voet 02 - 2 Water This chapter introduces you to the unique...

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2 Water This chapter introduces you to the unique properties of water and to acid–base reactions. The discussion of water begins with a look at its structure and how its polarity provides a basis for understanding its powers as a solvent. You are then introduced to the hydrophobic effect, osmosis, and diffusion. The chemical properties of water are then described, beginning with the ionization of water, which sets the stage for a discussion of acid–base chemistry and the behavior of weak acids and buffers. This discussion includes the Brønsted–Lowry definition for acids and bases, the definition of pH, and the derivation of the Henderson–Hasselbalch equation. As we shall see in subsequent chapters, a solid understanding of acid–base equilibria is fundamental to understanding key aspects of amino acid biochemistry, protein structure, enzyme catalysis, transport across membranes, energy metabolism, and other metabolic transformations. Essential Concepts 1. Water is essential to biochemistry because: (a) Biological macromolecules assume specific shapes in response to the chemical and physical properties of water. (b) Biological molecules undergo chemical reactions in an aqueous environment. (c) Water is a key reactant in many reactions, usually in the form of H + and OH ions. (d) Water is oxidized in photosynthesis to produce molecular oxygen, O 2 , as part of the process that converts the sun’s energy into usable chemical form. Expenditure of that energy under aerobic conditions leads to the reduction of O 2 back to water. Physical Properties of Water 2. The structure of water closely approximates a tetrahedron with its two hydrogen atoms and the two lone pairs of electrons of its oxygen atom “occupying” the vertices of the tetrahedron. 3. The high electronegativity of oxygen relative to hydrogen results in the establishment of a permanent dipole in water molecules. 4. The polar nature of water results in negative portions of the molecule being attracted to the positive portions of neighboring water molecules by a largely electrostatic interaction known as the hydrogen bond.
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Chapter 2 Water 14 5. Hydrogen bonds are represented as D H···A, where D H is a weakly acidic compound so that the hydrogen atom (H) has a partial positive charge, and A is a weakly basic group that bears lone pairs of electrons. A is often an oxygen atom or a nitrogen atom (occasionally sulfur). 6. Water is strongly hydrogen bonded, with each water molecule participating in four hydrogen bonds with its neighbors; two in which it donates and two in which it accepts. Hydrogen bonds commonly form between water molecules and the polar functional groups of biomolecules, or between the polar functional groups themselves.
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voet 02 - 2 Water This chapter introduces you to the unique...

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