ps2_key - Bi8 PROBLEM SET#2 Name _ TA _ Section # _ DUE BY...

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Bi8 PROBLEM SET#2 Name __________________________________ TA _________________________ Section # _______ DUE BY JAN 19, 1pm – turn in at the beginning of class No late homework will be collected. However, it can be turned in early in the box outside Prof. Stathopoulos’ office in Broad 261. Please answer questions directly on this sheet. Enough room has been allotted for appropriate answers. Answers taken directly from the textbook or another source will receive no credit. You MUST rephrase the answer into your own words. Problem 1: Henderson-Hasselbach Equation You have just synthesized a peptide that has shown promise to be a memory-enhancer! Upon testing it on your fellow Caltech students, however, you notice that it only seems to work when you ingest it with large amounts of Coca-cola or lemon juice, and that it does not work when it’s only dissolved in water. A useful table has been included at the end of this problem set. The sequence of the peptide is: Ile-His-Ser-Asn-Lys-Gly-Phe. A) (14 pts) Draw the chemical structure of the peptide (include all carbons and hydrogens) at pH 7.4. Remove 1 point for each structural mistake, 2 points for significant (in red). B) (8 pts) Describe how the peptide would change at the pH of Coca-Cola (pH 3.0)? What about when mixed with antacids (pH 11.0)? What would the net charge be in each case? In Coca-Cola, the histidine would gain a proton and become positively charged. This would cause the net charge to be +2. With antacids, the amino group of isoleucine would lose a proton C NH CH C NH CH C NH CH C NH CH C NH C C NH CH C C H N H 3 + O O O O O O O CH C H 3 CH 2 CH 3 CH 2 C C H N H CH N CH 2 OH C H 2 C O NH 2 CH 2 CH 2 CH 2 CH 2 NH 3 + CH 2 C C H C H CH CH CH O - H H
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and become uncharged; histidine would be uncharged; and lysine would lose its proton and become uncharged. The net charge would be -1. C) (5 pts) What must be true of the ionization state of the active peptide? Why? The histidine must be protonated in order for the peptide to be active. The difference between the peptide at pH 3.0 and pH 7.4 is that the histidine is not protonated at pH 7.4. Therefore, this must be the reason for which the peptide is active only when ingested with highly acidic substances. D) (10 pts) Using the Henderson-Hasselbach equation, calculate the ratio of active peptide to inactive peptide at pH 7.4 and at pH 3.0. Show your work for credit. ± pH = pKa + log( [ A ] [ HA ] ) The ionization state of His is critical to the activity of the peptide. Therefore, it is necessary to determine how much of the His is protonated (active) vs. how much is deprotonated (inactive). Student must show work; no work, no credit.
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This note was uploaded on 04/25/2010 for the course BI 8 taught by Professor Stathopolous during the Winter '08 term at Caltech.

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ps2_key - Bi8 PROBLEM SET#2 Name _ TA _ Section # _ DUE BY...

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