[mstrrogers]

These are the few questions I am stuck on. 

1. Aspirin is acetylsalicylic acid, which is shown below in its un-ionized form. The pK for the ionization of
acetylsalicylic acid is 3.5, and only the ionized form is absorbed in the body at a significant rate.

b. If the pH in the stomach 2.0, and the total concentration of aspirin is 1.7 mM, what is the concentration of acetylsalicylate?
c. What is the concentration of acetylsalicylate in the small intestine where the pH is 6.0?
d. Based on your answers, in what organ do you think ingested aspirin is absorbed?


2. Draw the tripeptide RAM in its predominant ionization state?? at pH 7.35, and calculate its pI.


5. . Where do new enzymes come from? Some bacteria have acquired the ability to metabolize man-
made chemicals. For example, tetrachlorophenol (TCP) did not exist in the environment before 1936
when it was introduced as a wood preservative, yet the bacterium Sphingomonas chlorophenolica
can metabolize TCP. One of the enzymes in the TCP degradative pathways is
tetrachlorohydroquinone dehalogenase (TCHQ dehalogenase). Examine the partial primary
sequences for TCHQ dehalogenase and several other enzymes and the structures of their substrates
shown on the following page. To which enzyme is TCHQ dehalogenase most closely related?
Explain your answer.

Not looking for answers, just some help explaining how to answer these questions.  Thanks

[Arkcon]

OK, lets start with the firts one.  You've been given two different pH values, and they're pretty far apart.  How can you relate your given info -- pKa of the active, and  pH of the environment, to structure?

[Yggdrasil]

For #2, you should start by identifying all of the acidic/basic groups on the tripeptide, looking up the pKa values for those groups, then deciding whether they are protonated or deprotonated at pH 7.35.

[mstrrogers]

I will attempt my best, but my teacher hasn't exactly covered this.  The pH of the stomach is pretty acidic and the ionized form of aspirin has a pK of 3.5(in this instance does pK=pH?).  Since the environment is more acidic does the concentration of aspirin is 1.7mM, I would assume that the concentration of acetylsalicylate would be a good percentage of aspirin?  Im just really lost on this question...

[mstrrogers]

For #2, you should start by identifying all of the acidic/basic groups on the tripeptide, looking up the pKa values for those groups, then deciding whether they are protonated or deprotonated at pH 7.35.

since 7.35 is pretty neutral and arginine has a high pK on its sidechain would it be deprotonated?

[Arkcon]

Ok, try this.  Look at the structures of acetylsalicylic acid, and it's ionized form,acetylsalicylate.  How are they different?  What would a free H+, which is abundant at low pH, do to each?  Since high pH is the opposite, what happens in that case?  Did you discuss concepts along those lines in class?

[mstrrogers]

Ok, try this.  Look at the structures of acetylsalicylic acid, and it's ionized form,acetylsalicylate.  How are they different?  What would a free H+, which is abundant at low pH, do to each?  Since high pH is the opposite, what happens in that case?  Did you discuss concepts along those lines in class?

The ionized form at the lower pH would want to bind to the free H+, and the acid form would not want too???....this is very frustrating...when teachers are not available for help....

[aHerraez]

To help clarify your thinking, this is the gold rule:

1. A compound does not have a pH. A solution has a pH, which means its concentration of H+, and that will be in equilibrium will all compounds present in the solution.

2. For any compound with an acid-base dissociation, it will be (mostly) protonated if pH of the solution is below the compound's pKa, and (mostly) unprotonated if pH of the solution is above the compound's pKa. (Think of amount of H+ available for the acid-base equilibrium)

3. The charge of the compound will depend on
a) how far is the pH from its pKa -- magnitude of the charge
b) whether the compound's chemical structure makes it ionized when protonated or when unprotonated -- sign of the charge. Examples: a carboxylic acid goes from neutral to negative when pH is raised, while an amine goes from positive to neutral.