L21to26Bio_Key

L21to26Bio_Key - MIT OpenCourseWare http:/ocw.mit.edu 5.111...

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MIT OpenCourseWare http://ocw.mit.edu 5.111 Principles of Chemical Science Fall 2008 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms .
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5.111 Principles of Chemical Science Solutions to selected biology-related questions from problem sets for lectures 21-26 Acid-base problems A. A pharmaceutical molecule with antifungal properties is only active when deprotonated and negatively charged (A - ). The protonated state (HA) is inactive. If the pK a of this drug is 10.0, (a) calculate the ratio of protonated to deprotonated compound at physiological pH (7.4). ( b ) Is this drug likely to be a useful pharmaceutical agent? (a) calculate the ratio of protonated to deprotonated compound at physiological pH (7.4). Since we are given both the pH and pK a of the compound, we can use the Henderson- Hasselbalch equation to solve for the ratio of [HA] to [A - ]. pH = pK a - log([HA] / [A - ]) log([HA] / [A - ]) = pK a – pH log([HA] / [A - ]) = 10.0 – 7.4 log([HA] / [A - ]) = 2.6 ([HA] / [A - ]) = 3 98.11 ([HA] / [A - ]) = 400 (correct sig figs) The ratio of protonated (inactive) compound to deprotonated (active) compound is 400 to 1 at physiological pH. ( b ) Is this drug likely to be a useful pharmaceutical agent? Since the vast majority of the compound is in the inactive form at physiological pH, it is unlikely to be a useful pharmaceutical agent.* Ideally, most of the compound would be active in the body. *However, if the active compound is highly potent, it is possible that a small fraction of active compound is sufficient for useful antifungal activity. B. Absorption of aspirin (acetylsalicylic acid, C 9 H 8 O 4 ,) into the bloodstream occurs only when the molecule is in its conjugate base form. (a) If a patient takes two tablets of aspirin (325 mg each), how many grams of aspirin are available for immediate absorption in the stomach? The pH of the stomach is 1.6, and the pK a of aspirin is 3.5.
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L21to26Bio_Key - MIT OpenCourseWare http:/ocw.mit.edu 5.111...

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