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03ab - CHAPTER 3 Water The Matrix of Life Biochemistry The...

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Biochemistry: The Molecular Basis of Life, 4/e McKee/McKee Copyright © 2009 by Oxford University Press, Inc. C H A P T E R 3 Water: The Matrix of Life
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FIGURE 3.1 Tetrahedral Structure of Water 3.1 Molecular structure of water Overview Unique physico-chemical properties of water are essential for formation and evolution of life: - chemical stability - best solvent - hydration (noncovalent interactions of water with solutes) Tetrahedral structure of H 2 O: sp 3 orbitals of O: 2 are filed with 2 e - from 2 H and 2 lone pairs of e - Partial charges: δ +, δ -. Dipole (charges are separated). FIGURE 3.2 Charges on a Water Molecule 104.5° FIGURE 3.4 Molecular Dipoles in an Electric Field
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Hydrogen bonds Hydrogen bond: electrostatic interaction & elements of a covalent bond (electropositive nucleus & e - ) (sharing an e - pair). !! 460 kJ/mol 20 kJ/mol (< 5% strong) FIGURE 3.5 The Hydrogen Bond Each H 2 O molecule can form up to 4 H-bonds (weak but many). FIGURE 3.6 Tetrahedral Aggregate of Water Molecules
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FIGURE 3.7 Dipolar Interactions 3.2 Noncovalent bonding Noncovalent bonds – due to electrostatic interactions (< 4 - 80 kJ/mol vs . >210 kJ/mol for covalent). Weak (but many): stability to macromolecules and flexibility of rearrangements. Noncovalent bonds in cells : ! ionic: between oppositely charged ions; 4-80 kJ/mol COO - + H 3 N (salt bridge; no water) ! van der Waals: between neutral dipoles: permanent or induced <4-11.3 kJ/mol ! H bonds mixed 12-29 kJ/mol strength
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3.3 Thermal properties of water H-bonds: exceptionally high melting & boiling t (°C).
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