Sapling Learning macmillan learning Amino Acid Chain pK The isoelectric point (p

Question

Sapling Learning macmillan learning Amino Acid Chain pK The isoelectric point (pl) of a peptide is the pH at which the peptide does not migrate in an electric field. Since the peptide is zwitterionic, there are the same number of positive charges as negative charges on the peptide population. The pl can be estimated fairly accurately (within 0.1 or 0.2 pH units) from the pK values of all the proton dissociable groups in the peptide. Using pK values from the table at the right, estimate the pl value of the following hexapeptide Side rg Asp Cys Glu His Lys yr peptide- NH3 peptide- 12.5 3.7 8.2 4.3 6.0 10.5 10.5 8.0 Trp-His-Glu-Tyr-Gly-Asp STRATEGY Step 1: Determine the total positive charge on the peptide (n) when all acidic Step 2: List the pK values of all ácidic and basic groups in order from lowest Step 3: Calculate the pl as the average of the values for pK(n), the proton and basic groups are fully protonated (pK1) to highest dissociation forming a neutral species from a +1 species, and pKn+1) 3.4 the proton dissociation forming a 1 species from the neutral species COOH Step 1: Determine the total positive charge on the peptide (n) when all acidic andAmi basic groups are fully protonated. (Enter as a whole number without the sign.) and carboxy terminal values differ from the amino and carboxy values of a single amino acid Number n = | | 4.85 Previous Give Up & View Solution V Check Answer Next Exit

Explanation / Answer

when all the basic and acidic groups are protonated, the charge on the peptide should be +3 at the location of HIS, TYR and NH3+ each.

And the pI would come out to be

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