This is a biochemistry question! I\'m trying to figure out why you can sometimes

Question

This is a biochemistry question! I'm trying to figure out why you can sometimes ignore minor ionic species that are well away from the pH under consideration when looking at a solution.

For example, we have a 0.1 M solution of the tripeptide AlaArgLeu at pH 4, and I've already figured out which are the major species for each peptide: zwitterion for Ala, and anion for both Arg and Leu. I believe the pHs of the major species for each peptide are 2.6 for Ala, 4 for Arg, and 1.8 for Leu. Is it telling me to disregard both Ala and Leu then? I'm confused.

Thanks in advance!

Explanation / Answer

I am also confused are you talking about individual amino acids that are dissolve in water or tripeptide is dissolved. It is because when peptide bond in water it arranges itself but it did not dissociate into individual amino acids.Only charges will change on peptide.

Based on the pKa values for all the ionizable groups in peptides its net charge will change.

Ionizable Group pKa

Carboxy-terminus (COOH) ~2.33

Arg -amino group (NH3 + ) ~12.48

Amino-terminus (NH3 + ) ~9.7

At pH 4 total net charge on tripeptide will be +1

At pH 2.6

Alanine and leucine amino acids will be in their zwitter ion with net charge 0

Arg will be in protonated form with net charge of +1

At pH 1.8

Leucine will be in non-protonated form with net charge of +1 and so will be alanine

Arg will have net chrge of +2.

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