pH

[manifesto]

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pKa of carbonyl group, amino group, and thiol group are 3.1, 8.0, 8.3 respectively.
At pH=7, left compound looks like right. Why?

Also how do I calculate isoelectric point?

Any help would be appreciated. Please and thanks!

 

[larrylu]

for starter when neutral the acid will not dissociate nor will the amine be protonated.

 

[youngy]

Considering you are slowly increasing the pH, when you are below pH3, all of your functional groups will be in protonated forms.
NH3+ / COOH / -SH.

When your pH reaches 3.1, the functional group with its pKa corresponding to pH=3.1 will be deprotonated, which in this case will be carboxylic acid group.
For the next functional group to be deprotonated, you would have to raise your pH up to 8.0. Therefore, at pH=7, you've raised pH high enough only to deprotonate the carboxylic acid group, but not the thiol and amine group.

Therefore, at pH=7, your functional groups will look like
COO-, SH, NH3+


I have forgotten a lot about these ph, pI stuff, so if I am wrong, please someone correct me.

 

[DogeDDS]

for starter when neutral the acid will not dissociate nor will the amine be protonated.

Uh no, don't listen to him. The structure on the right is correct.

Think of it like this. pKa is essentially how quick that atom will give up its proton (the lower, the quicker). A low pKa (like the carboxylic acid) means that it'll still give up it's proton even in the face of an already acidic solution with tons of protons floating around. Those with higher pKas (such as the thiol and amino groups) need less protons in solution (higher pH) for the equilibrium to shift in favor of dissociation.

I can't remember exactly but I think you find the 2 functional groups that revolve around the atoms neutral charge and average their pKa's. So in this example, the atom has an overall neutral charge when: COO-, SH, NH3+ (-1 + o + 1= 0). The carboxyl group and and the amino group allow the molecule to have either +1, 0, or -1 overall charge depending on whether they're protonated or deprotonated, thus, you average their pKa's: (3.1+8.0)/2= the pH at which the atom carries 0 charge.

 

[larrylu]

Uh no, don't listen to him. The structure on the right is correct.

Think of it like this. pKa is essentially how quick that atom will give up its proton (the lower, the quicker). A low pKa (like the carboxylic acid) means that it'll still give up it's proton even in the face of an already acidic solution with tons of protons floating around. Those with higher pKas (such as the thiol and amino groups) need less protons in solution (higher pH) for the equilibrium to shift in favor of dissociation.

I can't remember exactly but I think you find the 2 functional groups that revolve around the atoms neutral charge and average their pKa's. So in this example, the atom has an overall neutral charge when: COO-, SH, NH3+ (-1 + o + 1= 0). The carboxyl group and and the amino group allow the molecule to have either +1, 0, or -1 overall charge depending on whether they're protonated or deprotonated, thus, you average their pKa's: (3.1+8.0)/2= the pH at which the atom carries 0 charge.

I apologize for misleading the OP. Y'all are correct. I wasn't thinking straight.