Flavors

Flavor is the sensation produced by a material and perceived principally by the senses of taste and smell. Over the years, scientists have attempted to relate chemical senses to the molecular structures of various compounds in an effort to develop a coherent theory. In 1967 it was proposed the glucophore unit, which is responsible for sweetness in compounds, should consist of an AH, B hydrogen bonding system where H is an acidic proton and B is an electronegative atom or center (63). In 1972 the two-component system was extended to include a dispersion bonding component designated as y component. The distance parameters of the resulting tripartite structure are A, B = 2.6, B, y = 5.5, and A, y = 3.5 A. The orbital dis tance between the AH proton and B is 3.0 A. In a three-dimensional picture, the glucophore binds to the receptor site and the sweet taste is initiated by intermolecular hydrogen bonding between the glucophore and a similar AH, B unit on the receptor. The y component acts to align the molecule to the receptor. The locations of AH, B, y components in many sweet compounds are known. For example, in aspartame, the protonated a-amino group and the ionized y?-carboxyl group of the aspartyl residue represent the AH and B unit, respectively (Fig. 10). The phenyalanine end of the molecule represents the y component. Only the L-L isomer of aspartame is sweet, and this type of enantiomeric effect in sweetness is also shown in several simple amino acids. To account for this fact, the receptor AH, B site is believed to consist of a spatial barrier. Hence, the L-D isomer of aspartame has the methyl ester group so positioned that the molecule cannot fit into the pocket for interactions (64). A similar concept is also applied to the aminosulfonates, such as saccharin and acesulfame K. Ring substitution experiments on acesulfame K indicate a loss of sweetness when the length of the hydrophobic group on the nitrogen exceeds 0.7 A, suggesting that a bulky substituent would create a steric hindrance (65).

Another group of sweeteners worthy of attention are the sweet proteins, which are approximately 100,000 times sweeter than sucrose on a molar basis and several times on a weight basis. Thaumatin I, the most studied sweet protein, consists of a single polypeptide of 207 amino acids (My/ 22,000), with 8 disulfides and no histidine. The largest domain is a flattened /? barrel formed by 11 antiparallel ji strands, with 6 residues per strand on the average. Attached to this are domains II and III, with loops linked and stabilized by disulfide bonds (66). Monellin, the other known sweet protein, consists of subunits I and II of 50 and 44 amino acid residues, respectively. The protein consists of a 5-stranded, antiparallel ยก1 sheet (2 [i strands from

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