Triacylglycerols A and B are shown below. Which of the triacylglycerols would rotate plane-polarized light?

Question

Triacylglycerols A and B are shown below.  Which of the triacylglycerols would rotate plane-polarized light?

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11ecba2d_12be_2b4d_a3bf_331a3378cd43_MD0008_00 A chiral molecule contains at least one atom (typically carbon) that has four different substituents. This atom is known as a stereocenter, or chiral center. If the central atom is bonded to two or more of the same atoms, then the subsequent atoms are compared to determine whether the substituents are different. This process is repeated until a difference is found. If no difference is found in the two substituents, the central atom is not a stereocenter. Plane-polarized light is generated when normal light (waves oscillating in all directions) passes through a polarizer. The polarizer allows only light that oscillates in a particular direction to pass through it. Chiral molecules interact with plane-polarized light, causing the angle of oscillation to rotate. Triacylglycerols are lipids made up of three fatty acids attached to a glycerol molecule through ester linkages. All three fatty acids may be identical, two of the three may be identical to each other, or all three may be distinct. If at least one fatty acid is different from the other two, then the middle carbon may be a stereocenter. Both Triacylglycerol A and Triacylglycerol B have two identical fatty acids and one distinct one, but Triacylglycerol A is arranged symmetrically, so it is not chiral and will not rotate plane-polarized light (Choice C). Triacylglycerol B is not arranged symmetrically, so it is chiral and will rotate plane-polarized light. (Choices A and B) Only Triacylglycerol B has a stereocenter; therefore only it, and not Triacylglycerol A, will rotate plane-polarized light. Educational objective: Chiral molecules contain at least one atom that has four different substituents, known as a stereocenter, or chiral center. If the atom is bonded to two or more of the same atom, the subsequent atoms are compared to determine if the substituents are different. Chiral molecules can interact with and rotate plane-polarized light.

Triacylglycerols a and B Are Shown Below