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Tutor profile: Mary W.

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Mary W.
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Questions

Subject: Organic Chemistry

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Question:

Nucleophilic substitution reactions are very common in organic synthesis, and when doing reactions it is important to predict what kind of steroechemisty your products will have. An SN1 reaction on a purely R solution will yield both R and S stereoisomers, but an SN2 reaction in the same solution will only yield S isomers. Why is this?

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Mary W.
Answer:

In SN2 reactions, the nucleophile attacks the electrophile in a backside attack, which kicks off the leaving group and inverts the stereochemistry of the molecule. Therefore, all of the product in the above scenario will be in the S conformation. However, in SN1 reactions, the leaving group leaves before the nucleophile attacks. This creates a tertiary carbocation intermediate with trigonal planar geometry. Since there is a 50/50 chance that the nucleophile will attack from the top or the bottom of the planar molecule, the SN1 reaction will yield an approximately equal blend of products in the R and S conformation.

Subject: Biochemistry

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Question:

Ammonia is a toxic waste product of amino acid catabolism, so it is disposed of via the urea cycle. Why is ammonia toxic to humans?

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Mary W.
Answer:

Ammonia is one of the products of the glutamate dehydrogenase reaction, which uses glutamate to produce alpha-ketoglutarate and ammonia. When ammonia concentrations are high, equilibrium shifts to favor glutamate. While this decreases the amount of ammonia in the system, it also decreases the amount of alpha-ketoglutarate since equilibrium was shifted. Alpha-ketoglutarate is a key component of the Krebs cycle, which is vital for aerobic metabolism and eventual energy production via oxidative phosphorylation. Removing alpha-ketoglutarate breaks the Krebs cycle, and therefore too little energy is produced to maintain homeostasis. Because of this, ammonia is said to be metabolically toxic.

Subject: Biology

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Question:

Antibiotic resistance is a major health concern in our world today, and it often arises as a result of incomplete antibiotic treatment. Why would antibiotic resistance develop, and how does it relate to natural selection?

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Mary W.
Answer:

Natural selection is the concept that organisms that are the most fit to survive and reproduce will have offspring, and over time the population will begin to exhibit the characteristics that made them more fit more frequently. This is often thought of as happening in animals, but it happens at a microscopic level too. If you have a pathogenic bacteria that you are trying to get rid of and expose it to an appropriate antibiotic, the antibiotic should kill all susceptible bacteria. However, if one of the bacteria has a random mutation that allows it to evade the antibiotic killing mechanism, it will survive and reproduce so that the new bacterial population is resistant to the antibiotic treatment. Thus, resistance occurs.

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