Chemical Forums
Specialty Chemistry Forums => Biochemistry and Chemical Biology Forum => Topic started by: JackBlack on April 20, 2012, 02:49:08 PM
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Q. Calculate the value of Eo' for the reaction NAD+ + H+ +2e- :rarrow: NADH at PH7 ,given that the Eo= -0.320V at 298K
I am not too sure how i would answer this question. Do i need to use the value of Eo' for the O2/H2 half cell reaction: O2 + 4H+ +4e- :rarrow: 2H2O which is + 0.816V ?And would i need to use the Eo cell= Eoright- Eoleft equation ?
Help please
Thanks!
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No, you are just asked about the formal potential of the NAD+/NADH half cell. Simple plug and chug with the Nernst equation.
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No, you are just asked about the formal potential of the NAD+/NADH half cell. Simple plug and chug with the Nernst equation.
The nernst equation requires some concentrations, but i am not given that in the question ???
Do i assume that the NAD+ and NADH are 1 moldm-3?
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Yes, that's what I would do in this case.
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Yes, that's what I would do in this case.
thank u !!
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The use of E°' means that we are dealing with the biochemical standard state. The only difference between this standard state and the ordinary standard state is the assumption of pH 7, where [H+] = 1 x 10^-7 M. A commonly used Biochemstry textbook (Nelson and Cox, page 515) gives E°' for NAD being reduced to NADH as -0.320 volts, and that is the value I have always seen. E° for NAD (the value at pH 0) would be a very different number, because the half-cell reaction involves a proton.
To be honest, the question does not make sense to me. I only see a half cell, not a full cell. If I were to assume another half-cell to make a full cell, I might be tempted to choose the hydrogen ion:
2H^+ + 2e^- ===> H2. The E°' value for this is -0.414 volts (not 0.00 volts). The only other thing I can think of is that the question is asking the student to convert E° to E°'. However, the value given in the problem is not E°, if I understand things correctly.
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To be honest, the question does not make sense to me. I only see a half cell, not a full cell.
That's not a problem - tables of standard potentials contain only half cell potentials, measured against 0 - and 0 is by definition standard hydrogen electrode (not that anyone uses it in lab practice).
The only other thing I can think of is that the question is asking the student to convert E° to E°'.
That was my understanding.
However, the value given in the problem is not E°, if I understand things correctly.
And that can be a problem. Perhaps whoever designed the question was not aware of the fact something like "biochemical standard state" exists. I wasn't, for me the standard state means pH=0, period. I am not stating biochemical approach is wrong, I can understand why it is convenient to use pH 7 as a reference, it is just confusing when someone uses term will well know definition, but in different meaning.
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I had a look in the second edition of Metzler's book, Biochemistry, starting around page 293, and it seems as if the situation is in a bit of flux in the biochemical literature regarding the best choice for the standard state. The biochemical standard state was something I learned about in graduate school (which was a long time ago). It is my understanding that it is identical to the regular standard state, except for the hydronium ion concentration.
Metzler discusses electrochemistry starting on page 300. If I take E°' to be -.0320 volts for NAD/NADH, I can calculate that E° is -0.113 volts for NAD/NADH (the difference being related to the difference in hydronium ion concentration. Both of these values appear in his table on page 301.
Putting in another way, I would have formulated the question as follows: Given a value of E° of -0.113 volts, calculate E°' for the NAD/NADH half cell. I know just enough about this subject to be dangerous, so please let me know if anything was unclear.