Water structure

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Water structure

  1. 1. Water STRUCTURE
  2. 2. WHY DO WE NEED TO DO THIS AGAIN!!!
  3. 3. Properties of water Very polar Oxygen is highly electronegative H-bond donor and acceptor High b.p., m.p., heat of vaporization, surface tension
  4. 4. Water dissolves polar compounds solvation shell or hydration shell
  5. 5. Non-polar substances are insoluble in water Many lipids are amphipathic
  6. 6. How detergents work?
  7. 7. Hydrogen Bonding of Water One H2O molecule can associate with 4 other H20 molecules Ice: 4 H-bonds per water molecule Water: 2.3 H-bonds per water molecule
  8. 8. Crystal lattice of
  9. 9. Biological Hydrogen Bonds
  10. 10. non-covalent interactions
  11. 11. Relative Bond Strengths Bond type kJ/mole H3C-CH3 88 H-H 104 Ionic 40 to 200 H-bond 2 - 20 Hydrophobic interaction 3 -10 van der Waals 0.4 - 4
  12. 12. Ionization of Water
  13. 13. Ionization of Water H20 + H20 H2 0 Keq= [H+] [OH-] [H2O] H3O+ + OHH+ + OHKeq=1.8 X 10-16M [H2O] = 55.5 M [H2O] Keq = [H+] [OH-] (1.8 X 10-16M)(55.5 M ) = [H+] [OH-] 1.0 X 10-14 M2 = [H+] [OH-] = Kw If [H+]=[OH-] then [H+] = 1.0 X 10-7
  14. 14. pH Scale  Devised by Sorenson (1902)  [H+] can range from 1M and 1 X 10-14M  using a log scale simplifies notation  pH = -log [H+] Neutral pH = 7.0
  15. 15. Weak Acids and Bases Equilibria •Strong acids / bases – disassociate completely •Weak acids / bases – disassociate only partially •Enzyme activity sensitive to pH • weak acid/bases play important role in protein structure/function
  16. 16. Acid/conjugate base pairs HA + H2O HA A- + H3O+ A- + H+ HA = acid ( donates H+)(Bronstad Acid) A- = Conjugate base (accepts H+)(Bronstad Base) Ka = [H ][A ] [HA] + - pKa = - log Ka Ka & pKa value describe tendency to loose H+ large Ka = stronger acid small Ka = weaker acid
  17. 17. pKa values determined by titration
  18. 18. Phosphate has three ionizable H+ and three pKas
  19. 19. Buffers Buffers are aqueous systems that resist changes in pH when small amounts of a strong acid or base are added. A buffered system consist of a weak acid and its conjugate base.
  20. 20. The most effective buffering occurs at the region of minimum slope on a titration curve (i.e. around the pKa). Buffers are effective at pHs that are within +/-1 pH unit of the pKa
  21. 21. Henderson-Hasselbach Equation 1) Ka = [H ][A ] [HA] + - HA = weak acid A- = Conjugate base 2) [H+] = Ka [HA] [A-] 3) -log[H+] = -log Ka -log [HA] [A-] 4) -log[H+] = -log Ka +log [A-] [HA] 5) pH = pKa +log [A-] [HA] * H-H equation describes the relationship between pH, pKa and buffer concentration
  22. 22. Case where 10% acetate ion 90% acetic acid • pH = pKa + log10 [0.1 ] ¯¯¯¯¯¯¯¯¯¯ [0.9] • pH = 4.76 + (-0.95) • pH = 3.81
  23. 23. Case where 50% acetate ion 50% acetic acid • pH = pKa + log10 [0.5 ] ¯¯¯¯¯¯¯¯¯¯ [0.5] • pH = 4.76 + 0 • pH = 4.76 = pKa
  24. 24. Case where 90% acetate ion 10% acetic acid • pH = pKa + log10 [0.9 ] ¯¯¯¯¯¯¯¯¯¯ [0.1] • pH = 4.76 + 0.95 • pH = 5.71
  25. 25. Cases when buffering fails • pH = pKa + log10 [0.99 ] ¯¯¯¯¯¯¯¯¯¯ [0.01] • pH = 4.76 + 2.00 • pH = 6.76 • pH = pKa + log10 [0.01 ] ¯¯¯¯¯¯¯¯¯ [0.99] • pH = 4.76 - 2.00 • pH = 2.76

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