Amino Acid PH and Pka

[mrmilad]

Heres a question for you guys:
Say you have an amino acid with:


COOH group pKa = 2.2
NH3 group pKa = 10.1


So if the pH of the solution the amino is in is at pH = 9.0
What will the charges be on the amino acid.


So I understand that since the Ph > Pka of the COOH group it will be deprotonated and have a negative charge. My main question is that does the NH3 group become protonated because its pka > the PH?


Thanks in advance.

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[lsk87]

as far as I know, yes, the NH3 is protonated bc the pH is lower than its pH...but don't take my word for it

 

[axp107]

I don't think you can conclude anything from that.. you need to know its pH at its isoelectric point..

If its +ive and -ive at the same time, its neutral.. meaning its pI is 9... there's no info on here to show that.. I don't think you can directly compare pKa and pH values just like that.. unless there's something I'm missing.

If pH > pI, COOH will be deprotonated resulting in a negative amino acid
If pH < pI, NH3 will be protonated resulting in a positive amino acid
If ph = pI, it will be neutral (nh3 will be protonated and cooh will be deprotonated)

Can you post the whole question?

 

[BlackSails]

You can conceptualize the whole process as an equilibrium between the protonated and deprotonated states. At a certain proton concentration (the pKa) the equilibrium works out so that the half the molecules are protonated. As the proton concentration increases (pH > pKa), more molecules get protonated because due to le'chatlier's principle, the equilibrium shifts the other way. The opposite happens when pH < pKa. If there were no protons in solution for example, nearly all of the molecules would be deprotonated.

 

[lsk87]

but if you look at a titration curve, you see that what is protonated at each pH

 

[mrmilad]

Actually it appears that this is an easier problem that I thought. According to kaplans explanation and what I originally thought when:

PH > Pka then the species is deprotonated.
PH < Pka then the species is protonated.


In the case of NH3 when the PH < Pka then it exists as NH4+, when PH > Pka however it simply exists as neutral NH3.

 

[werd]

You can conceptualize the whole process as an equilibrium between the protonated and deprotonated states.

exactly. when pH = pKa, the species is 50% protonated, 50% deprotonated. for each pH point away from the pKa, the ratio changes by an order of magnitude. so at one pH point above the pKa, it's 9.1% protonated/90.9%deprotonated, 2 pH points above the pKa, it's 0.99/99; 3 pH points above it's 0.0999/99.9 ect. so, considering your NH3 group with a pKa of 10.1...

pH 7 --> 99.9% protonated
pH 8 --> 99% protonated
pH 9 --> 91% protonated
pH 10 --> about 50% protonated
pH 11 --> 9% protonated
pH 12 --> 1% protonated

at a pH of 9, the NH3 group will be "91% protonated", giving an average charge to the molecule of "+0.91." the OH group will be pretty much entirely deprotonated, giving an average charge of "-1." the net charge of this amino acid, neglecting any side chain, will then be (-1 + 0.91) = -0.09.

this rule about protonation as you near the pKa is useful, straightforward, and probably worth memorizing.