What is the isoelectric point (pI) of the polypeptide given the following pKa values: alpha-COOH pka=...

What is the isoelectric point (pI) of the polypeptide given the following pKa values:

alpha-COOH pka= 9.67

alpha-NH₃⁺ pKa= 2.17

guanidium R group pKa= 12.48

carboxylic acid R group pKa= 4.25

Expert Solution

You have mentioned as polypeptide, but sequence has not been given, therefore i am considering it as a single amino acid.

If the side chain does not have an ionizable group, then the pI is simply the average of the α-NH3 and α-COOH pKa values.

If the side chain has an ionizable group then all three pKa values must be considered.

If the side chain is acidic (asp and glu), then average the side chain pKa with the α-COOH pKa.

If the side chain is basic (his, arg, and lys), then average the side chain pKa with the α-NH3 pKa.

For present case guanidium is basic side chain and carboxylic acid R group is acidic.

So, (4.25 + 9.67) / 2 = 6.96 (OR) (12.48 + 2.17) / 2 = 7.325

If pH of solution is < pI of peptide, the peptide will have a net positive charge. The given pKa set represents overall net positive charge i.e from alpha-NH₃⁺ pKa= 2.17, carboxylic acid R group pKa= 4.25.

Hence pI = 6.96 (acidic medium will always imparts a positive charge due to protonation).

Hope this helped you!

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queen_honey_blossom answered 2 years ago

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