NADH, why strong electron donor?

[Monique]

NADH, why is this reduced molecule a strong electron donor (it likes to give its electrons away)? The instability has something to do with the high-energy electron it is carrying.. but why?

[nautica]

Is it an electon donor??? Or a Proton (H+) donor?

Looks to me that going from NADH to NAD it is an elimination reaction, which give off an H and forms a double bond in the ring.

http://www.csun.edu/~hcchm007/334nadh.pdf

Nautica

[GCT]

In converting from NADH to NAD+ a hydrogen from the c4 carbon is lost. The nitrogen, which is within the ring, facilitates this process by contributing its nonbonding electrons to the ring; there is a localized shift of electrons. By contributing its electrons it lowers the activation energy of the reaction since; otherwise a charge will develop from the loss of hydrogen and thus the activation energy will be high to form a molecule with such unstable transition state.

[Chemicalsuperfreak]

NADH is an electron donor. More specifically a hydride donor, it's certainly not a proton donor. I believe we had a thread on this several months back.

[Monique]

Yes, I know :) a H- is formed, which exists in water as H3O- right? [t)] due to the hydrogen bonding action of water molecules, the hydride is moved through the water without it itself actually having to diffuse, it is the bond energy that is shifted.

But what I am wondering about is what makes NADH and NADPH such strong electron donors, and why NAD+ and NADP+ are weak electron acceptors?

GeneralChemTutor, are you saying that the activation energy is low for the transition state between NADH and NAD+ due to the action of nitrogen in the ring that is able to redistribute its electrons?.

And since the activation energy is low, it is a strong donor right?

[GCT]

GeneralChemTutor, are you saying that the activation energy is low for the transition state between NADH and NAD+ due to the action of nitrogen in the ring that is able to redistribute its electrons?.

Yes. Nitrogen has a lone pair which it can distribute to stabilize the charge that is developing when the hydrogen is being pulled off. Without this contribution, the transition state of the activation complex would be unstable since a full positive charge is being developed.

[Monique]

Ok, so if this charge distribution is taking place due to the free electrons of nitrogen, why is NAD+ a weak electron acceptor?

[GCT]

Well in a generalized sense, the molecule NAD+ which is formed by the process that I had explained to you, would be relatively stable due to the electron contribution of N to the ring, and thus relatively unreactive . This is exactly the case. NAD+ is actually more stable than its precursor; this is another reason why it does not react under standard temperatures because the activation energy for the reverse process is much higher. Just tell me if you want to know the reason. At this time I'm a little preoccupied.

[GCT]

NAD+ is aromatic while NADH is not. Aromatic compounds are more stable than its standard counterparts due to delocalization of pi electrons. Thus energy input is required to convert to NADH; it has a higher free energy. This storing of energy can later be used in helping alcohol dehydrogenase convert alcohol to the aldehyde; NAD+,NADH is a coenzyme.