pH pKa Stuff...

[sugarbabee0]

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129. Which of the following statements supports the idea that DNA is made up of nucleic acids?


A. When the pKa < pH, the molecule is neutral.
B. When the pKa < pH, the molecule is negative.
B is the best answer. An acid generally carries a negative charge when its pKa is less than the pH. When the pKa = pH, an acid is 50% neutral and 50% negatively charged. When the pKa is less than the pH, the molecule is now in a more acidic environment, and more of it carries a negative charge. Choice A is a false statement. Choices C and D apply to a base and not an acid. For a base, when the pH is less than the pKa, the molecule retains an extra proton and is positively charged. When the pKa is less than the pH, the base (i.e., an amino group) donates its proton and becomes neutral. The best answer is B.

C. When the pKa > pH, the molecule is positive.
D. When the pKa > pH, the molecule is neutral.


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I thought that when pH>pKa, the acidic form predominates (and that would be either neutral or +). Am I wrong?

 

[Rabolisk]

When pH > pKa, the basic form predominates. The acidity of nucleic acids come from the phosphate group, which are negatively charged in basic form and neutral in acidic form.

Hendersen-Hasselbach equation says that pH = pKa + log[base]/[acid]. If pH> pKa, [base]/[acid] would have to greater than 1.

 

[nUgnoY]

I think the simplest way to think about this is:

1. if pH > pKa, solution is basic relative to the molecule. The solution will deprotonate the molecule. Molecule will exist in deprotonated state (conj base). The functional group will be negatively charged

2. if pH < pKa, solution is acidic relative to the molecule. The solution will protonate the molecule. Molecule will exist in a protonate stated (acid). The functional group will be neutral.

The question seems a bit flawed to me, because an acid which is in a more acidic environment (pKa > pH) will exist in protonated state and can be neutral. But, it does make some sense, because the question specifically gives DNA and nucleic acid which implies physiological pH.

 

[web2linc]

There is no need to make this question complicated.

DNA is a monoprotic acid

When pH = pKa, the acidic molecule is neutral.
When pH > pKa, the acidic molecule is deprotonated (losing H+), making the molecule carry negative charge.
When pH < pKa, the acidic molecule remains protonated (due to the excessive H+ concentration), and remains it neutral.

The best evidence is when pH > pKa the molecule is deprotonated.

 

[web2linc]

taking biochemistry really helps with this kind of stuff and dealing with amino acids etc.