Lecture 3 - final review.docx

6 Pages

Biochem & Molecular Biology
Course Code
BIOC 2300
Dr.Carmichael Wallace

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Lecture 3 Water = essential for life Water’s important properties: 1) Chemical stability 2) Remarkable solvent properties 3) Role as a biochemical reactant 4) Hydration Molecular structure of water - Tetrahedral geometry - Oxygen is more electronegative than hydrogen o Larger oxygen atom has partial negative charge, and hydrogen atoms have partial positive charges o Bond between oxygen and hydrogen is polar o Water is a dipole because the positive and negative charges are separate - An electron-deficient hydrogen of one water is attracted to the unshared electrons of water forming a hydrogen bond o Can occur with oxygen, nitrogen, and fluorine o Has electrostatic (opposite charges) and covalent (electron sharing) characteristics - Noncovalent interactions are electrostatic (includes ionic interactions, and van der waals forces) o Weak individually, but play a vital role in biomolecules because of cumulative effects o Three most important noncovalent bonds:  Ionic interactions  Van der Waals forces  Hydrogen bonds Ionic Interactions - Oppositely charged ions attract one another - Ionized amino acid side chains can form salt bridges with one another - Biochemistry primarily investigates the interaction of charged groups on molecules, which differs from ionic interactions like those of ionic compounds (eg. NaCl) Hydgrogen Bonds - Electron deficient hydrogen is weakly attracted to unshared electrons of another oxygen or nitrogen - Large numbers of hydrogen bonds lead to extended network Van der Waals Forces - Occur between neutral, permanent, and/or induced dipoles - Three types: o Dipole-dipole interactions = occur between molecules containing electronegative atoms cause molecules to orient themselves so that the positive end of one polar group is directed toward the negative group of another. Hydrogen bonds are a strong type of dipole-dipole interaction o Dipole-induced dipole interactions = a permanent dipole induces a transient dipole in a nearby molecule by distorting its electron distribution o Induced dipole-induced dipole interactions =London dispersion forces, extremely weak. Thermal Properties of Water - Water’s melting and boiling points are fairly high due to hydrogen bonding o Each water molecule can form four hydrogen bonds with other water molecules o Extended network of hydrogen bonds o Maximum number of hydrogen bonds form when water has frozen into ice  open, less-dense structure - Water has a exceptionally high heat of fusion and heat of vaporization o Helps to maintain an organism’s internal temperature o High heat capacity means that more energy must be added or removed to change the temperature by one degree Celsius Water is the Ideal Biological Solvent - Hydrophilic molecules, cell water structuring, and sol-gel transitions o Water can dissolve ionic and polar substances o Shells of water molecules form around ions forming solvation spheres - Structured water o Water is rarely free flowing o Water is associated with macromolecules and other cellular components o Forms complex three-dimensional bridges between cellular components - Sol-gel transitions o Cytoplasm has properties of a gel (colloidal mixture) o Transition from gel to sol important in cell movement  Amoeboid motion provides an example of regulated, cellular, sol-gel transitions o Sol = liquid state - Hydrophobic molecules and the hydrophobic effect o Small amounts of nonpolar substances are excluded from the solvation network when they are mixed with water, forming droplets o This hydrophobic effect results from the solvent properties of the water and is stabilized by van der waals interactions o Water forms a cage… This is responsible for the generation of stable lipid membranes and contributes to the fidelity of protein folding
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