Biochemistry: Henderson-Hasselbalch Problem

[cookiebookie]

Homework Statement

How many moles of base would be needed to titrate one mole of this fully protonated peptide to a pH of 4.00?

This polypeptide is V-A-Y-K-H.

Homework Equations

Henderson-Hasselbalch: pH= pKa + log[A]/[HA]

pKa of alpha COOH = 2.4
The alpha COOH (at the C-Terminus) is the only portion of the molecule that is deprotonated (basic) at pH 4.00, so we use its pKa for the calculation.

The Attempt at a Solution

pH = pKa + log [A]/[HA]
4.0 = 2.4 + log [A]/[HA]
1.6= log [A]/[HA]
39.8 = A/HA

I'm not sure how to go from the ratio of A/HA to the moles of base needed.

 

[Borek]

You need initial concentration of peptide. Then, knowing final ratio, you can easily calculate concentration of the neutralized peptide and amount of NaOH can be calculated from the neutralization stoichiometry.

 

[epenguin]

What that

39.8 = A/HA

is saying is a way of saying what should be evident from the fact that the pH 4 is substantially (by 1.6 units) above the pK, so the carboxyl will be almost all deprotonated (to within about 2.5% in fact) so the number of moles required is just under 1.

(Sorry Borek, the question was about moles so you do not need concentration.)

 

[Borek]

Yep, missed the "one mole" part. Sorry about that.

 

[DeeNos]

Can you provide more information on the peptide's structure and composition? Without this information, it is difficult to accurately calculate the number of moles of base needed for titration. Additionally, the pKa value given is for the alpha COOH group, but it is unclear if this is the only deprotonated group in the molecule. More information is needed to accurately solve this problem.