Question 20

[Guest]

Why is the answer pH>9.6 and not pH>2.3?

[admin]

You are thinking in reverse!

If the pH is low, that means there are tons of protons floating around (pH being negative log H concn). If there are lots of protons then chances go up that molecules that can be protonated will be.

pH over 7 is basic so the higher the pH, the greater the likelihood that the basic environment will remove protons or deprotonate molecules.

[Guest]

Isn't it that an amino acid is deprotonated when the ph is above its isoelectric point? Shouldn't the answer be PH>6?

thanks

[admin]

[Guest]

"magine for a moment that you have a million molecules of glycine and its isoelectric point pI = 6.0. Now add a tiny amount of OH- so that the pH goes to 6.001. Will all those zwitterions now be deprotonated?"

Here is how I think this question:

The zwitterions are already deprotonated, if you consider the fact that pka1 is much lower than 6. The two words " deprotonated" and predominantly, take the clearity out of this questions, I think. Deprotonated is when only one of the acidic sites unless mentioned otherwise. Similarly, predominantly can also refer to the acidic site whose pka was in range 2-3.

[admin]

What you just stated is not considered to be the convention. The way you stated the matter actually makes things much more complicated especially if you are examining proteins with, clearly, long chains of amino acids.

The convention is that an amino acid at its pI (the pH at its isoelectric point) is considered to be in its neutral form. That neutral molecule, by convention, is not considered to be deprotonated (as an analogy, consider CO2: the bonds within the molecule are polar but the molecule itself is non polar and thus has a 0 dipole moment). Likewise, there may be rearangements within the amino acid but the molecule itself has the same number of protons and atoms that it started with. It is only considered to be deprotonated when the original molecule has, overall, lost one proton.

[Guest]

At isoelectric point amino acid carries NET NEUTRAL charge. carboxyl group is deprotonated (COO-) and amino group is protonated (NH3+). if you go above pI, the NH3+ looses proton and becomes NH2 (deprotonated) while COO- remains deprotonated.

the correct answer would be pH>6.0

if you consider admin's answer with minute amount of base added, remember that since amino acid has basic and acidic group it will buffer that pH change. No minute amount of acid or base will change the pH of the solution.

[mmcginl7986]

The explanation given in the GS7 CBT is as follows: "At pH = 9.6, the zwitterion and the deprotonated form are equal in concentration. Therefore, at pH > 9.6, the deprotonated form predominates."

But we are given that the pKa *not the pH* of the NH3 group is 9.6. I'm having trouble finding the answer in the GS book, so could someone please clarify this? Don't you have to convert pKa to pH? pH = pKa + log[A-]/[HA], right? pKa cannot simply be interpreted as the pH...right?

Even if you don't need to calculate it for this problem, could someone point me in the direction of an explanation on quick conversions of pKa to pH?

[admin]

Bump.