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Author Topic: Polar sites in chlorophyll-a and b  (Read 9064 times)

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bromidewind

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Polar sites in chlorophyll-a and b
« on: January 16, 2010, 07:45:36 AM »

We recently performed a separation of β-carotene and chlorophyll-a/b from spinach leaves in lab, and I'm currently in the process of writing up the lab report. However, we haven't discussed identifying polar sites in complex organic molecules. This is not a required part of the lab write up, but I would like to know where the polar sites are and why. Is it primarily the ketone and aldehyde groups that cause the polarity, or does it have anything to do with the magnesium in the middle? I know that it is an asymmetric molecule, which contributes to the polarity of it, but knowing the individual polar sites would help me better understand this. I've attached the molecules below.


chlorophyll-a                                            chlorophyll-b
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SanShouXMA

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Re: Polar sites in chlorophyll-a and b
« Reply #1 on: April 14, 2010, 05:43:38 PM »

We did the same lab experiment just 2 weeks ago. Can anyone answer this question please. I'm having the same problem.

Thanks
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tmartin

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Re: Polar sites in chlorophyll-a and b
« Reply #2 on: April 15, 2010, 01:22:24 AM »

The carbonyl compounds can add to the polarity somewhat.  I'm not sure how you have drawn it, as I can't see your structures; but the long carbon chain off the ester is the hydrophobic/non-polar area (just a bunch of C-C and C-H bonds), and the tetrapyrrole ring system is a major source of the molecule's polarity.  There are two negatively charged nitrogen atoms tied up with the magnesium as well as two other pyrroles (the two imines in the 5 membered rings) which add polarity significantly, I would think.

Remember a basic way to think of polarity is the difference between the atoms electronegativities, and a shift in electron density towards the more polar atom.  I try to remember the Pauling values for electronegativity and use those as guidelines (even though they are not completely accurate).  Carbon has a value of 2.6 versus nitrogen with 3, so there is going to be some polarization of that bond.  Also, as you speculated, the same goes for the C=O bond, (Oxygen with a value of 3.4).
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Smrt guy

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Re: Polar sites in chlorophyll-a and b
« Reply #3 on: April 16, 2010, 03:09:23 PM »

Just look for O, N, and halogens.
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