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# keytest1 - BCH 4053Summer 2001Chapter 4 Lecture Notes Slide...

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BCH 4053 January 31, 2001 HOUR TEST 1 NAME_______ KEY _________ (10) 1. Vitamin C (ascorbic acid) is a diprotic acid, with dissociation constants: pK 1 = 4.1; pK 2 = 11.8. If vitamin C were excreted in the urine of a patient, and the urine had a pH of 4.5, what fraction of the vitamin would carry a negative charge? (Note: I am asking for a fraction and not a ratio ). 5 pts ratio, 5 pts fraction, accept 72% Page Points 1 _ 33 _ 2 _ 24 _ 3 _ 25 _ 4 _ 18 _ Total __ 100 _ 12 p K pK 2 21 4. 1 11.8 p H p K 4. 5 4. 1 0.40 2 As H As H A s (needconsideronlypK) [AsH] 1 0 1 0 1 0 2.51 [AsH ] 2.5 1 2.51 fractionof As H 0.72 [AsH ] [AsH ]1 2.5 1 3.51 -- - - - -   → == == = = == ++ (14) 2. Draw a titration curve for cysteine on the graph below. (a) Locate and identify the points on the curve corresponding to 1 , 2 , and 3 . (b) Calculate the approximate pI value and locate its position on the curve. (c) Indicate the pH region of the graph in which the side chain functional group is more than 75% charged. 0 1 2 3 4 5 6 7 8 9 10 11 12 13 0 0.5 1 1.5 2 2.5 3 equivalents of base pH . . . 1 2245 2 2 2245 8.5 3 2245 9.5 pI = (pK1+pK2)/2 = (2 + 8.5)/2 2245 5.3 above pH 9 side chain is >75% charged. 2 pts ea: pK 1 , pK 2 , pK 3 ; 3 pts ea. pI, pH of 75% side chain charge, 2 pts shape and location of curve (9) 3. Underline the following peptides which are negatively charged at pH 7.0. Circle each amino acid which is aromatic . Put an X through each amino acid that contains a sulfur atom . 1 pt ea. Correctly underlined or not; 1 pt each circled, 1 pt each X, -0.5 for incorrect circling or X.

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keytest1 - BCH 4053Summer 2001Chapter 4 Lecture Notes Slide...

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