Goals Learn about waters 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.
Waters Central Biochemical Role 1.Nearly all biological molecules assume their shapes and functions as a result of the physical & chemical properties of water. 2.Water is the medium for the majority of biochemical reactions and transport. 3.Water and its components, H + and OH -, actively participates in the chemical reactions of life. 4.(The oxidation of water to produce O 2, a fundamental photosynthetic reaction converts light energy into chemical energy. Energy is also used to reduce O 2 back to water) Voet. Voet & Pratt, 2002 Chapter 2
Molecular Structure of Water Voet. Voet & Pratt, 2008 Fig 2.1 Dipole moment Lehninger, 2000 Figure 4.1 Due to its structure each water molecule is both a simultaneous hydrogen bond donor and acceptor
Water Properties Tables Matthews et al., 1999 Tables 2.4 & 2.5 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 waters dipolar nature and is why water is very effective is shielding the charges of other ions in solution.
Lehninger, 2000 Figure 4.1c Water: Hydrogen Bond Matthews et al.,, 1999 Figure 2.X ~20 kJ mole kJ mole -1 = 1.8Å The typical lifetime of an H-bond is 1 x s and is shorter as temperature increases.
Hydrogen Bonding in Ice Lehninger, 2000 Figure 4.2 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)
H-Bonds: Directionality Lehninger, 2000 Figure 4.5 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.
Molecules that H-Bond tend to be Soluble in Water Examples of Common Biological Hydrogen Bonds Lehninger, 2000 Figure 4.3 e.g why sugars are soluble Alcohols, aldehydes, ketones and compounds containing N-H bonds all form H- bonds with water molecules and therefore tend to be SOLUBLE in water.
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.
Ion Solvation by Water Voet. Voet & Pratt, 2013 Fig 2.6
H-Bonding By Functional Groups Diagram Voet. Voet & Pratt, 2013 Fig 2.7 hydroxylketo carboxyl amino What are some biological examples of these functional groups?
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
Transferring of Hydrocarbons from Water to Nonpolar Solvents at 25 °C Voet. Voet & Pratt, 2013 Table 2.2
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
Micelles and Bilayer Structure Voet. Voet & Pratt, 2013 Fig 2.11 Lehninger, 2000 Figure 4.7 Voet. Voet & Pratt, 2013 Fig 2.12 Space-filling model of a micelle composed of 20 octyl glycoside molecules oxygen
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
Osmosis and Osmotic Pressure ( ) Voet, Voet & Pratt, 2013 Figure 2.13 Vant Hoff eq. = icRT R= gas const. T = abs. temp C= solutes molar concentration i = vant Hoff factor –extent dissociates into two or more ionic species, e.g. NaCl i =2 Initial State Final State Measurement Piston (Semipermeable Membrane)
Plasma Membranes, Osmolarity & Water Movement Lehninger, 2000 Figure 4.11 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.