Chem/Biochem 471 Exam 2 11/6/08 Page 1 of 7 Name:____________________________ (Please print clearly on all pages) Please leave the exam pages stapled together. The formulas are on a separate sheet. This exam has 5 questions. You must answer at least 4 of the questions. You may answer more questions if you wish. Answering 5 questions can be an advantage if you are unsure of some of your answers (this will distribute the “risk”). Answering 4 questions is advantageous if you are very sure of your answers. Each page is worth 20 points. The total exam grade will be normalized so that the maximum number of course points for this exam will be 20. For example, getting 80 points on 4 questions equals 100 points on 5 questions equals 20 points toward the final grade. Getting 80 points on 5 questions would be worth 80% of the maximum grade. If you leave a page blank, it will not be included in the grading. If you work on a page and then decide that you do not want it to be graded, be sure to mark the “DO NOT GRADE THIS PAGE ” box at the bottom of the page. If you work on the page and fail to mark the box, the page will be graded. Work at least 4 problems (of your choosing) or more, as you prefer. Answers without explanations (where indicated) are not complete.Chem/Biochem 471 Exam 2 11/6/08 Page 2 of 7 Name:____________________________ (Please print clearly on all pages) 1. The conversion of β-hydroxybutyrate, β-HB-, to acetoacetate, AAc-, is an important biochemical redox reaction that uses molecular oxygen, O2(g), as the ultimate oxidizing agent. You may assume that T = 25 °C and that solutions are ideal. reaction ε°’ (V) O2(g) + 4 H+(aq) + 4 e- → 2 H2O(ℓ) 0.816 AAc-(aq) + 2 H+(aq) + 2 e- → β-HB-(aq) -0.346 a) What is the net chemical reaction for the oxidation of β-hydroxybutyrate to acetoacetate? b) What is ε°’ for this reaction? c) If this reaction is run at pH 7.0 with the concentration of β-HB- equal to 50 mM and the concentration of AAc- equal to 100 mM, what is the value of ΔG’ per mole of β-HB- for the reaction? d) If ½ mole of O2 is reduced in the reaction, how many moles of electrons move between AAc- and β-HB-? e) Do the electrons of part d move from AAc- to β-HB- or in the opposite direction? { From AAc- to β-HB- { From β-HB- to AAc- DO NOT GRADE THIS PAGE Chem/Biochem 471 Exam 2 11/6/08 Page 3 of 7 Name:____________________________ (Please print clearly on all pages) 2. Consider an aqueous solution of urea at a concentration of 0.3 molal. For the following, you may assume that the solution is ideal and that it is at atmospheric pressure of 1 atm. The molar mass of water is 18 g/mol. a) What is the vapor pressure of the urea solution at 100 °C? b) The vapor pressure of pure water at 25 °C is 0.03 atm. What is the enthalpy of vaporization for water? c) What is the boiling point temperature of the urea solution? d) Urea actually forms complexes in solution (i.e. urea2, urea3, etc.). Would this fact affect your estimate of Tboil in part c? Explain. DO NOT GRADE THIS PAGE Chem/Biochem 471 Exam 2 11/6/08 Page 4 of 7 Name:____________________________ (Please print clearly on all pages) 3. Two aqueous solutions are separated by a semipermeable membrane. The two solutions are connected to a voltage source. The membrane is freely permeable to CO32- ions, but not to other ions. The left solution contains 5 mM CO32- while the right solution contains 100 mM CO32-. The voltage applied between the two solutions is 225 mV, with the right solution at a negative potential compared to the left solution, as shown. You may assume the solutions are ideal and T = 25 °C. a) Indicate the concentration gradient and electrical driving force directions for CO32- across the membrane between the two solutions. concentration driving electrical driving { from left to right { from left to right { from right to left { from right to left b) Calculate the chemical potential difference due to the concentration gradient, Δμconc = Δμconc,r - Δμconc,ℓ, for CO32-. c) Calculate the chemical potential difference due to the electric potential gradient, Δμelec = Δμelec,r - Δμelec,ℓ, for CO32-. d) If the voltage source is removed and replaced with a voltmeter, what will the electric potential difference reading, ΔΦ = Φr - Φℓ , on the voltmeter be when the system comes to equilbrium? DO NOT GRADE THIS PAGE Chem/Biochem 471 Exam 2 11/6/08 Page 5 of 7 Name:____________________________ (Please print clearly on all pages) 00.050.10.150.20.250.30.350 200 400 600 800 1000 1200[L] (mM)[Lbound] (mM) 4. The macromolecule P has multiple binding sites for ligand L. The sites are identical and independent of each other. A dialysis tube that is impermeable to P but permeable to L is filled with a solution of 0.1 mM P and no L. This tube is placed in a large beaker containing 25 mM of L. The volume of the beaker is very large compared to volume of the dialysis tube. After the system comes to equilibrium, you measure the total concentration of ligand in the tube to be 25.088 mM. You then add a very high concentration of L, 1 M, to the beaker and measure the total concentration of ligand in the tube to be 1000.28 mM after the system re-equilibrates. a) What is the concentration of ligand bound to P in the dialysis tube when the free ligand concentration is 25 mM? b) How many binding sites for L does each P contain? (Hint: you don’t need to do a complicated calculation here.) c) What is the value of the P-L binding association constant? d) Plot the concentration of L bound to P in the dialysis tube vs. free ligand concentration over the range of free ligand concentration shown on the graph at right. e) Suppose that a modifier M is added to the dialysis tube. The modifier M causes the L binding sites on P to interact with each other so that binding of an L to one site on P caused the other L binding sites on P to increase their affinity for L. Sketch the plot of concentration of L bound to P in the dialysis tube vs. free ligand concentration for this new situation on the graph at right. Be sure to clearly label both plots. DO NOT GRADE THIS PAGE Chem/Biochem 471 Exam 2 11/6/08 Page 6 of 7