Lecture 15b - Wednesday Lecture 15 Announcements RasMol...

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Wednesday, September 27, 2006 Lecture 15 Announcements: RasMol assignment #4, Exploring the Protein Data Bank Any interested students: Meet this Friday 9/29 in Comstock B106 from 1:25 - 2:15PM: “Application to medical schools: writing the personal statement”. IMPORTANT: CLASS ON MONDAY 10/2 IN BAILEY HALL (NOT STATLER). Monday's lecture: All the chemical reactions in Life are catalyzed reactions. No exceptions. How do these catalysts work? Rxn rate intro Study of rxn rates = kinetics. We must be able to describe quantitatively rxn rate, also termed rxn velocity. We saw how rate constants are used to describe forward and reverse rxn rates, and how they are related at equilibrium, for example: k 1 [A] eq x[B] eq = k -1 [C] eq x[D] eq and K eq = k 1 /k -1 = [C] eq x[D] eq /[A] eq x[B] eq Definition of the transition state: highest point on the free energy diagram; the most unlikely, least stable structure between reactants and products Today’s lecture, There are many ways to change the reaction pathway. We will look at a very simple example of a reaction pathway uncatalyzed, and then catalyzed. Our example is ester hydrolysis: addition of water to methyl acetate. At the top of p. 103, for the uncatalyzed reaction notice that there are three intermediates between the reactants and the products. Two intermediates involve separation of positive and negative charge . This separation is unfavorable, i.e. unstable, i.e. high in free energy. Ester hydrolysis catalyzed by acid has a totally different reaction pathway . Every intermediate in the acid-catalyzed mechanism is different from the intermediates of the non-catalyzed pathway. Notice that no intermediates in the acid-catalyzed pathway require charge separation (which is unfavorable). Also, the last three structures are actually resonance forms of the same molecule, so their stability is enhanced.
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**Thus, in the acid-catalyzed mechanism compared with the uncatalyzed mechanism, there are different chemical species between reactants and products, including a different TS . The key point is that the catalyzed TS has lower free energy than the uncatalyzed TS.**
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Lecture 15b - Wednesday Lecture 15 Announcements RasMol...

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