Titration - would jump from 0 to-1 instead of 0 to-2 In that case the total charge would equal 0 at pH values between the pKs for the side chain

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Question – what is the pI of the peptide NH 2 Ala-Asp-His-Tyr-GluCOOH? Approach – make a table with the ionizable groups with their pK’s in the top row and a series of pH values which lie between all the pKs in the first column: αCOOH ~2 β and γCOOH ~5 (2 groups) Imidazole ~6 αNH2 ~9 Phenolic ~10 Total Charge <2 0 0 +1 +1 0 +2 2-5 -1 0 +1 +1 0 +1 5-6 -1 -2 +1 +1 0 -1 6-9 -1 -2 0 +1 0 -2 9- 10 -1 -2 0 0 0 -3 >10 -1 -2 0 0 -1 -4 The pI must be at the point where each of the side chain carboxyls is half ionized, so that they contribute -1 in toto. By definition, the pH at which an ionizable group is half ionized is the pK for that group, so the pI for this peptide is at the pK of the side chain carboxyls, ~5. If there had been only one acidic amino acid, say no glu and only asp, then the second column
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Unformatted text preview: would jump from 0 to -1 instead of 0 to -2. In that case, the total charge would equal 0 at pH values between the pKs for the side chain carboxyls and the imadozole side chain, i.e. 5-6. This approach ALWAYS gives you the pI – DO NOT try to figue it out by “inspection” or “intuition” and DON’T add up the pKs and average them. Remember that a peptide has only one alpha amino group and one alpha carboxyl group regardless of length, but all the ionizable side chains have pKs of their own....
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This note was uploaded on 09/28/2009 for the course BIO 361 taught by Professor Lake during the Spring '08 term at SUNY Stony Brook.

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