8885dc02_47-747/25/0310:05 AM Page47mac76mac76:385 chapter WATER 2.1 Weak Interactions in Aqueous Systems 47 and titration curves, and consider how aqueous solu 2.2 lonization of water Weak Acids, and tions of weak acids or bases and their salts act as buffers Weak Bases 60 against pH changes in biological systems. The water 2.3 Buffering against pH Changes in Biological molecule and its ionization products, h and oh, pro- roundly influence the structure, self-assembly, and prop erties of all cellular components, including proteins 2. 4 Water as a Reactant 69 nucleic acids, and lipids. The noncovalent interactions 2.5 The Fitness of the Aqueous Environment responsible for the strength and specificity of " recogni or Living Organisms 70 tion"among biomolecules are decisively influenced by the solvent properties of water, including its abilit Torm hydrogen bonds with itself and with solutes I believe that as the methods of structural chemistry ar further applied to physiological problems, it will be found 2.1 Weak Interactions in Aqueous Systems that the significance of the hydrogen bond for physiology Hydrogen bonds between water molecules provide the is greater than that of any other single structural feature cohesive forces that make water a liquid at room tem- Linus Pauling, The Nature of the Chemical Bond, 1939 perature and that favor the extreme ordering of mole- cules that is typical of crystalline water (ice). Polar bio- What in water did Bloom water lover, drawer of water, water molecules dissolve readily in water because they can carrier returning to the range, admire? Its universality, its replace water-water interactions with more energetically favorable water-solute interactions. In contrast, nono- democratic quality lar biomolecules interfere with water-water interactions -James Joyce, Ulysses, 1922 but are unable to form water-solute interactions- consequently, nonpolar molecules are poorly soluble in water. In aqueous solutions, nonpolar molecules tend to later is the most abundant substance in living sys- cluster together tems, making up 70% or more of the weight of most organisms. The first living organisms doubtless arose in Hydrogen bonds and ionic, hydrophobic (Greek an aqueous environment, and the course of evolution " water-fearing), and van der waals interactions are in- has been shaped by the properties of the aqueous dividually weak, but collectively they have a very sig- nificant influence on the three-dimensional structures medium in which life began. This chapter begins with descriptions of the physical of proteins, nucleic acids, polysaccharides, and mem- and chemical properties of water, to which all aspects rane lipids f cell structure and function are adapted. The attrac- tive forces between water molecules and the slight ten- Hydrogen Bonding Gives Water Its Unusual Properties dency of water to ionize are of crucial importance to the Water has a higher melting point, boiling point, and heat structure and function of biomolecules. We review the of vaporization than most other common solvents (table topic of ionization in terms of equilibrium constants, pH, 2-1). These unusual properties are a consequence ofchapter WATER 2.1 Weak Interactions in Aqueous Systems 47 2.2 Ionization of Water, Weak Acids, and Weak Bases 60 2.3 Buffering against pH Changes in Biological Systems 65 2.4 Water as a Reactant 69 2.5 The Fitness of the Aqueous Environment for Living Organisms 70 I believe that as the methods of structural chemistry are further applied to physiological problems, it will be found that the significance of the hydrogen bond for physiology is greater than that of any other single structural feature. —Linus Pauling, The Nature of the Chemical Bond, 1939 What in water did Bloom, water lover, drawer of water, water carrier returning to the range, admire? Its universality, its democratic quality. —James Joyce, Ulysses, 1922 O O C H C H – 2 47 Water is the most abundant substance in living sys￾tems, making up 70% or more of the weight of most organisms. The first living organisms doubtless arose in an aqueous environment, and the course of evolution has been shaped by the properties of the aqueous medium in which life began. This chapter begins with descriptions of the physical and chemical properties of water, to which all aspects of cell structure and function are adapted. The attrac￾tive forces between water molecules and the slight ten￾dency of water to ionize are of crucial importance to the structure and function of biomolecules. We review the topic of ionization in terms of equilibrium constants, pH, and titration curves, and consider how aqueous solu￾tions of weak acids or bases and their salts act as buffers against pH changes in biological systems. The water molecule and its ionization products, H
and OH, pro￾foundly influence the structure, self-assembly, and prop￾erties of all cellular components, including proteins, nucleic acids, and lipids. The noncovalent interactions responsible for the strength and specificity of “recogni￾tion” among biomolecules are decisively influenced by the solvent properties of water, including its ability to form hydrogen bonds with itself and with solutes. 2.1 Weak Interactions in Aqueous Systems Hydrogen bonds between water molecules provide the cohesive forces that make water a liquid at room tem￾perature and that favor the extreme ordering of mole￾cules that is typical of crystalline water (ice). Polar bio￾molecules dissolve readily in water because they can replace water-water interactions with more energetically favorable water-solute interactions. In contrast, nonpo￾lar biomolecules interfere with water-water interactions but are unable to form water-solute interactions— consequently, nonpolar molecules are poorly soluble in water. In aqueous solutions, nonpolar molecules tend to cluster together. Hydrogen bonds and ionic, hydrophobic (Greek, “water-fearing”), and van der Waals interactions are in￾dividually weak, but collectively they have a very sig￾nificant influence on the three-dimensional structures of proteins, nucleic acids, polysaccharides, and mem￾brane lipids. Hydrogen Bonding Gives Water Its Unusual Properties Water has a higher melting point, boiling point, and heat of vaporization than most other common solvents (Table 2–1). These unusual properties are a consequence of 8885d_c02_47-74 7/25/03 10:05 AM Page 47 mac76 mac76:385_reb: