Biomedical subjects
Publications and source records attributed to G G Birch.
The pmr spin-spin pulse relaxation times (T2 values) of the L-amino acids are examined in relation to their taste threshold values. There is an inverse trend between T2 value and threshold value with a good correlation for amino acids whose natural pH is close to neutrality. These results may indicate that taste receptors respond to perturbation of water structure.
Of all the senses, that of taste is perhaps the hardest to define and quantify and elucidation of the underlying mechanisms is unrespondingly difficult. This article describes a conceptual model--at the molecular level--for the four basic taste modalities, using apparent specific volume as a fundamental parameter. It also considers possible practical applications.
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Intensity-time studies of the sweetness of D-glucose solutions show that there are no major differences between alpha- and beta-anomers. Nor do the alpha- and beta-anomers exhibit any differences in apparent molar volumes. Contrary to previous reports, the anomeric centre of D-fructose may play no direct role in the sweetness response.
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.
Intensity/time studies of sweetness response in pure solutions of each of nine different sweet stimuli have been carried out. Both variables exhibit simple power functions of the form Intensity (S) = kscns and Persistence (P) = kpcnp. In binary mixtures of these nine stimuli a depression (or negative synergism) of both sweetness intensity and persistence is observed which is predictable from the low exponents of the power functions. Combination of both power functions allows the "effective concentration" of each stimulus in a binary mixture to be calculated from its observed intensity/time characteristics. All "effective concentrations" calculable in this way show absolute dominance of one stimulus in mixtures of two irrespective of the relative proportions of the two stimuli. It is suggested that the "effective concentrations" may reflect real concentrations of a single molecular species in the microenvironment of the receptor. Thus the accession of sweet molecules to ordered, localized concentrations at the receptor is ultimately dependent on chemical structure.
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The preferable structural feature for sweetness is a lipophilic moiety, e.g., a five- or six-membered ring with a polar substituent containing an A-H/B system outside the ring. In ketoses, this unit is probably the 1,2-glycol. If a third feature (the lipophilic, gamma, site) is required for the attainment of optimum sweetness and if it is C-6 in ketoses, then this site can accommodate quite a large constituent, both above and below the plane of the ring. The removal of the hydroxyl group from the C-6 hydroxymethyl substituent to yield the 7-deoxy derivatives causes bitterness, thus implicating the primary hydroxymethyl group with bitterness.Therefore, the creation of lipophilic site(s) in the sugar ring causes the realignment of the sugar molecule on the taste receptor surface. The distrubance of the proposed A-H/B system, e.g., the removal of the C-2 hydroxyl group, causes the ring alpha-glycol unit (most likely the ring C-3 and C-4 hydroxyl groups) to function as the A-H/B system.
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Selective benzoylation of HO-2 and HO-2' of 4,6-O-benzylidene-alpha-D-glucopyranosyl 4,6-O-benzylidene-alpha-D-glucopyranoside with N-benzoylimidazole led to the exclusive formation of 2-O-benzoyl-4,6-O-benzylidene-alpha-D-glucopyranosyl 2-O-benzoyl-4,6-O-benzylidene-alpha-D-glucopyranoside. Oxidation of either the dibenzoate or the corresponding ditosylate with methyl sulphoxide--phosphorus pentaoxide gave the 3,3'-diulose, and subsequent reduction with borohydride gave the 3,3'diepimers having the allo-allo configuration. De-esterification and hydrolysis of the benzylidene substituents gave alpha-D-allopyranosyl alpha-D-allopyranoside.
Chemical modification of sugars and their simple analogues indicates that these types of compound are almost always sweet, bitter, or bitter/sweet; hence, the two basic tastes may be intimately associated features of the same molecule. Stepwise modification at each chiral center around the sugar ring allows the sapid functions in these molecules to be mapped and leads to the inescapable conclusion that sugar molecules may be "polarized" on taste bud receptors, so that one end of the molecule elicits sweetness and the other bitterness. However, more extensive chemical modification evidently causes the molecule to realign itself in entirely different ways on the receptor. In most oligosaccharides only one sugar residue is likely to bind to the taste receptor, and this is probably a nonreducing end group, because the anomeric center of glucopyranose types of structure does not appear to affect sweetness. Sweetness depresses bitterness and bitterness depresses sweetness. Hence, it is not possible to make structural comparisons between analogues without correcting for these effects. However, some semiquantitative studies have established the value of current hydrogen bond theories of sweetness and the ideal oxygen-oxygen interorbital spacings for sweetness criteria in sugar molecules.
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Explore the source record for details and available documents.