Campbell6e lecture ch2

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  • 1.Chapter Two Water: The Solvent for Biochemical Reactions Paul D. Adams University of Arkansas

2. What makes water polar

  • What is a polar bond:
  • Electrons are unequally shared,more negative charge found closer to one atom.
  • Due to difference in electronegativity of atoms involved in bond.

3. Electronegativity

  • Electronegativity:a measure of the force of an atoms attraction for electrons it shares in a chemical bond with another atom
    • Oxygen and Nitrogen, more electronegative than carbon and hydrogen
    • Fluorine is most electronegative (4)

4. Polar Bonds & Molecules

  • Molecules such as CO 2havepolarbonds but, given their geometry, arenonpolarmolecules; that is, they have a zero dipole moments

5. Solvent Properties of H 2 O

  • Ionic compounds (e.g.,KCl) and low-molecular- weight polar covalent compounds (e.g., C 2 H 5 OH and CH 3 COCH 3 ) tend to dissolve in water
  • The underlying principle is electrostatic attraction of unlike charges; the positive dipole of water for the negative dipole of another molecule, etc.
    • ion-dipole interaction: e.g., KCl dissolved in H 2 O
    • dipole-dipole interactions: e.g., ethanol or acetone dissolved in H 2 O
    • dipole induced-dipole interactions: weak and generally do not lead to solubility in water

6. Hydration Shells Surrounding Ions in Water 7. Ion-dipole and dipole-dipole interactions help ionic and polar compounds dissolve in water Ion-dipole and Dipole-dipole Interactions 8. Solvent Properties of H 2 O

  • Hydrophilic : water-loving
    • tend to dissolve in water
  • Hydrophobic : water-fearing
    • tend not to dissolve in water
  • Amphipathic : has characteristics of both properties
    • molecules that contain one or more hydrophobic and one or more hydrophilic regions, e.g., sodium palmitate

9. Amphipathic molecules

  • both polar and nonpolar character
  • Interaction between nonpolar molecules is very weak
    • called van der Waals interactions

10. Micelle formation by amphipathic molecules

  • Micelle:a spherical arrangement of organic molecules in water solution clustered so that
    • their hydrophobic parts are buried inside the sphere
    • their hydrophilic parts are on the surface of the sphere and in contact with the water environment
    • formation depends on the attraction betweentemporary induced dipoles

11. Examples of Hydrophobic and Hydrophilic Substances 12. Hydrogen Bonds

  • Hydrogen bond:the attractive interaction between dipoles when:
    • positive end of one dipole is a hydrogen atom bonded to an atom of high electronegativity, most commonly O or N, and
    • the negative end of the other dipole is an atom with a lone pair of electrons, most commonly O or N
  • Hydrogen bond is non-covalent

13. Interesting and Unique Properties of Water

  • Each water molecule can be involved in 4 hydrogen bonds: 2 as donor, and 2 as acceptor
  • Due to the tetrahedral arrangement of the water molecule (Refer to Figure 2.1).

14. Hydrogen Bonding

  • Even though hydrogen bonds are weaker than covalent bonds, they have a significant effect on the physical properties of hydrogen-bonded compounds

15. Other Biologically Important Hydrogen bonds

  • Hydrogen bonding is important in stabilization of 3-D structures of biological molecules such as: DNA, RNA, proteins.

16. Acids, Bases and pH

  • Acid: a molecule that behaves as a proton donor
  • Strong base: a molecule that behaves as a proton acceptor

17.

  • One can derive a numerical value for the strength of an acid (amount of hydrogen ion released when a given amount of acid is dissolved in water).
  • Describe by K a :
  • Written correctly,

Acid Strength 18. Ionization of H 2 O and pH

  • Lets quantitatively examine the dissociation of water:
  • Molar concentration of water (55M)
  • K wis called the ion product constant for water.
  • Must define a quantity to express hydrogen ion concentrationspH

19. Henderson-Hasselbalch

  • Equation to connect K ato pH of solution containing both acid and base.
  • We can calculate the ratio ofweak acid, HA, to its conjugatebase, A - , in the following way

20. Henderson-Hasselbalch (Contd)

  • Henderson-Hasselbalch equation
  • From this equation, we see that
    • when the concentrations of weak acid and its conjugate base are equal, the pH of the solution equals the pK aof the weak acid
    • when pH < pK a , the weak acid predominates
    • when pH > pK a , the conjugate base predominates

21. Titration Curves

  • Titration:an experiment in which measured amounts of acid (or base) are added to measured amounts of base (or acid)
  • Equivalence point:the point in an acid-base titration at which enough acid has been added to exactly neutralize the base (or vice versa)
    • a monoprotic acid releases one H +per mole
    • a diprotic acid releases two H +per mole
    • a triprotic acid releases three H +per mole

22. Buffers

  • buffer:a solution whose pH resists change upon addition of either more acid or more base
    • consists of a weak acid and its conjugate base
  • Examples of acid-base buffers are solutions containing
    • CH 3 COOH and CH 3 COONa
    • H 2 CO 3and NaHCO 3
    • NaH 2 PO 4and Na 2 HPO 4

23. Buffer Range

  • A buffer is effective in a range of about +/- 1 pH unit of the pK aof the weak acid

24. Buffer Capacity

  • Buffer capacityis related to the concentrations of the weak acid and its conjugate base
    • the greater the concentration of the weak acid and its conjugate base, the greater the buffer capacity

25. Naturally Occurring Buffers

  • H 2 PO 4 - /HPO 4 2-is the principal buffer in cells
  • H 2 CO 3 /HCO 3 -is an important (but not the only) buffer in blood
    • hyperventilation can result in increased blood pH
    • hypoventilation can result in decreased blood pH
    • (Biochemical Connections p. 60)

26. Selecting a Buffer

  • The following criteria are typical
    • suitable pK a
    • no interference with the reaction or detection of the assay
    • suitable ionic strength
    • suitable solubility
    • its non-biological nature

27. Laboratory Buffers