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Adam H.
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Organic Chemistry
TutorMe
Question:

Which of the following compounds will not react with chromic acid? A. butanol B. pentanal C. ethanol D. phenol

Adam H.
Answer:

Phenol will not react with chromic acid, D is correct. Breaking down the question: The chromic acid will oxidize primary and secondary alcohols, as well as aldehydes. There must be at least one available hydrogen on the carbon bonded to oxygen in the compound in order for it to be oxidized and butanol, pentanal, and ethanol all have at least one available hydrogen atom on the carbon bonded to oxygen. Butanol and ethanol are primary alcohols and will be oxidized to carboxylic acids. Pentanal is an aldehyde and will be oxidized into carboxylic acid too. Phenol does not have an available hydrogen on the compound bonded to oxygen. Putting it all together: Since phenol does not have an available hydrogen on the carbon bonded to oxygen, it cannot lose its hydrogen to be oxidized. Thus, phenol will not react with chromic acid since chromic acid requires an available hydrogen on the carbon bonded to oxygen in any given compound.

MCAT
TutorMe
Question:

Which complex of the electron transport chain (ETC) do electrons from pyruvate enter? A. Complex 3 B. Complex 4 C. Complex 1 and 2 D. Complex 1

Adam H.
Answer:

Electrons from pyruvate enter the ETC at complex 1, D is correct. Breaking down the question: Pyruvate is created at the end of glycolysis in the cytoplasm and is then shuttled into the mitochondria. In the mitochondria, pyruvate is oxidized to acetyl-CoA. This oxidation process of pyruvate takes electrons from pyruvate and places (reduces) them on NAD+ to create NADH (an electron carrier molecule). The ETC accepts electrons from NADH into complex 1 and electrons from FADH2 into complex 2. Putting it all together: Since pyruvate is oxidized (loses its electrons) to acetyl-CoA and generates NADH in the process, then electrons from pyruvate must enter the ETC at complex 1 as this is where NADH is received in the ETC.

Biochemistry
TutorMe
Question:

A protein has an isoelectric point (pI) of 4.55. In a solution with pH = 6.0, would this protein bind to a dimethylamino ethoxy-cellulose (DEAE-cellulose) column?

Adam H.
Answer:

Yes. The protein would bind the DEAE-cellulose column because the pI of the protein is less than the solution pH; thus, the protein will carry a net negative charge and this charge will be attracted to the positive charge of the DEAE-cellulose column. Breaking down the question: The pI of a protein is the pH in which the protein carries a net zero charge (all opposite charge balance each other out). Pretend that the protein started at a pH of 1.0; at this hypothetical pH, the protein would be fully protonated. As we increase the pH of the solution it becomes more basic and begins to deprotonate acidic sites on the protein. The protein will lose acidic protons until it reaches a pH = 4.55. At pH = 4.55, the negative charges created from deprotonation will equal any positive charges and the protein will have net charge of zero. But, what happens when we increase the pH beyond the pI. If the pI of the protein is less than the pH of the solution, this means that the protein is in a basic environment (relative to itself). Thus, the basic solution will deprotonate acidic sites on the protein and the protein will now carry a net negative charge. A DEAE-cellulose column is a type of affinity column chromatography called ion-exchange and is used to separate proteins and amino acids which carry net negative charges. A DEAE-cellulose column contains an amino group with four groups bound to it, thus the nitrogen of the amino group carries a positive charge and will bind proteins carrying a net negative charge. Putting it all together: Since the pI of the protein is less than the pH of the solution, the solution will act as a base and deprotonate the protein, leaving it with a net negative charge. If the protein is passed through a DEAE-cellulose column, which carries a net positive charge, then the column at protein will bind each other as opposites attract.

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