TBR Bio Ch. 6 #89

[ajumobim]

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The chemical structures shown in the passage are a triglyceride (Figure 1), phosphatidylcholine (Figure 2), cholesterol (Figure 3), and arachidonic acid (Figure 4). The arrangement of these lipids in order of decreasing polarity is:

Arachidonic acid

Correct Answer: A) phosphatidylcholine>arachidonic acid>a triglyceride>cholesterol

The explanation in the back first says that the molecule that has a full charge or charges is the most polar, which would be phosphatidylcholine. Then, it says that O is very electronegative, so the molecule with the more O oxygen atoms would be more polar. But it says that triglyceride and arachidonic acid have two oxygen atoms and that is not the case.

 

[amy_k]

Arachidonic acid definitely has 2 oxygens, each of the glycerol part of the chains in a tryclyceride has 2 oxygens for a total of 6. What does the explanation go on to say? Only the head of the triglyceride is polar, so it is only partially polar.

 

[ajumobim]

Arachidonic acid definitely has 2 oxygens, each of the glycerol part of the chains in a tryclyceride has 2 oxygens for a total of 6. What does the explanation go on to say? Only the head of the triglyceride is polar, so it is only partially polar.

OK, yes arachidonic definitely has 2, but I didn't understand why they were saying triglyceride has 2. But, I guess they were just referring to each of the 3 glycerol parts attached to a fatty acid. The rest of the explanation basically says that arachidonic acid is more polar than the triglyceride because there is resonance so there is increased stability of the functional group (since the pH>pKa and arachidonic acid is deprotonated). Do you have any idea why it would be more polar if there is resonance and increased stability?

Then, it goes on to say that since one of the O atoms in triglyceride is tied up between the 2 carbons, then the polarity is reduced, but not as reduced as the polarity in cholesterol since there is only one oxygen atom.

 

[amy_k]

I believe it is saying that the arachidonic acid is able to be polar (with the negative charge on the oxygen and positive on the hydrogen) because it is stabilized by resonance in that polar state, does that make sense?

 

[amy_k]

With regards to the triglyceride, it cannot put a minus charge on the COO group because the oxygen is bonded to carbon not hydrogen, there can therefore be no resonance structures and no formal charges/polarity. I hope that helps.