ProbSet10 - Problem Set 10. MCB C100A/Chem C130 Fall 2010...

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Problem Set 10. MCB C100A/Chem C130 Fall 2010 Do ** problems, due Thurs 2 Dec. **1. For the reactions below at pH 7 and 25°C the standard half cell voltages are: 2H + + 1 / 2 O 2 + 2e - H 2 O ε o ʼ = 0.820 V 2H + + cystine + 2e - 2 cysteine ε o ʼ = -0.340 V a) Write the balanced reaction which will be spontaneous if two half cells with these reactions were connected. b) If the second half cell is made with 0.01 M cystine and 0.01 M cysteine both present, what will the cell potential be, and what is the overall Δ G for the full reaction under these conditions? c) If the pH in both half cells was decreased by two units, would the cell potential increase or decrease (explain you answer)? If the pH in just the cystine/cysteine half cell was decreased what would happen? 2 . A graduate student was interested in analyzing the oxidation-reduction reactions of two heme containing proteins, cytochrome C (CytC) and myoglobin (Myb). In each case the net reaction is Fe +3 + e - Fe +2 . She found the standard reduction potentials for these two proteins in a Table (superscript o indicates oxidized, Fe +3 , r indicates reduced, Fe +2 ) : CytC o + e - CytC r ε o ʼ = 0.254 V Myb o + e - Myb r ε o ʼ = 0.046 V She knew how to easily make pure Myb o and Myb r , and could make CytC solutions that were mostly oxidized or mostly reduced but were still mixtures. Using an electrochemical cell set up with platinum electrodes on each side she carefully measured out 5 μM Myb o and 5 μM Myb r and put them together as one half cell, and on the other side she put a CytC solution with total CytC of 10 μM. She hooked up the cell to a voltmeter and measured a cell voltage of -0.267V, with the CytC side positive. a) What were the concentrations of CytC o and CytC r in the cytochrome half cell? b) If she just connected a wire between the cells and let it go until no more current ±owed, what would be the concentration of each form of protein on each side of the cell? c) She had a cytochrome C taken from a different organism, which contained an extra negatively
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ProbSet10 - Problem Set 10. MCB C100A/Chem C130 Fall 2010...

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