2.1 Properties of Water 2.2 pH 2.3 Buffers 2.4 Water's Unique Role in the Fitness of the Environment

1. CHEM 330 Lecture 2 Water (G&G, Chapter 2) • 2.1 Properties of Water • 2.2 pH • 2.3 Buffers • 2.4 Water's Unique Role in the Fitness of the Environment

2. For a small molecule, water is weird Bulk Properties • Abnormally high b.p., m.p. • Abnormally high surface tension The Molecular Explanation • H-bond donor and acceptor • ~ tetrahedral bond angles • Potential to form four H-bonds per water molecule • Bent structure makes it polar

3. d+ d- d+ Water Close Up Dipole Moment : Two lone electron pairs Bond angle 104.3° : Covalent Bond Length Between H and O: 0.95 Å Potential to form four H-bonds per water molecule

4. Comparison of Ice and Water(or: what separates the frozen from the fluid?) Number of H-bonds • Ice: 4 H-bonds per water molecule • Water: 2.3 H-bonds per water molecule (on average) Lifetime of H-bonds • Ice: H-bond lifetime ~ 10-5 sec • Water: H-bond lifetime ~ 10-11 sec

5. The Dynamics of Liquid Water “Flickering” H bonds in water: a series of snapshots at 5 picosecond intervals Figure 2.3

6. Solvent Properties of Water • Interaction with electrolytes • Interaction with polar, uncharged molecules • Interaction with nonpolar molecules

7. Electrolytes Compounds yielding ions when added to water Strong electrolytes: ionization is complete, eg H2O salts strong acids strong bases NaCl Na+(aq) + Cl-(aq) H2SO42H+ (aq ) + SO42-(aq) NaOH Na+ (aq) + OH- (aq) Major biological strong electrolytes: Phosphates, KCl, NaCl, CaCl2 Note that a solution containing electrolytes, though rich in ions, is electrically neutral

8. Weak electrolytes: ionization* isincomplete: CH3COOH+ H2O CH3COO- + H3O+ organic acids organic bases CH3-NH2+ H2O CH3-NH3+ + OH- Major weak electrolytes in biology: Amines, imines, carboxylic acids * another term for ionization isdissociation

9. in solution What effect does the intervening solvent have? e1e2 e1e2 F  F = r2 Dr2 D: the dielectric constant of the solvent Solvent Dielectric constant (D) water 78.5 methanol 32.6 acetone 20.7 benzene 2.3 As D increases, ions in solution interact more weakly with each other & more strongly with the solvent Ionic interactions r - + charge e2 charge e1

10. - + + Interaction of water with ions:no naked ions Cl- Chloride anion Na+ Sodium cation water Dipoles of water screen the charges of the ions so they don’t sense one another- water has a high dielectric constant

11. Water & polar neutral molecules: hydrogen bonding Water forms extensive H-bonds with molecules such as glucose, rendering it highly soluble

12. Life’s trouble with solutions, and life’s solution Water: can pass through membrane; tendency is to dilute the cell contents causing cell to burst What to do? Cell, full of solutes, which cannot pass through membrane Countermeasures 1) Strong cell wall (bacteria, single-cell eukaryotes) 2) Surround cells with an isotonic environment (multicellular eukaryotes)

13. Water & nonpolar molecules: Hydrophobic Interactions • H-bond network of water reorganizes to accommodate the nonpolar solute • This is an increase in "order" of water (a decrease in entropy) • number of ordered water molecules is minimized by herding nonpolar solutes together Yellow blob: nonpolar solute (eg oil)

14. Solvent Properties of Water- Recap • Water forms H-bonds with polar solutes • Ions in water are always surrounded by a hydration shell (no naked ions) • Hydrophilic (polar): water-soluble molecules • Hydrophobic (nonpolar): water insoluble (greasy) • Hydrophobic interaction: fewer water molecules are needed to corral one large aggregate than many small aggregates of a hydrophobic molecule Hydrophilic, hydrophobic - anything else?

15. Amphiphilic Molecules Also called "amphipathic" • Contain both polar and nonpolar groups • Attracted to both polar and nonpolar environments • Eg - fatty acids Polar head (carboxylic acid) Nonpolar hydrocarbon tail What happens in water?

16. Amphiphiles in water Hydrophilic domains face water Hydrophobic domains shielded from water Variety of structures possible Wedge-shaped amphiphiles form micelles (spherical) Cylinder-shaped amphiphiles form bilayers (planar)

17. Protons in solution - why are they so important ? • Most biomolecules bear groups that can undergo reversible protonation/deprotonation reaction • The conformation and functions of these biomolecules may depend on their protonation state: • -Active sites of hydrolytic enzymes • -Overall fold of proteins • Establishment of proton concentration gradients across biological membranes is central to an understanding of cell energetics The study of acid-base equilibria lets us quantify these effects

18. H2O XH  X- + H+ BH+ B + H+ H2O Acid-base Equilibria: Dissociation of protons from molecules in aqueous solution Measure [H+] to indicate degree of acidity Simple, but cumbersome: eg “physiological” [H+] ranges from ~ 0.5 M (stomach) ~ 0.00000001 M (blood)

19. The pH Scale • A convenient means of writing low concentrations of protons: • pH = -log10 [H+] • If [H+] = 1 x 10 -7 M (0.0000001 M) • Then pH = 7 Low pH indicates a high proton concentration (high acidity) High pH indicates a low proton concentration High pH indicates a high concentration of hydroxide -OH (high basicity) Each difference of 1 pH unit is a ten-fold difference in proton concentration

20. _ Dissociation of Water: water as a source of ions H+ Proton Hydroxide Little tendency to dissociate under neutral conditions

21. No Naked Protons! H+ in aqueous solution exists as H3O+

22. Proton movement through water: faster than any other ions

23. [ H + ] [ A - ] Ka = [HA] Dissociation of Weak Electrolytes Consider a weak acid, HA: HA H+ + A- The acid dissociation constant, Ka, is given by:

24. [A-] [HA] pH = pKa + log10 The Henderson-Hasselbalch Equation For any acid HA, the relationship between its pKa, the concentrations of HA and A- existing at equilibrium, and the solution pH is given by: Given any two parameters, you can solve the third