BISC 330 Spring 2011 Lecture 4

BISC 330 Spring 2011 Lecture 4 - BISC 330L: Biochemistry...

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Unformatted text preview: BISC 330L: Biochemistry Spring Semester 2011 USC Lecture 4: Wednesday, 19 January 2011 1. Begin Chapter 2 material In his classic, Nature of the Chemical Bond , Linus Pauling showed for chemical reaction, Reactant(R) Product(P), H o is related to bond dissociation energies (D A-B ) by H o = (React. bond D A-B values) - (Prod. bond D A-B values) Comparing D A-B with D A-A and D B-B , he found D A-B = 0.5(D A-A + D B-B ) + AB = d A + d B + AB, where AB is additional bonding energy arising from the electronegativity difference (X A-X B ) , which contributes to bond polarity and stability by increasing attraction between atoms A and B in A-B bond. In a balanced reaction (same atoms on both sides), only the AB component contributes to H o . He then showed AB (in kcal/mole) = 23|X A-X B | 2 (approx.), so that when equation is balanced (same atoms on both sides) H o (kcal/mole)= 23{ (R bond |X A-X B | 2 ) - (P bond |X A-X B | 2 )} Summary of Pauling Method for Estimating H o Major Covalent Bonds in Biochemistry and Their Dissociation Energies (D A-B , kcal mol-1 )* *[ L. Pauling. Nature of the Chemical Bond , 3rd Ed., 1960, p.85] H-H 104.2 H-C 98.8 H-N 93.4 H-O 110.6 C-C 83.1 C-H 98.8 C-N 69.7 C-O 84.0 N-N 38.4 N-H 93.4 N-C 69.7 O-O 33.2 O-H 110.6 O-C 84.0 S-S 50.9 S-H 81.1 S-C 62.0 (P-P 51.3) (P-H 76.4) P-O (84.2) Electronegativity values: H 2.1 C 2.5 N 3.0 O 3.5 P 2.1 S 2.5 Example:...
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This note was uploaded on 02/15/2012 for the course BISC 330L taught by Professor Petruska,tower during the Spring '07 term at USC.

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BISC 330 Spring 2011 Lecture 4 - BISC 330L: Biochemistry...

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