UWorld Clarifications

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May 22, 2020
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1) On UWorld, it says dNTP cleavage is exergonic but phosphodiester bond formation is endergonic. I thought bond formation is exergonic since energy is released whereas cleavage is endergonic since energy needs to be put in? Unless this is the only exception?

2) Increasing carbon-oxygen bond dissociation energy is from single to triple bond, but UWorld also says that pi bonds have smaller dissociation energy so I'm sorta confused?


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Apr 25, 2020
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1) For example, in ATP the phosphate bonds are high-energy bonds because of the repulsions of the individual phosphates (because they all carry negative charges and they are very close together). Therefore, its hydrolysis would release energy, which is the reason that ATP hydrolysis can power other reactions through coupling. The same is true for the three phosphates in dNTP, they are high-energy bonds and cleavage would release energy (exergonic).
2) Yes pi bonds have a smaller dissociation energy than sigma bonds, but in double bonds there is 1 pi and one sigma, in triple bonds there are 2 pi and one sigma, making the bond dissociation energy of those bonds larger than that of a molecule with a single bond, which only is comprised of 1 sigma bond. Basically, although individual sigma bonds have a larger BDE than individual pi bonds, the BDE is single< double< triple because of the increasing number of pi bonds, the number of sigma bonds are the same for each bond type (1 sigma bond).

I hope this helps!
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