Phosphate Groups

[DingDongD]

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I know this question has been asked before, but why are most phosphate groups dissociated on all oxygens in biological molecules, such as for AMP? I looked up the pkas for phosphoric acid, pka3 is 12.4. So, shouldn't the hydrogen be there?

Thank you ahead.

 

[aldol16]

Phosphoric acid is probably not a good model for something like AMP. The pKa's of those protons vary drastically depending on the environment they are in.

 

[NextStepTutor_2]

I know this question has been asked before, but why are most phosphate groups dissociated on all oxygens in biological molecules, such as for AMP? I looked up the pkas for phosphoric acid, pka3 is 12.4. So, shouldn't the hydrogen be there?

Thank you ahead.

Generally when speaking of phosphorylation/de-phosphorylation, it is the severing of P-O bonds that is involved. With phosphoric acid, however, it is the gain/loss of protons that we are describing when using Ka values. those phosphate groups will act very differently than the standard H3PO4 molecule.

hope this helps, good luck!