If the pH > pKa of an amino acid, is it ALWAYS deprotonated? And if the pH < pKa, is it ALWAYS protonated? Didn't come across this at all during my review, just a random passage. thanks.
Is this true?
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It's not that simple.
http://www.wiley.com/college/pratt/0471393878/student/review/acid_base/buffer_titration.gif
That's true for any monoprotic compound.
http://www.onlinebiochemistry.com/obj-512/Chap6-titration-aspartate.gif
That's true for actual amino acids. As you can see, it varies depending on what amino acid you are referring to because different AAs have different numbers of protic functional groups. Amino acids have either 2 or 3 different pKa's.
http://www.wiley.com/college/pratt/0471393878/student/review/acid_base/buffer_titration.gif
That's true for any monoprotic compound.
http://www.onlinebiochemistry.com/obj-512/Chap6-titration-aspartate.gif
That's true for actual amino acids. As you can see, it varies depending on what amino acid you are referring to because different AAs have different numbers of protic functional groups. Amino acids have either 2 or 3 different pKa's.
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For an amino acid, it's a little tricky because there are always at least two acidic/basic functional groups due to the backbone of the structure. The phrase "pH > pKa" makes no sense applied to an amino acid, because there are multiple pKas for multiple acidic/basic groups-- which one are you referring to? In general though, when pH > pKa for a certain functional group, it doesn't necessarily mean that the group is 100% deprotonated-- it just means that there's more of the deprotonated form in solution. Recall that the equation you can use to figure this out quantitatively for a weak acid is pH=pKa + log (A-/HA). Notice that if pH>pKa, then log(A-/HA) must be a positive term, meaning that [A-] > [HA] in solution.
What you might be trying to think of is when pH > pI of the amino acid, where pI refers to the isoelectric point (the pH at which the amino acid is electrically neutral). In this case, majority of amino acid in solution would be negatively charged, since the pH is basic relative to it's "balance point," the pI.
What you might be trying to think of is when pH > pI of the amino acid, where pI refers to the isoelectric point (the pH at which the amino acid is electrically neutral). In this case, majority of amino acid in solution would be negatively charged, since the pH is basic relative to it's "balance point," the pI.
dont mean to hijack this thread but i am doing a tpr online questions and the solution says
Because the pH of the stomach will be higher than the pKa of aspirin after taking Brand #3, less aspirin will be protonated.
background info
the pH of the stomach acid is around 4-5 and (aspirin) has a pKa of 3.5
when the pH>pKa doesn't it remain protonated? why are they saying less is protonated...
i swear i think i have something down and then........i get things wrong lol
Because the pH of the stomach will be higher than the pKa of aspirin after taking Brand #3, less aspirin will be protonated.
background info
the pH of the stomach acid is around 4-5 and (aspirin) has a pKa of 3.5
when the pH>pKa doesn't it remain protonated? why are they saying less is protonated...
i swear i think i have something down and then........i get things wrong lol
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You've got it backwards. Remember, when pH = pKa (I'll call this the "balance point"), the protonated and deprotonated forms are present in equal quantities. If pH > pKa, the solution is more basic than the balance point, and you should see more of the DEPROTONATED form.
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The way I think about this and used all through BioChem was this. If PH > pka, that means I have a bunch of -OHs floating around. What do -OHs like to do? They like to take protons off of anything even remotely acidic, aka anything less basic than themselves.
In general, high PH equals, lots of -OHs to deprotonate everything. Low PH? Lots of protons floating around looking to hook up.
Granted, this is not the most sound explanation, but I could visualize it and it made physical sense to me and worked.
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