Chapter Two Water: The Solvent for Biochemical Reactions

1. Chapter TwoWater: 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 _________________________of atoms involved in bond.

3. Electronegativity • __________________: a measure of the force of an atom’s attraction for electrons it shares in a chemical bond with another atom • Oxygen and Nitrogen, ____________ electronegative than carbon and hydrogen • _______________ is most electronegative (4)

4. Polar Bonds & Molecules • Molecules such as CO2 have polar __________ but, given their geometry, are nonpolar _______; that is, they have a __________ dipole moments

5. Solvent Properties of H2O • _________ compounds (e.g.,KCl) and low-molecular- weight ________ covalent compounds (e.g., C2H5OH and CH3COCH3) tend to dissolve in ______________ • The underlying principle is electrostatic ________ of _____________ charges; the positive dipole of water for the negative dipole of another molecule, etc. • ____________ interaction: e.g., KCl dissolved in H2O • ____________ interactions: e.g., ethanol or acetone dissolved in H2O • ____________ 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 • ____________ and ____________ interactions help ionic and polar compounds dissolve in water

8. Solvent Properties of H2O • ____________ : water-loving • tend to dissolve in water • ____________ : water-fearing • tend not to dissolve in water • ____________ : characteristics of both properties • molecules that contain one or more ____________ and one or more ____________ regions, e.g., sodium palmitate

9. Amphipathic molecules • both ____________ and ____________ character • Interaction between ____________ molecules is very weak, called van der Waals interactions

10. Micelle formation by amphipathic molecules • Micelle: a ____________ arrangement of organic molecules in ___________ solution clustered so that • their ____________ parts are buried inside the sphere • their ____________ parts are on the surface of the sphere and in contact with the water environment • formation depends on the attraction between ____________ ____________ ____________

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 ______________________

13. Interesting and Unique Properties of Water • Each water molecule can be involved in ___ hydrogen bonds: ___ as donor, and ___ as acceptor • Due to the ____________ arrangement of the water molecule (Refer to Figure 2.1).

14. Hydrogen Bonding • Even though hydrogen bonds are ___________ 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 _______________ of 3-D structures of biological molecules such as: DNA, RNA, proteins.

16. Acids, Bases and pH • Acid:a molecule that behaves as a ____________ ____________ • Strong base:a molecule that behaves as a ____________ ____________

17. Acid Strength • 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 Ka: • Written correctly,

18. Ionization of H2O and pH • Lets quantitatively examine the dissociation of water: • Molar concentration of water (55M) • Kw is called the ion product constant for water. • Must define a quantity to express hydrogen ion concentrations…pH

19. Henderson-Hasselbalch • Equation to connect Ka to pH of solution containing both acid and base. • We can calculate the ratio of weak acid, HA, to its conjugate base, A-, in the following way

20. Henderson-Hasselbalch (Cont’d) • 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 pKa of the weak acid • when pH < pKa, the weak acid predominates • when pH > pKa, the conjugate base predominates

21. Titration Curves • ________________ : an experiment in which measured amounts of acid (or base) are added to measured amounts of base (or acid) • _______________________ : 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 releasesthree H+ per mole

22. Buffers • buffer: a solution whose ________ 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 • CH3COOH and CH3COONa • H2CO3 and NaHCO3 • NaH2PO4 and Na2HPO4

23. Buffer Range • A buffer is effective in a range of about +/- 1 pH unit of the pKa of the weak acid

24. Buffer Capacity • Buffer capacity is related to the ____________ 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 • ____________ is the principal buffer in cells • ____________ is an important (but not the only) buffer in blood • ____________ can result in increased blood pH • ____________ can result in decreased blood pH (Biochemical Connections p. 60)

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

27. Laboratory Buffers