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chem135-fall07-mt1-Francis-exam

chem135-fall07-mt1-Francis-exam - Chem 135 First Midterm...

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Chem 135: First Midterm September 28 th , 2007 Please provide all answers in the space provided. Extra paper is available if needed. You may not use calculators for this exam, but you are free to use (previously unassembled!) molecular model kits. Including the title page, there should be 6 pages in this exam booklet. Good Luck! Name: ___________________________________ (1) _____________ (17 points) (2) _____________ (15 points) (3) _____________ (37 points) (4) _____________ (31 points) TOTAL _____________ (100 points)
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1. Your Chem 135 GSIs have recently isolated a new pain killer from Peet’s coffee beans (yet another reason not to drink Starbucks!). As shown below, this compound exists as a ring, where the N- and C-termini of a polypeptide have joined to form a new amide bond. The stereochemistry of this structure has been omitted intentionally. The dotted outline is only for use in part b. NH NH HN HN HN HN O O O O O O NH O OH O NH 2 HO HO NH NH H 2 N + Start Here a) Starting from the site that is labeled with the arrow, provide the one letter abbreviations for the amino acids in this polypeptide. Write your sequence in the conventional order (7 points). b) Assume this cyclic peptide was synthesized from naturally occurring amino acids. Considering only the outlined portion of the structure shown above, redraw the side chains on the backbone segment below, clearly indicating the correct configuration of all stereocenters (10 points). NH NH HN HN O O O O
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2. Draw the dipeptide Trp-His with the amide bond in the cis conformation, as defined in class. Adjust the protonation state of the side chains to be what you would expect at pH 2 and indicate the pK a
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