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Water

Water. Properties of water. Very polar Oxygen is highly electronegative H-bond donor and acceptor High b.p., m.p., heat of vaporization, surface tension. Water dissolves polar compounds. solvation shell. Non-polar substances are insoluble in water. Many lipids are amphipathic.

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Water

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  1. Water

  2. Properties of water • Very polar • Oxygen is highly electronegative • H-bond donor and acceptor • High b.p., m.p., heat of vaporization, surface tension

  3. Water dissolves polar compounds solvation shell

  4. Non-polar substances are insoluble in water Many lipids are amphipathic

  5. 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 Crystal lattice of ice

  6. Relative Bond Strengths Bond typeKJ/mole H3C-CH3 88 H-H 104 Ionic 10-20 H-bond 3-6 Hydrophobic interaction 1-3 van der Waals 1

  7. Biological Hydrogen Bonds d+ d- H-Bond distance = ~0.2 nm d+ d- DNA strands held together by H-bonds

  8. H20 + H20 H3O+ + OH- Ionization of Water H20 H+ + OH- Keq=1.8 X 10-16M Keq= [H+] [OH-] [H2O] [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

  9. 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

  10. 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

  11. Conjugate acid conjugate base pairs HA + H2O A- + H3O+ HA A- + H+ HA = Conjugate acid ( donates H+)(Bronstad Acid) A- = Conjugate base (accepts H+)(Bronstad Base) Ka & pKa value describe tendency to loose H+ large Ka = stronger acid small Ka = weaker acid Ka = [H+][A-] [HA] pKa = - log Ka

  12. pKa values determined by titration

  13. Phosphate has three ionizable H+ and three pKas

  14. 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. • 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

  15. Henderson-Hasselbach Equation HA = Conjugate acid A- = Conjugate base 1) Ka = [H+][A-] [HA] 2) [H+] = Ka [HA] [A-] 3) -log[H+] = -log Ka -log [HA] [A-] * H-H equation describes the relationship between pH, pKa and buffer concentration 4) -log[H+] = -log Ka +log [A-] [HA] 5) pH = pKa +log [A-] [HA]

  16. Case where 10% acetate ion 90% acetic acid • pH = pKa + log10[0.1 ] • ¯¯¯¯¯¯¯¯¯¯ • [0.9] • pH = 4.76 + (-0.95) • pH = 3.81

  17. Case where 50% acetate ion 50% acetic acid • pH = pKa + log10[0.5 ] • ¯¯¯¯¯¯¯¯¯¯ • [0.5] • pH = 4.76 + 0 • pH = 4.76 = pKa

  18. Case where 90% acetate ion 10% acetic acid • pH = pKa + log10[0.9 ] • ¯¯¯¯¯¯¯¯¯¯ • [0.1] • pH = 4.76 + 0.95 • pH = 5.71

  19. 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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