EXAM 1, BICH 410( MWF 1:50 PM), Friday, Sept. 22, 1995

Write your name on each page. Write concise answers to demonstrate effectively your mastery of the subject material. Show your work in order to receive partial credit where applicable.

1. (15 pts) If 50 ml of 0.01 M HCl is added to 100 ml of 0.06 M phosphate buffer (pH 7.16), what is the resultant pH? For the reaction H2PO4- to HPO4-2 + H+, the pKa = 6.86. Show your work and your reasoning explicitly.

2. (10 pts) Consider the following tripeptides:
Which one of these tripeptides:

3. (20 pts) Fill in the blanks with an appropriate answer or circle the correct answer. In some cases, several answers are possible, but write just one.

4. (15 pts) Use the table below that describes several properties of some hypothetical enzymes in order to answer the following questions. Before purification, these enzymes are combined in a single tube.

enzymepIsubunit composition and molecular weightmass (mg)activity units
CUase8.0 one @ 25,000100300 field goals
ATMase8.0 one @ 50,000100800 touchdowns
TUase4.5 two@ 25,000, each150200 safeties
Ricease5.0 three @ 10,000, each5050 touchdowns

(a) What chromatography method could be used to separate ATMase from TUase? Why?
(b) What chromatography method could be used to separate ATMase from CUase? Why?
(c) After steps (a) and (b) would ATMase be separated from the other enzymes?
(d) What is the specific activity of purified ATMase if 50% of the starting activity was recovered?
(e) After purification, by what factor would the ATMase be purified when compared to the starting mixture?

5. (10 pts) In their optimal orientations, draw two types of hydrogen bonds (as dotted lines) that could form between the peptide backbone and water molecules.

6. (10 pts) Using a flow chart, briefly describe the sequence of steps involved in the chemical synthesis of a dipeptide by the Merrifield method.

7. (a) (12 pts) Draw the structure of the dipeptide aspartylproline as it would exist at pH 7. You do not have to depict the proper stereochemistry in this structure.
(b) (3 pts) The pKas for the ionizable groups in aspartylproline are 1.99, 3.65 and 9.60 (using values from the individual amino acids). Assign each pKa to the correct ionizable group (either in writing below or on your structure drawn above).
(c) (5 pts) What is the pI (isoelectric point) of aspartylproline using the pKas given in part (b)? Show your reasoning for full credit.