CHAPTER 2 (Figures)

CHAPTER 2 (Figures) - David L. Nelson and Michael M. Cox...

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Lehninger Principles of Biochemistry Fourth Edition Chapter 2: Water David L. Nelson and Michael M. Cox
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2 Introduction Water is the the most abundant substance in living system, making up 70% or more of the weight of most organism
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3 Weak Interactions in Aqueous System Hydrogen bonds (H-bonds) between water molecules provide the cohesive forces that make a liquid at room temperature and that favor the extreme ordering of molecules that is typical of crystalline water (ice). Polar biomolecules dissolves readily in water because they can replace water- water interaction with more energetically favorable water-solute interaction. In contrast, nonpolar 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. Very important interactions in aqueous systems include; H-bonds Ionic bonds Hydrophobic interactions Van de Waals (individually weak, but collectively very influential) i. H-bonding Water has very unusual melting point, boiling point and heat of vaporization than most solvents ( TABLE 2-1 ). WHY ? Because of the h-bonding between the oxygen atom of one water molecule and the hydrogen of another ( Figure 2-1 ), extra energy is require to destabilize these bonds.
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Weak Interactions in Aqueous System i. H-bonding (cont.) In ice, each water molecule is fixed in space and forms h-bonds with four other water molecules to yield a regular lattice structure (Fig. 2-2 ). Breakage of a sufficient number of H-bonds to destabilize the crystal lattice of ice requires much thermal energy, which accounts for the relatively high melting point of water. ii. H-bonding between and polar solvents H-bonding readily from between an electronegative atom (H, acceptor, usually O or N with lone pairs of electrons) and a hydrogen atom covalently bonded to another electronegative atom (hydrogen donor) in the same or another molecule ( Fig. 2-3 ). Hydrogen covalently bonded to C atoms do not participate in H-bonding. B.P of butanol is 117ºC whereas butane is only –0.5 ºC. WHY ? Butanol has a polar hydroxyl group thus can form intramolecular H-bonds. Alcohols, aldehydes, ketones, and compounds containing N-H bonds with water ( Fig. 2-4 ) and tends to be soluble. Hydrogen bonds are strongest when the bonded molecules are oriented to
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CHAPTER 2 (Figures) - David L. Nelson and Michael M. Cox...

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