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1. Using what you know intermolecular and intramolecular forces, describe why the followingphenomena occur:(a). Repulsive interactions are greatly decreased in water as opposed to in a vacuum.

Ionic repulsive interactions are greatly reduced in water as the partial charge separation in waterinteracts with the charges on the ions, and creates an electrostatic shield around the ions. Thisreduces the strength of the electric field, and is accounted for mathematically by the permittivityconstant in coulomb's law.

(b). Water has an unusually high boiling point when compared to similar molecules like H2S.

Water, H20, forms hydrogen bonds between each other due to the high electronegativity ofoxygen which draws the electrons of the hydrogen closer to itself, creating a very distinct dipolemoment. The sulfur atom in H2S, however, is not as electronegative as 0, and while creates aless distinct dipole moment within the molecule. This makes it less susceptible to hydrogenbonding, and reduces the amount of intermolecular interactions. Less intermolecularinteractions between molecules translates to requiring less energy to bring these moleculesapart, which again translates to a lower boiling point.

(c). A DNA strand binds stronger to its complementary strand if it has more G and enucleotides than if it has more A and T nucleotides.

DNA strands bind to each other through hydrogen bonding formed between the nucleotides, Ato T and e to G. Specifically, two hydrogen bonds form between A and T while three hydrogelbonds form between e to G. That's why DNA strands with more e and G nucleotides bindstronger to its complementary strand. This is important for bioengineers designing primers forpeR as it determines the temperature primers anneal to DNA (just a small tidbit for those whoare always hungry for more information on how what we learn here can be applied to thebiotech industry).

(d). Rank the different interactions discussed in class (ion-ion, ion-dipole, dipole-dipole, anddispersion interactions) with respect to the strength of interaction and justify your answer (howcan you tell which is strongest, weakest, etc?).

In descending order: ion-ion, ion-dipole, dipole-dipole, (ion-induced dipole), (dipole-induceddipole), dispersion. The strength of the interaction rests largely on the distance in which theycan act on each other.lon-ion interactions are proportionate to 1/r,ion-dipole interactions are proportionate to 1/r2,dipole-dipole interactions are proportionate to 1/r 3 ,and dispersion forces/interactions are proportionate to 1/r6.

(We did not give you equations for ion-induced dipole and dipole-induced dipole forces, nor didwe ask for them in the question, but do note that they are stronger than dispersion forces in theorder listed above. The actual relative strengths can be found on Dr. Michael Blaber's websiteon intermolecular forces, here:http://www.mikeblaber.org/oldwine/chm1046/notes/Forces/lntermol/Forces02.htm)

3. Why is perspiration the best way to cool a body in human physiology?

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Perspiration is thought of as the best way to cool the human body due to its large heat capacity(defined as amount of energy it takes to raise the temperature of a substance by 1 Kelvin),which means that it can store large amounts of energy without a significant change intemperature. Water has a large heat capacity as the hydrogen bonds that it forms within itselfhold it very strongly in place. As the body heats up, a large amount of heat energy is transferredto the water, which is subsequently expelled. This is all done without a large perturbation tobody temperature.

4. We all know Ethanol is miscible with water, but it is immiscible in hexane. Why do they notmix?

Ethanol is a small polar molecule that has a -OH group at one end. This group is capable offorming hydrogen bonds and hence, forms hydrogen bonds in water. It does not mix withhexane since hexane is non-polar. It preferentially interacts with itself, and hence partitionsaway from hexane in a mixture.

5. Nitrogen molecule has no permanent dipole. Yet you can liquefy nitrogen. Why is thispossible?

Nitrogen undergoes a change of state into liquid nitrogen at low temperatures. This is due to thefact that a lower temperature translates to lower (average) kinetic energy of the molecule, whichmeans that the physical vibrations of the molecule is reduced. At this point, the attractive forcesof the induced dipoles start to dominate and pull molecules together, resulting in the change ofstate from gaseous to liquid.

7. How might the amino acid composition for a transmembrane protein differ from a cytosolicprotein? How might one predict based on the amino acid composition of a transmembraneprotein how many transmembrane domains it contains?

Transmembrane proteins contain organized stretches of amino acids with hydrophobic residues(or r-groups). The hydrophobic residues interact with the lipid component of the bilayer, allowingthe protein to anchor itself to the membrane. The number of domains containing stretches ofhydrophobic amino acids can be a good indicator of how many times a transmembrane proteinspans the lipid bilayer. See "hydropathy index" if you're interested in more information.

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