102 QUIZ 2 KEY - MCB102 Quiz#2 9/20 Please write your name...

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MCB102 Quiz#2 9/20 Please write your name on the back of the quiz. You may use pencil or pen. Question 1 (8 points) 1a. Draw the peptide with the sequence EDR at pH 7. pKa of terminal amino group is 9.7 pKa of terminal carboxyl group is 2.2 pKa of E’s side chain is 4.2 pKa of D’s side chain is 3.6 pKa of R’s side chain is 12.5 H H O H O H O ││ ││ ││ H ─ N + ─ C ─ C ─ N ─ C ─ C ─ N ─ C ─ C ─ O - 4.5 points total for structure H CH 2 H CH 2 H CH 2 CH 2 C─O - CH 2 ││ C─O - O CH 2 ││ O NH + C = NH 2 NH 2 -1 if switch E and D -1.25 if draw K not R -0.25 if one or both terminal charges incorrect for pH 7 -0.25 if R group charge incorrect for pH 7 in Glu’s or Asp’s R group -0.25 if put in extra H’s on alpha carbons or on backbone nitrogens -0.5 if incorrect number of CH 2 ’s in Glu’s or Asp’s R groups -0.5 if carboxyl group of Glu’s or Asp’s R groups drawn incorrectly -0.25 if incorrect number of CH 2 ’s in Arg’s R group -0.25 if NH in Arg’s R group drawn incorrectly -0.25 if C=NH 2 in Arg’s R group drawn incorrectly -0.25 if C-NH 2 in Arg’s R group drawn incorrectly -0.25 if charge incorrect for pH 7 or on wrong NH 2 in Arg’s R group 1b. What is the net charge of the peptide at pH 7? Show your work. 1 point total -1: see charges drawn above 1c. What is the net charge of the peptide at pH 3? Show your work. 1 point total amino terminus (+1) + E (0) + D (0) + R (+1) + carboxyl terminus (-1) = +1 pH>pKa group loses proton pH<pKa group gains proton Charge can be zero. 1d. What is the net charge of the peptide at pH 11? Show your work. 1 point total amino terminus (0) + E (-1) + D (-1) + R (+) + carboxyl terminus (-1) = -2 1e. In which pH range does the isoelectric point (pI) of this peptide occur? 0.5 points total A. below pH 3 B. between pH 3 and pH 7 Charge must be 0 at some point between when it’s +1 and -1 C. between pH 7 and pH 11 D. above pH 11
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MCB102 Quiz#2 9/20 Question 2 (7 points) Chromatography is a widely used technique for separating proteins based on their different properties.
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