2 1 MCB 450 Lecture 2 Properties of Water Brownian Thermal Motion Hydrogen Bonds Hydrophilic Hydrophobic Amphipathic Molecules Weak Non Covalent Interactions Acid Base Chemistry pH Buffers and Titration Curves Homework 1 is now open on LON CAPA 2 2 Water Most abundant substance in living systems Forms hydrogen bonds and can ionize Solvent in which chemical reactions of cells occur Can participate as a reactant e g in hydrolysis 2 3 Properties of water Compound Melting point oC Boiling point oC H2O 0 100 H2S 83 60 NH3 78 33 These properties suggest that intermolecular attractive forces between water molecules are very high Water is denser as liquid than as a solid therefore has a negative volume of melting has a high heat capacity a lot of heat needs to be absorbed to break interactions These properties are explained by water s unparalleled ability to form hydrogen bonds 2 4 The water molecule is dipolar Oxygen nucleus attracts emore strongly than hydrogen e shared unequally e more often in vicinity of O unequal e sharing results in 2 dipoles unshared e pair unshared e pair DIPOLE MOLECULE WITH BOTH ve AND ve CHARGES 2 5 Hydrogen bond between 2 water molecules Electrostatic attraction between of O on one water molecule and of H on another water molecule hydrogen bond 2 6 Hydrogen bonds H bonds are l o n g e r and weaker than covalent bonds H atoms covalently bonded to C atoms do not participate in H bonding C H bond v weakly polar H bonds are strongest when the 3 atoms involved lie in a straight line Intramolecular H bonds are important in conferring 3D structure on proteins and nucleic acids 2 7 H bonding in liquid water Partly ordered structure H bonded clusters continually form in different directions break apart http biomodel uah es en water index htm 2 8 H bonding allows a column of water to be maintained Water is cohesive 2 9 Brownian Motion Thermal motions power biological interactions Water molecules bounce around randomly at a rate determined by the temperature and collide with other molecules or larger particles 2 10 Solubility of biological molecules in water Water is a polar solvent dissolves charged or polar compounds by replacing solute solute H bonds with solute water H bonds Compounds that dissolve easily in water are hydrophilic Non polar molecules such as lipids are hydrophobic side chains of some are referred to as hydrophobic amphipathic molecules have polar or charged regions as well as non polar regions alcohols aldehydes ketones compounds containing N H bonds all form H bonds with H2O molecules and tend to be soluble in water 2 11 Solvent properties of water Water is an excellent solvent for ionic substances ions are always hydrated in water and carry around a hydration shell nonionic but polar substances water forms H bonds with polar solutes Water is NOT a solvent for non polar substances Water forms a clathrate around hydrophobic molecules 2 12 Examples of polar non polar and amphipathic molecules Shown as ionic forms at pH 7 2 13 Non covalent weak interactions 1 H bonds polar interactions 2 Ionic interactions salt bridges 3 Van der Waals forces dipole dipole interactions 4 Hydrophobic interactions non polar These interactions are critical for the formation of biological structures interactions between enzymes their substrates 2 14 H bonds in biological systems FORM BETWEEN AN ELECTRONEGATIVE ATOM USUALLY O OR N WITH A LONE PAIR OF e IN SAME OR ANOTHER MOLECULE 0 2 nm 0 1 nm AND AN H ATOM COVALENTLY BONDED TO ANOTHER ELECTRONEGATIVE ATOM Electronegativity measure of the tendency of an atom to attract electrons towards itself 2 15 Some biologically important H bonds ABILITY OF A MOLECULE TO PARTICIPATE IN H BONDING TO WATER INFLUENCES ITS SOLUBILITY 2 16 Electrostatic interactions between opposite electrical charges a k a ionic bonds or salt bridges Ionic bonds between Na and Cl ions give rise to salt crystals But the Na Cl interactions are easily disrupted by water molecules Hydration shells of water molecules surround Na and Cl ions Cl binds to the of H Na binds to the of O 2 17 Why is water such a good solvent Water has a very high dielectric constant the ability of water molecules to surround ions and diminish the attraction of opposite charges for each other H bonding between water molecules stabilizes the ordered hydration shells around ions by reducing random thermal motion of the molecules Dielectric constant increases with polarity of the medium solvent 2 18 Van der Waals Forces a k a dipole dipole interactions Short range attractive interactions Between molecules that are neither polar nor charged Depend on transient asymmetry in electrical charge induced between closely approaching atoms Very weak 0 4 4 0 kJ mol ATOMS 3 5 APART Example interaction between permanent dipoles 2 19 Energy of van der Waals interactions as two atoms approach each other Too close and you get electron electron repulsion between the centers of two atoms 2 20 Van der Waals forces become important when large numbers of contacts can be made STABILITY IN NUMBERS http admet com blogposts mit researchers working to develop biomimetic medical adhesive 2 21 Hydrophobic interactions the hydrophobic effect Aggregation of non polar molecules results in an increase in randomness of water molecules Second Law of Thermodynamics Systems tend towards disorder randomness The entropy S of a system is a measure of its disorder randomness Systems tend to proceed from ordered low entropy small S value to disordered high entropy high S value states Example equilibration of NaCl molecules 1 M NaCl H2O S1 0 5 M NaCl 0 5 M NaCl S2 S S2 S1 2 22 Hydrophobic interactions between non polar groups Result from system s achieving greatest thermodynamic stability by minimizing the number of ordered water molecules required to surround hydrophobic portions of solute molecules Not intrinsic attaction Principle illustrated for a long chain fatty acid H2O molecules in immediate vicinity of the fatty acid are constrained in their orientations i e can t form H bonds in all directions So H2O molecules form a highly ordered shell around each fatty acid molecule S is low ve for dissolving a non polar solute in water 2 23 Changes in entropy of water caused by hydrophobic molecules Insertion of hydrophobic molecules unfavorable because H2O molecules less free to take up different positions near the solute S When 2 hydrophobic solute molecules come together the area of hydrophobic molecules in contact with H2O is
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