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A peptide has the sequence of Gly-Ser-Glu-Leu-Ala-His-Gly-Arg-Leu-Ala-PheCys-Leu. (pKR=4.25, 6.0, 8.2, 12.5. Assume pKa’s of amino terminus and carboxyl terminus are 9.6 and 2.3, respectively.) The PI of the peptide is close to:_______

a. 7.1
b. 7.8
c. 5.1
d. 8.2
e. 10.3

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Answer:

The correct answer is option a. "7.1".

Explanation:

One easy way to determine if a peptide sequence is acidic, basic or neutral is to check for the number of amino acid residues that are acidic, basic or neutral. In this case, most amino acid residues are neutral, which mean that under neutral conditions they have a pKa close to 7.0. Particularly, the content of 3 leucine, 2 alanine and 2 glycine residues determines that the peptide have a pI of around 7.1.