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Chapter 4 Water

Chapter 4 Water. The electronegativity of oxygen polarizes its covalent bond to H. The lone pair electrons on oxygen make intermolecular hydrogen bonding (h-bonding) possible. Intermolecular hydrogen bonding gives water unique properties, including high melting point, high boiling point

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Chapter 4 Water

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  1. Chapter 4 Water The electronegativity of oxygen polarizes its covalent bond to H. The lone pair electrons on oxygen make intermolecular hydrogen bonding (h-bonding) possible.

  2. Intermolecular hydrogen bonding gives water unique properties, including high melting point, high boiling point and a high heat of vaporization.

  3. Liquid water forms “flickering clusters”, lasting a nanosecond, with 3.4 bonds per molecule on average Ice forms an ordered hydrogen bonded network with 4 bonds per molecule of water

  4. Hydrogen bonds are weak (20 kJ/mol vs hundreds of kJ/mol for covalent bonds). However, since water is the main component of all living things, the interaction of biomolecules with the hydrogen bonds in water is a major factor in biomolecular structure. For example, Lipids-Form membranes, micelles and organelles Proteins-Fold into a native, active conformation Nucleic Acid- complementary DNA strands spontaneously form a double helix in water

  5. The hydrogen bond acceptor is usually oxygen or nitrogen.

  6. A hydrogen bond is most stable when the donor, the hydrogen and the acceptor are in a straight line. This gives them directionality and allows them to dictate the geometry of their constituents.

  7. Examples of important hydrogen bonds

  8. The polar nature of the covalent O-H bonds in water make it an excellent solvent for other polar molecules

  9. The polar, partially charged nature of water makes it a good solvent for charged particles

  10. Uncharged, nonpolar molecules, such as alkyl chains, prevent contacted waters from forming the normal number of hydrogen bonds. This forces more order on the system, decreasing Entropy.

  11. Segregation of alkyl chain releases waters, increasing entropy

  12. Membrane bilayers are formed the same way

  13. Release of ordered water from substrates can help drive formation of an enzyme-substrate complex.

  14. Complexes are stabilized by complementary Noncovalent interactions

  15. Ionization of Water and the meaning of pH H2O H+ + OH- [H+][OH-] Keq = = 1.8 x 10-16 M [H2O] measured by electrical conductivity at 25 oC 1 L water weighs 1,000 g The molecular weight of water is 18.015g/mol [H2O] = 1000 g / 18.015 g/ mol = 55.5 M KW = Keq [H2O] = Keq (55.5 M ) = [H+][OH-] 1.8 x 10-16 M ( 55.5 M ) = 1.0 x 10-14 M2

  16. KW = [H+][OH-] = 1.0 x 10-14 M2 At neutral pH, [H+] = [OH-] 1.0 x 10-14 M2 = [H+] 2 1.0 x 10-14 M2 = [H+] 2 1.0 x 10-7 M = [H+] = [OH-] 1 = - log [H+] pH = log [H+] So at neutrality, pH = 7

  17. Buffers are important tools in biochemistry, allowing scientists to control the pH of their experiments. For any weak acid, HA H+ + A- HA [H+][A-] = Ka Keq = [HA] 1 pKa = log = -log Ka Ka A buffer consists of a weak acid (HA) and its conjugate base (A-).

  18. Weak acids and bases have characteristic dissociation constants CH3COOH CH3COO- + H+ A- + H+ HA [A-] [H+] = Ka Keq= [HA] Dissociation constant 1 pKa = - log Ka = log Ka Stronger the acid Higher the Ka Lower the pKa

  19. Conjugate acid-base pairs consist of a proton donor and a proton acceptor

  20. Buffers work from 1 pH unit below to 1 pH unit above their pKa

  21. Different buffers have different pKas

  22. Buffering occurs due to the simultaneous balancing of water and buffer dissociation reactions as governed by the constants Kw and Ka.

  23. The Henderson-Hasselbach equation is used to (1) calculate pKa , given pH and molar ratio of proton donor and acceptor (2) calculate pH, given pKa and molar ratio of proton donor and acceptor (3) calculate the molar ratio of proton donor and acceptor, given pH and pKa [proton acceptor] pH= pKa+ log [proton donor] When [proton acceptor] = [proton donor], pH= pKa

  24. Enzyme activity varies with pH as amino acids within the enzyme are protonated or deprotonated. Enzymes evolve to function best at the pH of their unique environment.

  25. Water is also an important reactant in biology. ATP hydrolysis, oxidation of fuels and photosynthesis all involve water. C6H12O6 + 6O2 6 CO2 + 6 H2O light O2 + 2 AH2 2H2O + 2A A is an electron acceptor.

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