pH<=>pka

[laczlacylaci]

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Assuming the COO- group pKA=2; NH2 group pKa=9
@ pH=3>pKa=2 | deprotonated COO-
@pH=3<pKa=9 | protonated NH3+

@pH=7>pKa=2 | deprotonated COO-
@pH=7<pKa=9 | protonated NH3+

@pH=9>pKa=2 | deprotonated COO-
@pH=9=pKa=9 | protonated NH3+ (to balance out - charge on COO-)
or COOH and NH2

Questions:
1. Should I approach this question with more of pH>=<pKa analysis or acidic=protonated; neutral=zwitterion; basic=deprotonated.
2. The key states at pH=3, the COO- group is protonated. On the MCAT, should we just assume that under acidic conditions this will be protonated, what is the cut off mark for that 3-4?
3. If the pH goes to 10, what it look like this:
@pH=10>pKa=2 | deprotonated COO-
@pH=10>pKa=9 | deprotonated ?

 

[theonlytycrane]

1. These approaches are all using similar underlying logic, so go with an approach that you like best. I go with -COOH pKa = 2, NH3+ pKa = 9.

When pH < pKa -> + (proton on)
When pH = pKa -> neutral (equal acid and base forms)
When pH > pKa -> - (proton off)

You just have to be a bit careful with an amino acid like His, where proton off is neutral instead of negative.

2. At pH = 3, there is more unprotonated COO- form than COOH. Using pKa = 2, we can figure out the exact ratio using the h.h. equation, but that statement is incorrect in the key (assuming pKa = 2 for COOH).

3. Yes

This question is just bad because at pH = 9, there's an equal amount of NH2 and NH3+. A better answer would be pH > 9 for choice (C) to be correct.

 

[majikarp]

My approach here would be...
Amino Acids are zwitterions at physiological pH = about 7
Zwitterion means that amino group will be R-NH3+ and carboxylic acid group will be R-COO-, thus net charge = 0
If net charge increases or decreases, this is typically due to change in pH.
If pH goes up, concentration H goes down, deprotonation of molecules increases, thus AAs are going to become more negative.
If pH goes down, concentration H goes up, protonation increases, thus AAs are going to become more positive
In this case, the AA became more negative, thus pH should be greater than 7.
9 is the only answer greater than 7.