5.111 Lecture Summary #17 Friday, October 17, 2014

Transcription

1 5.111 Lecture Summary #17 Friday, ctober 17, 2014 Reading for today: Sections 8.8, 8.12, 8.13, 8.15, and 8.16 (same sections but in hapter 7 in 4 th ed): Entropy and Gibbs Free Energy, and Free-Energy hanges in Biology. Reading for Lecture #18: Sections (same sections but in hapter 9 in 4 th ed): hemical Equilibrium Topics: Thermodynamics I. Effect of temperature on spontaneity II. Thermodynamics in biological systems A. ydrogen-bonding B. ATP-coupled reactions I. EFFET F TEMPERATURE SPTAEITY onsider the decomposition of sodium bicarbonate at 298 versus 450. K. 2a 3 (s) a 2 3 (s) + 2 (g) + 2 (g) º = kj/mol Sº = kj/ (K mol) G r = r T( S r ) At T = 298K Gº = kj/mol -298 K( kj/ (K mol)) = kj/mol The reaction is at room temperature. But at baking temperatures of 350ºF or 450.K Gº = (450.)(0.334) = kj/mol The reaction is at baking temperature. When º and Sº have the sign, it is possible to control spontaneity with T. Assuming that º and Sº are independent of T, a reasonable first-order assumption, then Gº is a function of T. 1

2 Gº = º T Sº or Gº = Sº(T) + º alculate T* (at which Gº = 0 ) for the decomposition of sodium bicarbonate. Gº = º T Sº 0 = º T* Sº T* = T* = kj/mol = K kj/(k mol) onsider the plot of temperature dependence when both º and Sº are negative Gº = º T Sº 2

3 Gº = º T Sº º < 0 Sº > 0 spontaneous Gº < º > 0 Sº < 0 spontaneous Gº > º > 0 Sº > 0 spontaneous Gº < T* º < 0 Sº < 0 spontaneous Gº < T* Temperature is important! It can influence both spontaneity and rate of reaction. II. TERMDYAMIS I BILGIAL SYSTEMS A) YDRGE BDIG A hydrogen bond is an interaction between a hydrogen bond donor (a hydrogen atom in a polar bond, typically a -F, - or - bond), and a hydrogen-bond acceptor (an electronegative atom X Y where X =,, F δ+ δ- For example, hydrogen bonds form between water molecules: δ+ δ+ δ Mean bond enthalpies of hydrogen-bonds (-bonds): -bonds are the strongest type of intermolecular interaction. owever, -bonds are weaker than covalent or ionic bonds. mean bond enthalpy (in kj/mol) bond covalent bond -bond -bond covalent bond bonding can be intermolecular (as in the water molecules above) or intramolecular. Intramolecular -bonds in proteins are required for a protein s 3-dimensional shape. 3

4 ydrogen Bonding in DA ydrogen bonding binds together complementary strands of DA to form a double helix. 3 Guanine ytosine Adenine Thymine The lower bond enthalpies of hydrogen bonds compared to covalent bonds facilitate the separation of DA strands during DA replication. I TEIR W WRDS: WY YDRGE BDS ARE IMPRTAT RA interference (RAi) is a process in which small pieces of silencing RA (sira), typically 19 to 25 base pairs in length, inhibit gene expression. Target RA from gene omplimentary sira Image from "Behind the Scenes at MIT. The Drennan Education Laboratory. Licensed under a reative ommons Attribution-onommercial-ShareAlike License. Dr. Lourdes Aleman discusses how understanding hydrogen bonding is essential to her research on RAi in the lab of Prof. Phil Sharp, and how scientists hope to use their understanding of RAi to design treatments for genetic and viral diseases. B) ATP-UPLED REATIS Many biological reactions are non-spontaneous, meaning they require energy to proceed in the forward direction. The hydrolysis of ATP (ATP ADP), a spontaneous process, can be a non-spontaneous reaction to drive the reaction forward. to The resulting Gº of the coupled reaction is the sum of the individual Gº values. 4

5 First, let s calculate the Gº for ATP hydrolysis at 310 K (body temperature). = -24 kj/mol (from Lecture #16) S = +22 J/K mol 2 2 P P 2 P P P (l) +P 4 2 (aq) (aq) G = -24 kj/mol (310 K)(0.022 kj/(k mol)) = kj/mol ote: the calculated free energies are under standard conditions. This is an approximation since these molecules are T under standard conditions in cells. Example of an ATP-coupled reaction: the conversion of glucose to glucose-6-p. Adding a phosphate (P) group to glucose gives the glucose a negative charge, which prevents the glucose molecule from diffusing back out of the cell through the greasy cell membrane. - - P 2 2 Gº = +17 kj/mol for glucose to glucose-6-p Gº = kj/mol for ATP hydrolysis An enzyme couples the glucose-to-glucose-6-p reaction to ATP hydrolysis. The net change in free energy = G total º = - = kj/mol If ATP hydrolysis is spontaneous, why is it not occurring unregulated in the cell? Kinetics. A reaction can be thermodynamically spontaneous, but kinetically very very slow. 5

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