AAMC sample test C/P #47

[lostnconfused]

Can somebody please explain how we are suppose to know the -OH on the phosphate group is suppose to be deprotonated at PH of 7? Is this just something we are just suppose to know in addition to knowing the pKas for the AAs?

Thank you!

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[theonlytycrane]

The -POOH group is kind of like -COOH, and since -COOH is deprotonated, -POOH would be as well. (Admittedly, this isn't a great answer as I just got used to drawing the phosphate groups with proton off in biochem).

 

[lostnconfused]

The -POOH group is kind of like -COOH, and since -COOH is deprotonated, -POOH would be as well. (Admittedly, this isn't a great answer as I just got used to drawing the phosphate groups with proton off in biochem).

Oooh hey I never thought of it that way but that makes sense. Thank you!!

 

[aldol16]

You should also know the general structure of ATP and the phosphates on ATP are deprotonated. You don't need the pKas memorized but you should have a general feeling at this point for which groups will be charged/uncharged at physiological pH.

 

[Sai111394]

@aldol16 why isn't the OH group deprotonated? Wouldn't it be able to do resonance stabilization?

 

[aldol16]

Which OH group? On a phosphate? 2 of them are deprotonated. The third one stays on because the pKa is higher than physiological pH.

 

[Sai111394]

Which OH group? On a phosphate? 2 of them are deprotonated. The third one stays on because the pKa is higher than physiological pH.

The OH group that is next to the double bonded O, not the phosphate or carboxylate.... Did you memorize the pka?

 

[Sai111394]

I though it would be easily deprotonated because it could do resonance with the amide

 

[aldol16]

I though it would be easily deprotonated because it could do resonance with the amide

It can't do resonance with the amide. Try drawing a resonance structure.

I know the pKas because of chemistry grad school and general exams but you don't need to know the pKa of these per se. For your question, look at the hydroxyl in question. It's an alcohol. It can't be delocalized. Is that going to be deprotonated at physiological pH? Or, simply put, do you think an alcohol is very acidic?