[eikosan]

Hi everyone,
this problem is giving me a headache so I kindly ask you for help.
We have this peptide: Glu-Met-Glu-Pro.
The question is: How much hydrochloric acid (conc.) should be added to the solution of this polypeptide (c=0,5 mol/dm3; 100 mL) to lower pH from 7.5 to 4.5.

The thing is...I don't know where to start. Should I calculate the net charges at pH=7.5 and pH=4.5 and the difference is H+ used from the acid?

Thank you in advance.

[Babcock_Hall]

Can we assume that there is no buffer present, outside of the buffering action of the peptide itself?  I am not the pH guy here, but your approach sounds reasonable.  The pK_a of glutamate side chains look like the only thing that will begin to protonate by pH 4.5.  I might try to find reasonable estimates for the pK_a of glutamyl side chains in a random coil of polypeptide, which are probably going to be in the neighborhood of 4.

[BetaAmyloid]

Won't the net charges be the same at pH = 7.5 and pH = 4.5?

Approximate pKa
Secondary Amine: 9
Carboxylic Acid: 2
Glutamic Acid: 4
Methionine: 10.5

pH = 7.5 Amine protonated (+), carboxylic acid deprotonated (-), glutamic acid deprotonated x 2(-), methionine protonated (+) = -1 charge

pH = 4.5 Amine protonated (+), carboxylic acid deprotonated (-), glutamic acid deprotonated x 2(-), methionine protonated (+) = -1 charge

Don't know how to do the problem...but was just commenting on the net charge question.

[Babcock_Hall]

In free glutamic acid, the side-chain carboxylate has a pKa of 4.07 (Zubay, Biochemistry, 4th ed.)