PowerPoint PresentationSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40Slide 41Slide 42Slide 43Slide 44Slide 45Slide 46Slide 47Slide 48Slide 49Slide 50Slide 51Slide 52Slide 53Slide 54Slide 55Slide 56Slide 57Slide 58Slide 592-1MCB 450Lecture 2Properties of WaterBrownian (Thermal) MotionHydrogen BondsHydrophilic, Hydrophobic & Amphipathic MoleculesWeak, Non-Covalent InteractionsAcid-Base Chemistry: pHBuffers and Titration CurvesHomework 1 is now open on LON-CAPA• 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)Water2-2Compound Melting point (oC) Boiling point (oC)H2O 0 100H2S -83 -60NH3-78 -33Properties of water2-3These properties suggest that intermolecular, attractive forcesbetween water molecules are very highWater…..• 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 interactionsThese properties are explained by water'sunparalleled ability to form hydrogen bondsThe water molecule is dipolar2-4- Oxygen nucleus attracts e- more strongly than hydrogen- e- shared unequally, e- more often in vicinity of O- unequal e- sharing results in 2 dipolesunshared e- pairunshared e- pairDIPOLE = MOLECULE WITHBOTH +ve AND -ve CHARGESHydrogen bond between 2 water molecules2-5Electrostatic attraction between of Oon one water molecule and+ of H on another water molecule= hydrogen bond+-Hydrogen bonds• H-bonds are l o n g e r and weaker than covalent bonds• H-bonds are strongest when the 3 atoms involved lie in a straight line:• Intramolecular H-bonds are important in conferring 3D structureon proteins and nucleic acids• H-atoms covalently bonded to C-atoms do not participate in H-bonding(C-H bond v. weakly polar)2-6H-bonding in liquid water2-7Partly ordered structure:H-bonded clusters continually formin different directions & break aparthttp://biomodel.uah.es/en/water/index.htmWater is cohesive2-8H-bonding allows acolumn of water tobe maintainedWater moleculesbounce around randomlyat a rate determined bythe temperature, andcollide with…other molecules, or larger particlesBrownian Motion Thermal motions power biological interactions2-9• Water is a polar solvent:dissolves charged or polar compounds by replacing solute-solute H-bondswith solute-water H-bonds Non-polar molecules such as lipids are hydrophobic; side chains of some are referred to as “hydrophobic”. Compounds that dissolve easily in water are hydrophilic.2-10 Solubility of biological molecules in water• alcohols• aldehydes• ketones• compounds containing N-H bondsall form H-bonds with H2O moleculesand tend to be soluble in water.amphipathic molecules have polar or charged regions, as well as non-polar regions.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 solutesWater is NOT a solvent for :- non-polar substances:Water forms a clathrate around hydrophobic moleculesSolvent properties of water2-112-12Examples of polar, non-polar, and amphipathic moleculesShown as ionic forms at pH 7Non-covalent ("weak") interactions2-131. 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 betweenenzymes & their substratesH-bonds in biological systems 2-14FORM BETWEEN AN ELECTRONEGATIVE ATOM,USUALLY O, OR N WITH A LONE PAIR OF e-AND AN H-ATOM COVALENTLY BONDEDTO ANOTHER ELECTRONEGATIVE ATOMIN SAME ORANOTHERMOLECULE~0.2 nm~0.1 nmElectronegativity = measure of the tendency of an atom to attract electrons towards itselfSome biologically important H-bonds2-15ABILITY OF A MOLECULE TOPARTICIPATE IN H-BONDINGTO WATER INFLUENCES ITS SOLUBILITYElectrostatic interactions between opposite electrical chargesa.k.a. ionic bonds or salt bridges2-16Ionic bonds between Na+ and Cl- ionsgive rise to salt crystals:++-But the Na+ - Cl- interactionsare easily disrupted bywater moleculesHydration shells of water moleculessurround Na+ and Cl- ions:Cl- binds to the + of HNa+ binds to the - of ODielectric constant increases with polarity of the medium (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 otherWhy is water such a good solvent?2-17H-bonding between water molecules stabilizes the ordered hydration shells around ions by reducing random thermalmotion of the moleculesVan der Waals Forces (a.k.a. dipole-dipole interactions)ATOMS ~3-5 ÅAPART2-18•Short-range attractive interactions•Between molecules that are neither polar nor charged•Depend on transient asymmetry in electrical chargeinduced between closely approaching atoms•Very weak (0.4-4.0 kJ/mol)Example:interaction betweenpermanent dipoles+-+-between the centers of two atomsToo close, and you getelectron-electronrepulsionEnergy of van der Waals interactionsas two atoms approach each other2-19Van der Waals forces become importantwhen large numbers of contacts can be made2-20http://admet.com/blogposts/mit-researchers-wo rking-to-develop-biomimetic-medical-adhesive/STABILITY IN NUMBERSHydrophobic interactions(the hydrophobic effect)2-21Aggregation of non-polar moleculesresults in an increase in randomness of water moleculesSecond Law of Thermodynamics:Systems tend towards disorder / randomnessThe entropy (S) of a system is a measure of its disorder / randomnessSystems tend to proceed from ordered (low entropy, small S value)to disordered (high entropy, high S value) statesH2O1 M NaCl0.5 M NaCl0.5 M NaClExample:equilibrationof NaCl moleculesS1S1S2S2∆S = S2 –S1Principle 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
View Full Document
Unlocking...