HCS 112: Chemistry for the Bioscience. Dr Aimee Kibondo Biological Molecules Intermolecular Forces: - Covalent bonds are intermolecular. - Intermolecular forces arise electro static attractions/repulsions as a result of partial changes. For example: Dipole-dipole, dispersion and steric repulsion, these are Van der Waals. Also, hydrogen bonding, ionic forces and hydrophobic forces. Van der Waals Interaction: - Permanent dipoles: between opposite partial charges on polar molecules. - Dispersion forces: Weak, short lived and short distance. Uneven distribution of charge occurs in all molecules at an instant in time (e continuous motion).Induced dipoles. - Steric repulsion: repulsion experience by 2 areas of negative charge (e clouds). Molecules have to be very close. Dipole-dipole interactions: - In a polar bond, the +VE end of one molecule attracts the -VE end of another (permanet dipole- dipole). - This attraction is between molecules gives rise to increases in boiling points. Electronegativity Table H 2.1 Li 1.0 Be 1.5 8 c 2.5 2.0 N 30 O 3.5 F 40 Influenced by: Na 0.9 Mg 1.2 K Ca 0.8 1,0 Rb Sr 0.8 1.0 AI 1.5 Ga 1.6 In 1.7 1.8 Si 1.8 Ge 1.8 Sn P 2.1 S 2.5 As 2.0 Se 2.4 Sb 1.9 Te 2.1 CI 3.0 Protons in nucleus (+ve) charge Br 2.8 Distance of valence electrons from nucleus 1 2.5 Cs Ba 0.7 0.9 TI 1.8 Pb 1.8 Bi 1.9 Po 2.0 8 V × + Attractive dispersion forces predominate
Hydrogen bonding: - Hydrogen bonds are attractions between a + hydrogen on one molecule (H bonded to a very electronegative atom) and a lone pair on the another very electronegative atom (N,O, or F) on another molecule. Induced partial positive charge 8- 8+ x . . . H X Hydrogen bonds exist between solvent molecules ... Hô+ 1 Hô+ Electron distribution O / s Solute molecules dissolve in (mix completely with) the solvent. 8- ... and between solvent and solute molecules. 9 1 Polar = Hydrophilic 00 (b) Insoluble molecules aggregate and form a separate layer ... No hydrogen bonds form between the solvent and insoluble molecule. Fats and Lipids: Non-polar = Hydrophobic 8 8 ... while the solvent molecules also interect with each other to form a separate phase. 12 - Lipids: biomolecules which are insoluble in water. - Large hydrocarbon components. - Not many functional groups. - Three important types: Steroids, Triacylglycerols, Glycerophspholipids. Triacylglcerols: - Fats and oils. - Derived from: 3 carbon alcohol, glycerol and fatty acids. - Esterfication. - Note the carboxyllic acid. Insert drawing here:
Fatty acids: - Sturated, single C-C bond. - Monosaturated, one C=C double bond. - Polyunsaturated, one C=C double bond. Glycerophospholipids: - Central component of cell membranes. - One hydroxyl group attached to a phosphoiester group. - Glycero - Phospho - Lipid Polar (hydrophilic) head ... which we can represent like this. Non-polar (hydrophobic) tail . to give a polar head and a non-polar tail ... Polar phosphodiester group orients away from fatty acid groups ... Hydrophilic heads are exposed to water-based surroundings.