## 4 Mechanism for Synergism in Mixtures of Sweeteners
This hypothesis has been supported by some sweetener combinations with synergistic effects that interact with different monomers/domains of human T1R2/T1R3, as demonstrated by a cell-based receptor function analysis [26]. Specifically, two high-intensity artificial sweeteners, NHDC and cyclamate, which have been identified as binding at the TMD of T1R3, significantly enhance the sweetness of various sweeteners including sucrose, aspartame, neotame, rebaudioside-A, saccharin, sucrose and thaumatin, which have been identified as interacting with the VFTM or CRD of T1R2/T1R3. Figure 1 shows the distinct binding sites of various sweeteners in the human sweet taste receptor. Based on these results, the authors suggest that NHDC and cyclamate act as positive sweet modulators to exert their synergistic effects with other sweeteners [50; 61]. This finding is in line with the previous results that in mixtures, structurally dissimilar sweeteners interacting with different sites in the receptor have the potential to exhibit synergy, whereas structurally similar sweeteners interacting with parallel sites in the receptor have the potential to show suppression [22; 32]. Furthermore, the relatively less effective synergism in polynary mixtures than binary mixtures could be partly explained by the distinct binding sites for various sweeteners, as the more sweeteners appear in the mixtures, the more possibilities for overlapping binding and contact with the re  
Figure 1: Graphic representation of the modeled structure of the human sweet taste receptor T1R2/T1R3 and its binding sites for different sweeteners. The extracellular VFTM (Venus flytrap module), CRD (cysteine rich domain) and TMD (transmembrane domain) are colored in red, light-green and blue, respectively. This figure was made with the PyMOL software. The binding sites of various sweeteners in the receptor are indicated by arrow lines. ceptor, resulting in an overall counteracted or alleviatively effect in sweeteners' synergism [41]. Moreover, a recent study has used molecular dynamics simulations to illuminate the potential structural basis of receptor activation by full synergistic combinations of sweeteners [14]. It was also proposed that synergy occurs when the components in the sweetener mixtures have identical hydration characters (e.g., hydrophobic or hydrophilic), which was explained by increases in the mobility of water molecules and reductions in the volume of hydrated sweetener molecules, but its link to the receptor activation and origin of sweetener synergy has not been elucidated [62]. Another study investigated the packing characteristics of blend sweetener molecules in water to account for their synergistic effect [63]. The results indicate that sweetener-sweetener, sweetener-solvent and solvent-solvent interactions have essential effects on the properties of binary sweeteners. The bulk sweeteners (sucrose and malitol) appear to dominate the properties of the mixtures due to their molar excess over the intense sweeteners (acesulfame K, aspartate and cyclamate), while the intense sweeteners play an important role in modifying the structure of water in solution. The molecular volumes, hydrophobicity, ionic character and isentropic compressibility of the sweeteners in the binary mixtures are of obvious relevance to their interaction as well as the solvent's packing, thus affecting their accession to the sweet taste receptor and their sweet intensities [63]. Another interesting study pointed out that in some sweetener mixtures, across-adaption and synergism display opposite relationships, with the most across-adaption combinations showing the least synergism, and vice versa, suggesting a complex interplay between the sweetener components [64,65]. It is worthy to note that the sweeteners' synergism at the level of molecular interactions are sweetener-dependent, which should be analyzed carefully when specific sweetener combinations are considered or applied [63,65]. Other explanations of the molecular mechanisms of sweetener synergy have also been proposed.