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# 07Fexam2key - Name_KEY_ EID_ CH 339K Exam 2 20 Sept 2007...

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Name____________ KEY __________________________ EID__________________________________________ 1 CH 339K Exam 2 20 Sept 2007 100 points total 1 . Five ml of a 20 mM solution of aspartate was adjusted to pH 4.0. 10 pts. a . Draw the structures of the principal ionization states present in solution (i.e. just those forms that would be present in any signficant amount at this pH). At pH 4, the α -NH 3 + group will remain completely protonated, thus only 3 species could possibly exist to any significant extent: COOH C CH 2 H 3 N + H H + H + 1.88 3.65 I II III COOH COO - C CH 2 H 3 N + H COOH COO - C CH 2 H 3 N + H COO - At pH 4.0, will species I be present to any significant extent? Calculate the I:II ratio from the Henderson- Hasselbalch equation (I = HA; II = A - ): 4.0 = 1.88 + log II I " 2.12 = log II I " 10 2.12 = II I " [ II ] = 132[ I ] So, we can neglect the concentration of I at this pH. Species II and III represent the principal forms of asp at pH 4.0. 10 pts. b . Calculate the concentration of each principal form drawn in part a. simply calculate the ratio of II and III (II = HA; III = A - ): 4.0 = 3.65 + log III II " 0.35 = log III II " 10 0.35 = III II " 2.24 = III II " [ III ] = 2.24[ II ] Since total [asp] = 20 mM = [II]+[III] 20 mM = [II] + 2.24[II] 20 mM = 3.24[II] [II] = 6.2 mM; [III] = 13.8 mM 5 pts. 2 . Estimate the molecular weight of a protein composed of 373 amino acids. State any assumptions you make. The average molecular weight of amino acid residues in proteins is 110 (see p. 87), thus: 373 aa x 110/aa = 41,030 molecular weight

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Name____________ KEY __________________________ EID__________________________________________
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## This note was uploaded on 03/22/2008 for the course CH 339K taught by Professor Appling during the Fall '08 term at University of Texas at Austin.

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07Fexam2key - Name_KEY_ EID_ CH 339K Exam 2 20 Sept 2007...

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