Interpretation of psychophysics response curves using statistical physics.

Experimental gustatory curves have been fitted for four sugars (sucrose, fructose, glucose and maltitol), using a double layer adsorption model. Three parameters of the model are fitted, namely the number of molecules per site n, the maximum response RM and the concentration at half saturation C1/2. The behaviours of these parameters are discussed in relationship to each molecule's characteristics. Starting from the double layer adsorption model, we determined (in addition) the adsorption energy of each molecule on taste receptor sites. The use of the threshold expression allowed us to gain information about the adsorption occupation rate of a receptor site which fires a minimal response at a gustatory nerve. Finally, by means of this model we could calculate the configurational entropy of the adsorption system, which can describe the order and disorder of the adsorbent surface.

Effect of sucrose on the properties of caffeine adsorption layers at the air/solution interface.

Sweet and bitter tastes are known to be mediated by G-protein-coupled receptors. The relationship between the chemical structure of gustable molecules and their molecular organization in saliva (aqueous solution) near the surface of the tongue provides a useful tool for elucidating the mechanism of chemoreception. The interactions between stimulus and membrane receptors occur in an anisotropic system. They might be influenced by the molecular packing of gustable molecules within an aqueous solvent (saliva) close to the receptor protein. To investigate the molecular organization of a sweet molecule (sucrose), a bitter molecule (caffeine), and their mixture in an aqueous phase near a "wall", a hydrophobic phase, we modeled this using an air/liquid interface as an anisotropic system. The experimental (tensiometry and ellipsometry) data unambiguously show that caffeine molecules form an adsorption layer, whereas sucrose induces a desorption layer at the air/water interface. The adsorption of caffeine molecules at the air/water interface gradually increases with the volume concentration and is delayed when sucrose is added to the solution. Spectroscopic ellipsometry data show that caffeine in the adsorption layer has optical properties practically identical to those of the molecule in solution. The results are interpreted in terms of molecular association of caffeine with itself at the interface with and without sucrose in the subphase, using the theory of ideal gases.

Hydration properties and the role of water in taste modalities of sucrose, caffeine, and sucrose-caffeine mixtures.

Solution properties of sapid molecules are informative on their type of hydration (hydrophobic or hydrophilic) and on the extent of the hydration layer. Physicochemical properties (intrinsic viscosity and apparent specific volume) and nuclear magnetic resonance (NMR) relaxation rates R(1) and R(2) for pure sucrose, bitter molecule caffeine, and their mixture were found to be relevant in the interpretation of the effects of these solutes on water mobility. Likewise, surface tension, contact angles with a hydrophobic surface, and the adhesion forces to this type of surface of the aqueous solutions of sapid molecules were found to discriminate between their effects on water cohesion and also between their taste qualities. The interpretation of the two sets of independent experimental results, namely physicochemical and spectroscopic data, helps in the elucidation of the role of water in sweet and bitter taste chemoreception.

Hydration number and water activity models for the sucrose-water system: a critical review.

The nature of various molecular interactions occurring in the sucrose-water system and the structure of the aqueous solution are outlined briefly. The thermodynamic implications of these interactions are discussed, with special emphasis placed on the sucrose hydration number and the water activity coefficient. The existing "chemical" models of water activity, due to Scatchard (1921), Stokes and Robinson (1966), Schönert (1986), and Van Hook (1987), are evaluated in detail, including their predictive ability for dilute solutions and their limiting behavior in the region of high sucrose content. Some suggestions, based on the sucrose association mechanism, are made with regard to possible reformulations of the existing activity models that could result in a powerful thermodynamic model of the system.

FTIR and laser-Raman spectra of oligosaccharides in water: characterization of the glycosidic bond.

Changes in conformation of oligosaccharides, and the constraints imposed by hydrogen bonding with the solvent, were studied by means of vibrational spectroscopy (FTIR-ATR and laser-Raman). Oligosaccharides differing in positions of the glycosidic bond, such as trehalose, sucrose, maltose, melibiose, lactose, maltotriose, raffinose, and stachyose, were investigated. FTIR spectra of oligosaccharides in aqueous solution at different concentrations allow differentiation of these molecules according to the types of glycosidic bonds present and the changes in conformation of their constituent monosaccharides. Characteristic spectral ranges influenced by the glycosidic linkage position, overall hydration, and concentration of the aqueous solution were found. Tentative assignment of the observed IR and Raman bands was achieved.

Modulation of egg-white lysozyme activity by viscosity intensifier additives.

The activity of egg-white lysozyme was measured in the presence of carbohydrate additives in the reaction medium. These additives show a significant affinity for water. They depress water activity and increase the viscosity of the medium. Solute-solvent interactions in aqueous solutions of the additives are characterized by properties such as the intrinsic viscosity, Huggins constant apparent molar volume and hydration number. It was found that, despite the lowering of enzyme activity when the concentration of additive is increased, the behavior remains Michaelian and neither modification of Km nor inhibition by excess substrate is observed. On the other hand, the effect of the viscosity of the medium on enzyme activity was determined. This effect is independent of the nature of the additive at high viscosities (greater than 4 mPa s-1) for which enzyme activity is very low and appears to vary according to the kind of additive in dilute solution at low viscosities (less than 2 mPa s-1).

Lysozyme kinetics in low water activity media. A possible hydration memory.

The influence of water activity on initial lysozyme kinetics is studied. Enzyme catalytic capacity exponentially increases with thermodynamical water activity of the reactant medium; this relation seems independent of the chemical structure of the water activity depressors but dependent on the structuration of water molecules they induce. This influence of water organization on initial enzyme activity is immediate and may be preserved even after a large dilution, thus lysozyme presents a "hydration memory" phenomenon. This effect is in accordance with sorption/desorption isotherms, an hysteresis loop being observed.

F.t.-i.r. spectra of oligo- and poly-nucleotides.

Fourier-transform infrared (F.t.-i.r.) spectra of some synthetic mono-, oligo-, and poly-nucleotides and a natural DNA extracted from crab gonad have been recorded. Assignments of the observed frequencies are proposed by reference to our previous interpretation of laser-Raman and F.t.-i.r. spectra of constituents of nucleic acids. The spectra reflected structural differences between poly[d(A-T).d(A-T)] and crab gonad DNA (which is extremely rich in d(A-T) base-pairing). Such differences are mainly due to the effect of hydration on the conformational properties of natural DNA. Although the degree of complexity of the molecules investigated is higher than that of mononucleotides, most of the characteristic i.r. bands remain recognizable and assignable by comparison with the spectra of nucleic acid constituents previously studied.

Relationship between the structure and the properties of carbohydrates in aqueous solutions: sweetness of chlorinated sugars.

The structural basis of the sweet taste of D-galactose, D-glucose, D-mannose, sucrose, and some of their chlorinated derivatives has been derived from an interpretation of their F.t.-i.r. spectra. AH-B glucophores are proposed in the light of the observed OH vibrations, and an explanation of the differences in sweetness of the monosaccharides is proposed. The hydrophobic character of the CH2Cl, "gamma" centre in the tripartite template does not seem to play the same role in monosaccharides and 4,1',6'-trichloro-4,1',6'-trideoxy-galacto-sucrose. The enhancement of sweetness in the disaccharide derivative is due to the enhanced hydrophobicity of the CH2Cl groups as well as to specific interactions with water. A sharp i.r. absorption characteristic of free hydroxyl is found in the spectra of most of the very sweet polyhydroxy compounds.

F.t.-i.r. and laser-Raman spectra of cytosine and cytidine.

Fourier-transform infrared (F.t.-i.r.) and laser-Raman spectra of cytosine and cytidine in the solid state have been recorded and assignments of the frequencies made. Comparison of the observed frequencies for cytosine with those for cytidine permits identification of the bands characteristic of the sugar on the one hand, and of the pyrimidine base on the other.

F.t.-i.r. and laser-Raman spectra of guanine and guanosine.

Fourier-transform infrared (F.t.-i.r.) and laser-Raman spectra have been obtained for solid guanine. The F.t.-i.r. spectrum of guanosine in the solid state was also recorded. Assignments are proposed for the i.r. bands. The molecular basis of the spectral differences between guanine and guanosine are discussed.