2/14/2016 08:oo AM A 65-7/100 2/13/2016 o2:00 AM Print Calculator had Periodic T

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2/14/2016 08:oo AM A 65-7/100 2/13/2016 o2:00 AM Print Calculator had Periodic Table Question 21 of 21 Map 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: Acid Arg Ng Asp 12.5 3.7 8.2 4.3 6.0 10.5 Cys Glu Met-Asp-Lys-Arg-Ala-Tyr STRATEGY Step 1: Determine the total positive charge on the peptide (n) when all acidic Step 2: List the pK values of all acidic and basic groups in order from lowest (pK) Step 3: Calculate the pl as the average of the values for pKpy the proton Lys and basic groups are fully protonated. to highest (pKe). dissociation forming the neutral species from the +1 species, and pKnpeptide- Ty 10.5 peptide- 8.0 NH3 3.4 the proton dissociation forming the -1 species from the neutral species.COOH Step 1: Determine the total positive charge on the peptide (n) when all acidic and "Amino and carboxy basic groups are fully protonated. Noice there are two basic amino acids in the peptide, Lys and Arg So the total from the a amino and terminal values differ positive charge is 3. carboxy values of a single amino acid. 8.0 HINCH CNHCH C NH CH CNH CH CNH CH CNH CH C OH 34 OH CH 3.7 H2NH NHNH 10.5 10.5 NH 12.5 Step 2: List the pK values of all acidic and basic groups in order from lowest (pKi) to highest. and basic groups in order from lowest (pK) to highest O Previous Give Up & View Solution ) Check Answer Next Exit Hint

Explanation / Answer

Step I: At pH 7, Arg has a fully protonated side chain and is capable of being only a hydrogen bond donor Any amino acid side chain capable of accepting a hydrogen bond can hydrogen bond with Arg at pH = 7. and same for Lysine.

Hence the total positive charge o0n the peptide (n) will be +3

Step II: Ala < Met < Asp < Tyr < < Lys < Arg

Step III : The average value for pKa (n) and pKa (n+1)

= 3.5 + 10.5 + 12.5 + 10.5 = 9.25

.................................................................................

Step I: At pH 7, Arg has a fully protonated side chain and is capable of being only a hydrogen bond donor Any amino acid side chain capable of accepting a hydrogen bond can hydrogen bond with Arg at pH = 7. and same for Lysine.

Hence the total positive charge o0n the peptide (n) will be +3.

Step II: Met < Ser < Ala < Tyr < < Lys < Arg

Step III : The average value for pKa (n) and pKa (n+1)

= 10.5 + 12.5 + 10.5 = 11.16

PI = 11.16

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