[GeLe5000]

Hello.

It is well known that:
"Bond energies (enthalpies) can be used to indicate how stable a compound is or how easy it is to break a particular bond.
The more energy that is required to break a bond, the more stable the compound will be.
A larger bond energy implies the bond is harder to break, so the compound will be more stable."

So, I don't understand the following sentence from "Energy flow in Biology, Harold J. Morowitz, Ox Bow Press, 1979), page 145:

"Certain of the chain-extending bonds, such as C-C and C-N, are stable under ordinary conditions and thus make an important contribution to the system".

It's in a chapter about order information and entropy in living systems. He tries to demonstrate that in biological systems, order as thermodynamically defined corresponds to macromolecular complexity.
He compares in a Table different bonds, categorized as chain-terminating and chain extending.
C--O (double bond  !) and O-H are chain terminating and are characteristic of an equilibrium distribution of rather low average molecular weight (small molecules like CO2 and H2O). Chain extending bonds like C-C, C-N, C-H,.. increase the stored energy in more complex compounds in nonequilibrium systems, with a rise in average molecular size.

It's a rather complicated subject and I could not explain more here. However, I think that I can understand enough to suspect a dramatic mistake.

The two bonds that he mentions, C-C and C-N, are those that have the lowest bond energies among 13 others!
He doesn't give the bond energies in his Table, but they are well-known. C-C : 82.6 kcal/mole, C-N : 72.8 kcal/mole compared with C--O (double bond in CO2 !) : 182 kcal/mole, O-H : 110.6 kcal/mole.
He insists that C-C and C-N are particularly stable, whilst they are actually the most unstable since they are easily broken.

So, is this renowned biophysicist a crook and nobody can explain the relation between stored energy and molecular complexity, or is there something that I don't understand?

[Corribus]

C=O actually contains two bonds, so it's not a fair comparison to the other species. You also can't just consider single bonds in isolation, because carbon atoms are also bonded to other things. Compare a ketone to a carbon bonded to two other carbons and two hydrogens. Ketone(...C-C(=O)-C...) has total bond energy of 1491 kJ/mol whereas ...C-C(H)(H)-C... has 1556 kJ/mol. I'd say it's the C-H bonds that count a lot toward stability, directly, not the C-C bonds, but long chains of C-C bonds contain lots of C-H bonds. Two C-H bonds (411 kJ/mol each) have more total bond energy than a single C=O bond (799 kJ/mol).

(With this information you should also be able to explain why carbohydrates are excellent energy storage molecules.)

[GeLe5000]

So, you probably mean that when he makes the list of chemical bonds, he omits to say that they can give rise to structures in which they can appear several times, whilst of course in simpler molécules (CO2, H2O) they appear only one or or two times.
I agree with that but I still have a problem with "C-C and C-N are stable under ordinary conditions". It's strange that he doesn't see the necessity to underly that this stablity stems from the fact that they are present several times in the molecule, and doesn't concern the bond itself.
It's not the first time that I have a strange feeling when I meet the whish of scientists (chemists, biochemists, and here a biophysicist) to explain the relation between energy, order, complexity or even "information". For example a few words of Dr Lehninger : "Information is a form of energy". But no demonstration !
They wish to explain, but it's not clear.
I will keep reading.

Thank you for your reply.

[Enthalpy]

Hi,

I don't like too much the term "stable" in relation with heats of formation. For instance cubane is stable while nitroglycerine is not.

Biology uses ways to operate at room temperature and it's all a matter of intermediates, where all steps have a very small activation energy. To my understanding, life reactions act on molecules where the functions are: alcohols, acids and so on, but hardy on the hydrocarbon part.

The limit of comparing with bond strength is that the biological reaction (...and chemical reactions too) doesn't break the bond, wait for a minute, and then make the new compounds - here the bond energy would be meaningful. It goes immediately through intermediate species; only this makes a reaction possible, especially at room temperature. So the ability to go through other species at tiny energy cost defines if a bond can break or not.

At functions, a bond can easily make such an intermediate, while the hydrocarbon part is more difficult to transform. Not very different from non-bio chemistry.