Q.7
The standard free energy change (ΔG°) of the binding reaction between different sweet
molecules and a common sweet taste receptor are given. Which amongst these molecules
is the sweetest at the same molar concentration?
(A) Sucrose (- 6.7 kcal mol-1 )
(B) Saccharin (- 9.7 kcal mol-1 )
(C) Alitame (- 11.1 kcal mol-1 )
(D) Neotame (- 12.1 kcal mol-1 )
Neotame exhibits the most negative standard free energy change (ΔG°) for binding to the sweet taste receptor, making it the sweetest at the same molar concentration. This principle stems from thermodynamics: a more negative ΔG° indicates stronger, more favorable binding, leading to higher receptor activation and perceived sweetness.
Sweetness and ΔG° Principle
Sweet taste arises from agonist binding to the T1R2/T1R3 G-protein-coupled receptor, where binding strength correlates with sweetness intensity at fixed concentrations. The relationship follows ΔG∘=−RTlnK, with more negative ΔG∘ yielding higher equilibrium binding constant K and affinity. Stronger binding displaces the receptor’s Venus flytrap domain into a closed, active conformation, amplifying signal transduction via gustducin and PLCβ2 pathways.
Option Comparison
| Sweetener | ΔG° (kcal mol⁻¹) | Relative Sweetness | Notes |
|---|---|---|---|
| Sucrose (A) | -6.7 | Baseline (least sweet) | Natural disaccharide; weakest binding here, matches moderate affinity. |
| | |||
| Saccharin (B) | -9.7 | Higher than sucrose | Artificial; ~300-500x sweeter than sucrose in practice, intermediate ΔG°. |
| | |||
| Alitame (C) | -11.1 | Strong binder | Dipeptide analog; ~2000x sucrose sweetness, tighter binding than saccharin. |
| | |||
| Neotame (D) | -12.1 | Sweetest (strongest) | Most negative ΔG°; ~7000-13000x sucrose sweetness due to optimal receptor interaction. |
| |
Correct Answer: Neotame (D)
Neotame’s ΔG° of -12.1 kcal mol⁻¹ is the most negative, signifying highest binding affinity and thus greatest sweetness potency at equal molarity. This aligns with structure-activity trends where neotame’s modifications enhance hydrophobic and hydrogen-bonding interactions at the receptor’s primary binding site (LB1/LB2). Other options bind less avidly, reducing activation efficiency.
