Week 3 Practice Quiz

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The University of Oklahoma *

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9/15/22, 5:42 PM Quiz: Week 3 Practice Quiz F22 https://canvas.ou.edu/courses/249882/quizzes/357868/take 1/15 Week 3 Practice Quiz F22 Started: Sep 15 at 5:42pm Quiz Instructions 1 pts Question 1 essentially all PO essentially all HPO none of the above 50:50 H PO and HPO essentially all H PO 50:50 H PO and H PO essentially all H PO 50:50 HPO and PO Consider the titration curve of phosphate (PO ) ion shown below. When 1.5 equivalents of H has been added to a solution of phosphate ion, what species predominates? 4 3− + 4 3− 4 2− 2 4 − 4 2− 2 4 − 3 4 2 4 − 3 4 4 2− 4 3−

9/15/22, 5:42 PM Quiz: Week 3 Practice Quiz F22 https://canvas.ou.edu/courses/249882/quizzes/357868/take 2/15 1 pts Question 2 75:25 H PO to HPO 25:75 H PO to H PO essentially all HPO essentially all H PO none of the above 25:75 H PO to HPO 50:50 H PO and HPO 75:25 H PO to H PO essentially all H PO essentially all PO 50:50 H PO and H PO 50:50 HPO and PO Consider the titration curve of phosphoric acid (H PO ) shown below. What species will be present when 0.25 equivalents of OH has been added? 3 4 − 2 4 − 4 2− 3 4 2 4 − 4 2− 2 4 − 2 4 − 4 2− 2 4 − 4 2− 3 4 2 4 − 3 4 4 3− 3 4 2 4 − 4 2− 4 3−

9/15/22, 5:42 PM Quiz: Week 3 Practice Quiz F22 https://canvas.ou.edu/courses/249882/quizzes/357868/take 3/15 1 pts Question 3 L D The structure below is that of a generalized amino acid. Assuming that the R group is not H, then what is the relative configuration (D or L) of the structure that is shown? Note that the alpha-carbon is at the intersection of the two lines, the horizontal bonds are projecting out of the plane of the paper (or screen), and the vertical bonds are projecting behind the plane of the paper (or screen). 1 pts Question 4 The structure below is that of a generalized amino acid. Assuming that the R group is not H and that it has lower priority than the alpha-amino and alpha-carboxyl groups, what is the absolute configuration ( R or S ) of the structure that is shown? Note that the alpha carbon is at the intersection of the two lines, the horizontal bonds are projecting out of the plane of the paper (or screen), and the vertical bonds are projecting behind the plane of the paper (or screen).

9/15/22, 5:42 PM Quiz: Week 3 Practice Quiz F22 https://canvas.ou.edu/courses/249882/quizzes/357868/take 4/15 R S 1 pts Question 5 7.6 8.2 3.9 9.3 8.8 6.1 10.4 A compound used to make a buffer is known to have two aminium functional groups and a carboxyl functional group. The respective p K s of the groups are 8.2, 10.4, and 3.9. A biochemist has 100.0 mL of a 0.25 M solution of this compound at an initial pH of 8.2. She adds 50.0 mL of a 0.15 M solution of HCl to the solution. What will be the final pH of this solution after the addition of the HCl? a 1 pts Question 6 3.4 4.4 A compound used to make a buffer is known to have two aminium functional groups and a carboxyl functional group. The respective p K s of the groups are 8.2, 10.4, and 3.9. A biochemist has 100.0 mL of a 0.10 M solution of this compound at an initial pH of 8.2. She adds 50.0 mL of a 0.15 M solution of HCl to the solution. What will be the final pH of this solution after the addition of the HCl? a

9/15/22, 5:42 PM Quiz: Week 3 Practice Quiz F22 https://canvas.ou.edu/courses/249882/quizzes/357868/take 5/15 3.9 8.2 10.4 9.3 6.1 1 pts Question 7 Yes, the statement is true. No, the statement is false. In a normal alpha-helix, there are 3.6 residues per turn, and the pitch is 5.4 angstroms. 1 pts Question 8 Yes, the statement is true. No, the statement is false. Intra-helix hydrogen-bonding in an alpha-helix occurs between the amide (carbonyl) oxygen of one residue (call it the n residue) and the hydrogen that is bonded to the amide nitrogen of the residue that occurs four residues beyond (call it the n +4 residue). (Assuming that there is an amino acid residue in the alpha helix four residues ahead of the n one.) th th th 1 pts Question 9

9/15/22, 5:42 PM Quiz: Week 3 Practice Quiz F22 https://canvas.ou.edu/courses/249882/quizzes/357868/take 6/15 The statement is false. The statement is true. The hydrogen bonding pattern between adjacent beta-strands that run antiparallel in a beta sheet tend to be off-axis such that the amide nitrogen (to which the hydrogen is covalently bonded) is not in line with the amide oxygen (hydrogen bond acceptor) of the neighboring beta-strand. 1 pts Question 10 10.3 7.0 What is the isoelectric point (pI) of the following peptide? To facilitate answering this question, assume that the N-terminal aminium group has a p K of 8, the C-terminal carboxyl group has a p K of 3.5, and that the ionizable side chains have p K s as follows: Asp = 4.0, Glu = 4.0, His = 6, Cys = 8.4, Tyr = 10, Lys = 10, and Arg = 12.5. Also, assume that if the pH is at least two units above or below the p K of a particular group, then that group is completely unprotonated or protonated, as the case may be. Hint: you might want to imagine what the net charge is at pH 7, and then if the net charge is positive you know that the pI will be greater than 7; if the net charge is negative at pH 7, you know that the pI is less than 7. What you should find is that there will be two groups that bracket the point at which the net charge is zero. The pI will be the average of the p K s of these two groups. a a a a a

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