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.