Bis102- MT1- W'03 KEY- Gasser

As done above ph 62 3 20 histidine can be used as

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Unformatted text preview: ) (20) Histidine can be used as a buffer in solutions used in biochemistry. How much of a 1 M solution of the monosodium salt of histidine (NaHis) and how much 1 M HCl or 1M NaOH would you have to add to make one liter of 0.1 M histidine buffer at pH 6.0? His+2 <–> His+1 <–> His0 <–> His-1 1.8 6.0 9.2 are the pKa s Must add 0.1 liter of 1 M NaHis to make 1 liter of 0.1 M buffer. This is the His-1form. Final buffer is at pH 6.0 so it will have equal concentrations of His+1 and His0 . Therefore [His+1] = [His0 ] = 1/2 • 0.1 M = 0.05 M Must add HCl to interconvert forms of His. Add 0.1 mole to convert all His-1 to His0 , then add additional 0.05 mole to convert 0.05 moles of His0 to 0.05 moles of His+1. This produces the correct ratio of the different forms to make the buffer at pH 6.0. Added HCl = 0.1 mole + 0.05 mole = 0.15 mole , since it is 1 M, this is 0.15 liter 1.0 M NaHis = 0.1 liter Units 1.0 M HCl or NaOH = (circle one) 0.15 liter Units Name KEY Last, First BIS102-03, Winter '03, Page 4 of 7 C. S. Gasser 4. Write the one and three letter codes for all of the amino acids whose side chains (R-groups) fit into the following categories. a) (4) The neutral form can act as a proton donor. Asp D, Glu E, Cys C, Tyr Y b) (4) Contains sulfur. Cys C, Met M c) (4) Is aromatic and can also form hydrogen bonds. Tyr Y, Trp W d) (4) Can have a positive charge. His H, Lys K, Arg R 5. (25) Place an X next to the ONE correct answer to each of the following questions. a) Which of the following could form an H-bond with the carbonyl oxygen of a peptide bond? X The hydrogen in an alcohol group. One of the hydrogens in the side chain of amino acid F. A nitrogen in the side chain of Asparagine One of the hydrogens in the side chain of Leu An oxygen in the side chain of amino acid N b) The reaction A <––> B will always go in the forward direction if: DG°' for the reaction is less than 0 The concentration of A is greater than that of B B is an amino acid X DG for the reaction is less than zero at the current concentrations of A and B DG for the reaction is greater than zero at the current concentrations of A and B Name KEY Last, First BIS102-03, Winter '03, Page 5 of 7 C. S. Gasser (problem 5, continued) c). The weak acid HA, like all weak acids, undergoes the following reaction to a limited extent in water : HA + H2 O <––> A- + H3 O+ The ratio between the Ka for this acid and the Keq for this reaction (Ka/Keq) would be equal to: Kw X 55.6 M [H + ][A - ] [H+ ][A - ] K a /Keq = = [H2 O] = 55.6M [HA] [HA][H2 O] 1/55.6 M Kb for NaA cannot tell from this information d) The reaction A <––> B will go in the forward direction if: DS is positive. X TDS is positive and greater than DH (this ensures that DG is negative) DH is positive and DS is positive DH is positive and DS is negative A is more organized than B e) Van der Waals (induced dipole) interactions: are most important for charged molecules act only between h...
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This note was uploaded on 09/19/2013 for the course BIS 102 taught by Professor Hilt during the Spring '08 term at UC Davis.

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