1. Wonders of Water Student Edition 5/23/13 Version
Dr. Brad Chazotte
213 Maddox Hall
Pharm. 304 Biochemistry
Fall 2013
Web Site:
Original material only © B. Chazotte

2. Goals Learn about water’s central role in biochemistry.
Review the properties of water and Hydrogen bonding of water.
Review the concept of solvation and what make molecule soluble in water.
Review the hydrophobic effect for micelles and membrane structure.
Review the colligative properties of aqueous solutions.
Review the concepts of osmosis and osmotic pressure and their importance for biological membranes.

3. Water’s Central Biochemical Role
Nearly all biological molecules assume their shapes and functions as a result of the physical & chemical properties of water.
Water is the medium for the majority of biochemical reactions and transport.
Water and its components, H+ and OH-, actively participates in the chemical reactions of life.
(The oxidation of water to produce O2, a fundamental photosynthetic reaction converts light energy into chemical energy. Energy is also used to reduce O2 back to water)
Voet. Voet & Pratt, 2002 Chapter 2

4. Molecular Structure of Water
Due to its structure each water molecule is both a simultaneous hydrogen bond donor and acceptor
Dipole moment
Voet. Voet & Pratt, 2008 Fig 2.1
Lehninger, 2000 Figure 4.1

5. Water Properties Tables
for a molecule of its size water has a high heat of vaporization, a high boiling point and a high melting point
WHY?
The high dielectric constant results from water’s dipolar nature and is why water is very effective is shielding the charges of other ions in solution.
Matthews et al., Tables 2.4 & 2.5

6. Water: Hydrogen Bond =1.8Å ~20 kJ mole-1 460 kJ mole-1
The typical lifetime of an H-bond is 1 x s and is shorter as temperature increases.
Lehninger, 2000 Figure 4.1c
Matthews et al.,, Figure 2.X

7. Hydrogen Bonding in Ice
In ice each water molecule interacts tetrahedrally with four other water molecules to form a regular lattice structure
Ice has a lower density than water.
(Important property)
Lehninger, 2000 Figure 4.2

8. H-Bonds: Directionality
The attraction between the partial electrical charges is greatest when the three atoms involved lie in a straight line.
Of biological importance because it confers precise three-dimensional structures on proteins and nucleic acids.
Lehninger, 2000 Figure 4.5

9. Molecules that H-Bond tend to be Soluble in Water Examples of Common Biological Hydrogen Bonds
Alcohols, aldehydes, ketones and compounds containing N-H bonds all form H-bonds with water molecules and therefore tend to be SOLUBLE in water.
e.g why sugars are soluble
Lehninger, 2000 Figure 4.3

10. Solvation
The solubility of a molecule depends on the ability of the solvent to interact more strongly with the solute than the solutes to interact with each other.
Water makes an excellent solvent for polar and ionic materials, i.e. hydrophilic.
Water is a poor solvent for nonpolar substances, i.e. hydrophobic.

11. Ion Solvation by Water
Voet. Voet & Pratt, 2013 Fig 2.6

12. H-Bonding By Functional Groups Diagram
What are some biological examples of these functional groups?
hydroxyl
keto
carboxyl
amino
Voet. Voet & Pratt, 2013 Fig 2.7

13. Hydrophobic Effect I
Definition: The tendency of water molecules to minimize their contact with hydrophobic molecules.
Responsible for the shapes of many large biomolecules and molecular aggregates.
Entropically driven process.
H-bond
Voet. Voet & Pratt, 2013 Fig 2.8

14. Transferring of Hydrocarbons from Water to Nonpolar Solvents at 25 °C
Voet. Voet & Pratt, 2013 Table 2.2

15. Hydrophobic Effect II
Net result: Due to the unfavorable G of hydration of a nonpolar substance from the ordering of the surrounding water molecules, nonpolar substances tend to be excluded from the aqueous phase
Why?: The surface area of the cavity containing the aggregate of nonpolar molecules is less than the sum of the cavities individually occupied by the nonpolar molecules.
Aggregation of nonpolar groups minimizes the surface area of the cavity and therefore maximizes the entropy of the entire system

16. Micelles and Bilayer Structure
oxygen
Space-filling model of a micelle composed of 20 octyl glycoside molecules
Voet. Voet & Pratt, 2013 Fig 2.12
Voet. Voet & Pratt, 2013 Fig 2.11
Lehninger, 2000 Figure 4.7

17. Colligative Properties of Aqueous Solutions
All kinds of dissolved solutes alter certain physical properties of the solvent, e.g. water.
Vapor pressure
Boiling point
Melting point (freezing point)
Osmotic pressure
Colligative - “tied together”
Depends on numbers of solute particles not their chemical properties
Lehninger, 2000 Figure 4.9

18. Osmosis and Osmotic Pressure ()
Van’t Hoff eq.
= icRT
R= gas const.
T = abs. temp
C= solutes molar concentration
i = van’t Hoff factor –extent dissociates into two or more ionic species, e.g. NaCl i =2
Piston
(Semipermeable Membrane)
Initial State
Final State
 Measurement
Voet, Voet & Pratt, 2013 Figure 2.13

19. Plasma Membranes, Osmolarity & Water Movement
Hypotonic
Isotonic
Hypertonic
Osmosis is defined as the movement across a semipermeable membrane of solvent molecules from a region from high concentration to a region of lower concentration.
Lehninger, 2000 Figure 4.11

20. Dialysis
Voet. Voet & Pratt, 2013 Fig 2.14

21. End of Lecture