different taste categories.
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Discussion |
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As would be expected, apparent molar volumes increase as molecular weight increases (Figure 1). A plot of the molar volume of all 65 tested substances against molecular weight also shows good correlation, with R2 = 0.8009. Apparent molar volumes reflect the size of the hydrated molecules in solution, and hence the extent of interaction of the solute molecules with water structure. This property has previously been investigated in relation to the taste quality of sugars and some polyols (Birch and Catsoulis, 1985
; Birch and Shamil, 1986;
Hoiland, 1986a,b; Kemp et al., 1990; Birch and Kemp, 1989).
Division of the apparent molar volume value by the molecular weight of the substance yields
the apparent specific volume, which accounts for ways in which the different forms of molecular
architecture are interactive with water structure. Shamil et al. (1987) previously reported
the parameter ASV as being a broad determinant of taste quality, with the four basic tastes
occupying predominantly certain ranges of ASVs as listed in Figure 5.
The range of ASVs
reported in this experiment are 0.13–0.49 cm3/g for salty molecules,
0.55–0.68 cm3/g for sweet molecules, 0.53–0.88 cm3/g
for sweet–bitter molecules and a much wider range (0.16–0.85 cm3/g) for sour molecules. The sweet-tasting molecules fit nicely in the ASV range of
0.52–0.71 cm3/g defined by Shamil et al. (1987), with those
substances exhibiting a clean sweet taste in the range 0.60–0.64 cm3/g
(Birch, 1991, 1996; Birch et al., 1996). The calculated ASV values of the molecules
investigated broadly fit these categories except for the sour-tasting substances, which can have
unusually high values; the latter point will be discussed later.
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Low apparent molar volumes and low apparent specific volumes show that the solute molecule is heavily hydrated, and therefore hydrophilic. Salts are assumed to be completely dissociated in solution into their respective ions, which by electrostriction pull in water molecules closer to themselves. This results in a small apparent molar volume (Table 1). This high solute–solvent affinity also seems to exist in sour-tasting molecules. The higher the interaction or affinity between the solute and the solvent (in this case water), the higher the probability of the ease and rapidity of transport of the solute molecule to the receptor. The solute may thus be transported to deep layers of the lingual epithelium, which leads to the conclusion that salty and sour receptors lie deeper in the lingual epithelium than sweet and bitter ones (Birch et al., 1993
). Green and Frankmann (1987) provide physiological evidence that salty
and sour receptor sites lie deep in the epithelium, and Hiji and Ito (1977)
have shown that sweet
receptors lie close to the surface of the tongue. Further evidence that salty and sour molecules
exert a higher interactive effect on water structure than sweet and bitter ones is provided by
Frank and Korchmar (1985), who showed that salty and sour substances
have shorter reaction
times.
Is it logical to expect those molecules possessing more than one basic taste, also known as
multisapophoric molecules, to have ASVs close to the ASV of the dominant taste of the
compound? This conclusion does seem to apply to most of the multisapophoric molecules (e.g.
potassium iodide, salty–bitter, 0.2755 cm3/g; sodium tartrate,
salty–bitter, 0.4126 cm3/g; citric acid, sour–sweet, 0.5996 cm3/g). In the case of the sweet–bitter tasting compounds studied above, the ASVs lie
on the borderline between the ASVs reported for the sweet and the bitter substances. It is not
always possible to predict the solution behaviour of multisapophoric molecules from their tastes
and vice versa. Which sapophores on the molecule will govern its taste properties will depend on
the medium used, the hydrophilic–hydro phobic balance of the molecule and the
dissociation constant of the molecule, all of which will influence the solution behaviour of the
solute. It should also be noted that when more than one taste sensation is experienced, it can
either be tasted simultaneously or in isolation, an example of the latter being the effect of
aftertaste exhibited by some of the common artificial sweeteners (Birch, 1996). Sometimes the
taste quality is too complex for any taste modality to be experienced individually. Other
substances are also known to change their taste in solution, a well-investigated one being D-glucono-1,5-lactone (Parke et al., 1997). The change in
taste of the solution
from sweet to increasingly sour can also be followed using solution properties and can be
explained in terms of changes in solute volume and its hydration layer.
Some sweet molecules, such as artificial sweeteners, have opposed hydrophilic and
hydrophobic sides, and this has been associated with increased sweetness (Mathlouthi
et al., 1993). Bitter molecules, on the other hand, are very hydrophobic in
character (Birch,
1987), and Sheridan et al. (1983) have shown that
the intensity of bitterness is related to
the region of the molecules in which hydrophobicity resides. Bitter receptors have also been
shown to be hydrophobic in nature (Venanzi, 1984). In substances with
large apolar surfaces, it is
thought that hydrophobic hydration is the main form of solute–solvent interaction. This is
structurally enhanced water, also called `stiffened' water, formed from the
rearrangement of the molecules in an organized fashion with strong hydrogen bonds between
them, and is less compressible than bulk water.
Isentropic partial molar compressibilities [K(s)] can be
expressed as the extent to which the water of hydration around the solute molecule can be
compressed. Galema and Hoiland (1991) have reported the
compressibility values of some
carbohydrate molecules. Figure 6 gives an overview of the concept of
isentropic compressibility.
Solutions become less compressible as K(s) values become
more negative. The method used to calculate K(s) assumes that
the solutes themselves are incompressible. The threedimensional hydrogen-bonded structure of
water is such that it has a large positive K(s) value, meaning
that the structure around each water molecule can be collapsed to a fairly large extent. The
hydration layer of a hydrophobic solute is less compressible than that of water. The hydrophobic
end of the solute molecule causes the water molecules to form strong hydrogen bonds amongst
themselves. This restructuring of the water of hydration is often referred to as hydrophobic
hydration (Arnett et al., 1965). In the case of carbohydrate
molecules, the water
structure is slightly disturbed by the hydrogen-bonded network around the solute; this holds the
water around the solute firmly, making the hydration layer even less compressible. Ions, with
their electrostrictive forces, cause the water structure to collapse around the solute. This water of
hydration is tightly held to the ions, reducing compressibility even further.
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In general, isentropic apparent molar compressibilities do not correlate well with molecular weight, but tend to become increasingly negative as molecular weight increases, implying that the bigger the molecule the more tightly water molecules are held around it. Salty molecules, consisting of ionic structures, which almost completely dissociate in solution, have the smallest K
(s) values (ranging from –3.089 x 10–3 to –8.058 x 10–3 cm3/mol.bar)
caused by electrostriction. Sweet molecules also give negative compressibilities (the largest
value
reported is –1.707 x 10–4 cm3/mol.bar) but the
isentropic apparent molar compressibility never reaches a more negative value than
–5.390 x 10–3 cm3/mol.bar. Sweet–bitter
molecules cover a wider range of compressibilities (+3.941 x 10–4–4.844 x 10–3 cm3/mol.bar), between values
reported for sweet and for bitter molecules. This study shows that isentropic compressibility
values can also be grouped by the taste quality exhibited by the molecules, in the same order as
for ASV, with salty showing the largest negative values and bitter at the top of the range (see
Table 2 and Figure 7).
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The equation used to calculate compressibility hydration numbers in this paper shows the number of water molecules which are disturbed by the presence of the solute in solution. The equation, however, assumes that the hydration layer around the solute is incompressible, which is clearly not the case in this study; therefore the numbers obtained can only be used as an approximation of the overall picture in solution. If the compressibilities of the hydration layers were to be included in the hydration numbers, the values would only change slightly, but the same trend would be obtained. Hydration numbers vary with the method used to calculate them, so that numbers obtained through other methods cannot be directly compared. Figure 2 shows that, the bigger the molecule, the higher the number of water molecules disturbed, therefore the better the fit of the solute into the hydrogen-bonded structure of water. A plot of compressibility hydration number against molecular weight for the different taste categories shows good correlation (R2 = 0.7860). Compressibility hydration numbers also seem to relate fairly well to apparent molar volumes as shown in Figure 3. The salty, sour and sweet–bitter molecules studied also correlate well with isentropic apparent molar compressibilities (Figure 4). However, sweet molecules do not (R2 = 0.0735). Among compounds with the same apparent molar volumes (Figure 3), salty molecules displace a bigger number of water molecules than sour and sweet molecules. Therefore the salty structures, which break water structure with their ions, have higher compressibility hydration numbers than the other molecules. In addition, for the same number of water molecules disturbed, salty molecules display lower apparent molar volumes and more negative isentropic apparent molar compressibilities (Figure 4) than sour molecules, which in turn have lower values than sweet molecules. These findings agree with the hydrophilicity of the molecules and their degree of interaction with the water structure, both of which decrease from salty to sour to sweet.
Limiting the ASV range of sour-tasting molecules to 0.33–0.52 cm3/g,
as quoted by Shamil et al. (1987), may not be truly appropriate.
Analyses of acids are
very sensitive to structural behaviour. Acids are made up of three components, the anion, the
cation and the undissociated molecule. The proportions of each of these components in solution
depend upon the dissociation constant of the molecule and the temperature at which
measurements are taken, so that the apparent volumes and the apparent compressibilities of their
hydration layers are averages of the properties of all three components in solution. This explains
the wide range of values obtained for sour molecules (ASVs lie between 0.16 and 0.85 cm3/g, and isentropic specific compressibilities range from +6.131 x 10–6 to –2.99 x 10–5 cm3/g.bar). For
some of the low molecular weight acids (<75 g/mol), namely formic acid, hydrochloric acid,
lactic acid, nitric acid and propanoic acid, the calculated isentropic compressibility values are
positive (with molar compressibilities of 2.822 x 10–4, 8.176 x
10–4, 1.777 x 10–4, 8.585 x 10–4 and 3.568 x 10–4 cm3/mol.bar
respectively). Hydrochloric acid and nitric acid solutions have molar compressibilities that are
higher than that reported for water (the isentropic molar compressibility of pure water is +8.17
x 10–4 cm3/mol.bar). These acids have relatively
simple, open structures compared with the other acids analysed, and therefore probably fit easily
within the structure of water. The low hydration numbers (1–4) associated with these
particular acids also support this fact.
Molecules with two tastes, such as artificial sweeteners, which taste both sweet and bitter,
could be polarized on taste receptors (Birch et al., 1977) and
span both receptor sites at
the same time (Birch and Mylvaganam, 1976). Chemical modification of
sugars (Birch, 1976)
has shown that one end of the molecule (the 3,4-
glycol group of glucopyranoside types of
structures) elicits sweetness and the other end (first and second hydroxyl groups, ring oxygen
atom and primary alcohol group of glucopyranoside types of structures) elicits bitterness. This
also suggests that at least some of the sweet and bitter receptor sites might be extremely close to
one another, probably within 3–4 Å. It is, however, essential to note that sweet and
bitter receptors are separate. Support for this is provided by the action of gymnemic acid, which
has the ability to block sweetness without affecting the bitter response (Bartoshuk,
1977). The
same is true of the sodium salt of 2(-4-methoxyphenoxy)propanoic acid, another sweetness
inhibitor (Johnson et al., 1994).
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Conclusion |
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Isentropic apparent compressibilities, as opposed to apparent volumes, seem a more sensitive parameter for measuring structural changes in solution as they allow precise monitoring of changes in the hydration layer of the solute. The whole concept of compressibility accords well with that of apparent molar and apparent specific volumes. Isentropic molar compressibility values can also be grouped by the taste quality exhibited by the molecules in the same order as for ASV (with salty showing the largest negative values and bitter at the top of the range), although no clear boundaries can be given based on this experiment. A low K
(s) also reflects a heavily hydrated solute molecule, and therefore high
solute–solvent affinity. Low isentropic apparent molar and specific compressibilities as
well as low apparent molar and specific volumes (as in the case of ionic structures, which impart
saltiness and/or sourness) show that the solute is heavily hydrated and therefore hydrophilic. This
in turn implies that there is high solute–solvent interaction, and therefore the molecule
can
be more easily and rapidly transported to the deeper layers of the lingual epithelium wherein the
appropriate receptors lie (Birch et al., 1993). Ions (produced
from salty and sour
structures) also cause a big spread in isentropic compressibility values, with large negative values
exhibited by salts. It is still very difficult to relate hydration to the stereochemistry of any molecule. There are several considerations to be taken into account: intramolecular hydrogen bonding, which makes certain functional groups unavailable for bonding with water molecules; the fact that equatorial hydroxyl groups of carbohydrate structures are better hydrated than axial hydroxyl groups; rotating C–C bonds, which make measurements very difficult; the anomeric forms of different sugars, which cause their hydration layers to be different; and the steric dispositions of substituents of ring systems, which affect bonding distances. Nevertheless, the solution measurements reported help to illuminate hydration mechanisms which mediate taste quality.
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Accepted January 4, 1999
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