Chapter 2: Water - Interactions and Properties PDF

CHAPTER 2: Water Learning goals: What kind of interactions occur between molecules Why water is a good medium for life Why nonpolar moieties aggregate in water How dissolved molecules alter properties of water How weak acids and bases behave in water...

CHAPTER 2: Water Learning goals: What kind of interactions occur between molecules Why water is a good medium for life Why nonpolar moieties aggregate in water How dissolved molecules alter properties of water How weak acids and bases behave in water How buffers work and why we need them How water participates in biochemical reactions  Water Is the Medium for Life Life evolved in water due to the protection it provides from UV light. Organisms typically contain 70–90% water. Chemical reactions occur in aqueous milieu. Water is a critical determinant of the structure and function of proteins, nucleic acids, and membranes.  Structure of the Water Molecule The octet rule dictates that there are four electron pairs around an oxygen atom in water. These electrons are in four sp3 orbitals. Two of these pairs covalently link two hydrogen atoms to a central oxygen atom. The two remaining pairs remain nonbonding (lone pairs). Water geometry is a distorted tetrahedron. (VSEPR) The electronegativity of the oxygen atom induces a net dipole moment. Because of the dipole moment, water can serve as both a hydrogen bond donor and acceptor. (amphoteric compound) Water Bound to Proteins Is Essential for Their Function  Hydrogen Bonds Hydrogen bonds are strong dipole-dipole or charge-dipole interactions that arise between a covalently bound hydrogen and lone pair of electrons. They typically involve two electronegative atoms (frequently nitrogen and oxygen). Hydrogen bonds are strongest when the bonded molecules allow for linear bonding patterns. Ideally, the three atoms involved are in a line.  Ionization of Water → H+ + OH⎼ H2O  O-H bonds are polar and can dissociate heterolytically. Products are a proton (H+) and a hydroxide ion (OH–). Dissociation of water is a rapid reversible process. Most water molecules remain un-ionized, thus pure water has very low electrical conductivity (resistance: 18 M cm). The equilibrium is strongly to the left (low Keq). The extent of dissociation depends on the temperature.  Proton Hydration Protons (H+) do not exist free in solution. They are immediately hydrated to form hydronium ions (H3O+). A hydronium ion is a water molecule with a proton associated with one of the nonbonding electron pairs. Hydronium ions are solvated by nearby water molecules. The covalent and hydrogen bonds are interchangeable. This allows for an extremely fast mobility of protons in water via “proton hopping.” Proton Hopping  Water Bound to Proteins Is e- Essential for Their Function Example of Proton hoping with cytochrome f e-  Ice: Water in a Solid State Water has many different crystal forms; the hexagonal ice is the most common. Hexagonal ice forms an organized lattice and thus has a low entropy. Hexagonal ice contains maximal hydrogen bonds/ water molecules, forcing the water molecules into equidistant arrangement. Thus: ice has lower density than liquid water ice floats  Physics of Noncovalent Interactions Noncovalent interactions do not involve sharing a pair of electrons. Based on their physical origin, one can distinguish between: O O = = Ionic (Coulombic) interactions R – NH3+ - - - -O-C-CH2-R Na+ - - - -O-C-CH2-R – electrostatic interactions between permanently charged species, or between the ion and a permanent dipole δ+ - δ+ δ- δ Dipole interactions H-Cl - - - H-Cl – electrostatic interactions between uncharged but polar molecules van der Waals interactions – weak interactions between all atoms, regardless of polarity – attractive (dispersion) and repulsive (steric) component Hydrophobic Effect – complex phenomenon associated with the ordering of water molecules around nonpolar substances Intramolecular Intermolecular  The Hydrophobic Effect Refers to the association or interaction of nonpolar molecules or components of molecules in the aqueous solution Is one of the main factors behind: – protein folding – protein-protein association – formation of lipid micelles – binding of steroid hormones to their receptors Does not arise because of some attractive direct force between two nonpolar molecules Water Surrounding Nonpolar Solutes Has Lower Entropy Low entropy is thermodynamically unfavorable, thus hydrophobic solutes have low solubility.  Origin of the Hydrophobic Effect Consider amphipathic lipids in water. Lipid molecules disperse in the solution; nonpolar tails of lipid molecules are surrounded by ordered water molecules. Entropy of the system decreases. The system is now in an unfavorable state.  Origin of the Hydrophobic Effect Nonpolar portions of the amphipathic molecule aggregate so that fewer water molecules are ordered and entropy increases. All nonpolar groups are sequestered from water, and the released water molecules increase the entropy further. Only polar “head groups” are exposed.  Origin of the Hydrophobic Effect With high enough concentration of amphipathic molecules, complete aggregation into micelles is possible. Vesicles Hydrophobic Effect Favors Ligand Binding Binding sites in enzymes and receptors are often hydrophobic. Such sites can bind hydrophobic substrates and ligands, such as steroid hormones, which displace water and increase entropy of the system. Many drugs are designed to take advantage of the hydrophobic effect.  Osmotic Pressure Water moves from areas of high water concentration (low solute concentration) to areas of low water concentration (high solute concentration). (less salty to more salty) Osmotic pressure (π) is the force necessary to resist the movement. Osmotic pressure is influenced by the concentration of each solute in solution. [Na] + [K] + [Mg] + [AA] + [NA] + … = Dissociated components of a solute individually influence the osmotic pressure. 1 Mg 2 Cl = 3 osmol MgCl2 1 M MgCl2 Osmotic Pressure Osmotic Pressure Effect of Osmotic Pressure on Cells 300 mOsm 300 mOsm “Tonics” refers to the movement of water. This depends on the osmolality of non-penetrating solutes. 300 mOsm 200 mOsm 400 mOsm 300 mOsm  Henderson- Hasselbalch Equation Example Question: For a buffer with 0.25 M formic acid and 0.80 M sodium formate (pKa of 3.75) : pH = 3.75 +log (0.25/0.80​) = 4.26  Chapter 2: Summary In this chapter, we learned about the: nature of intermolecular forces properties and structure of liquid water behavior of weak acids and bases in water way water can participate in biochemical reactions