How Much Energy Is Required To Unfold One Turn Of An Alpha-Helix?

Question

[Keith Callenberg]

Someone asked me this question during a presentation I was giving because they said the answer might help convince them that my model was correct.

How much energy is required to unfold one non-terminal turn of an alpha-helix?

Alpha-helices are held together mostly by hydrogen bonding in the backbone. I think a "turn" of the helix is composed of 3.6 residues on average. Is it just a matter of how much energy would be required to break these bonds? Or are there any other factors?

UPDATE: please consider a turn within the middle of the helix only, none of the residues in the turn are terminal residues.

Also, a range of values might be the only way to answer this question. For instance, Tobias & Brooks show with MD that they get different values for valine and alanine. I am wondering whether their 1991 MD simulation is outdated, whether it is supported by experiment, and whether valine and alanine represent the full range.

Answer 1

[Priyabrata]

If you are asking about only helix alone ,then the energy required depends on the position of the turn in the helix because

The terminal residue's both CO and NH do not participate for hydrogen bonding. For Nonterminal residues, each residue forms 2 hydrogen bond.

Still here we are not assuming any sidechain-sidechain hydrogen bonding and sidechain-mainchain hydrogen bonding. If you include these, then energy required will still be more.

Again here we are excluding interaction of Helix with near by secondary structures (Nearby helices, strands etc).

So not only just hydrogen bonding but there can be many more factors responsible, which depends on protein itself. We can not generalize this.

Best possible way to use any forcefield to calculate the energy for unfolding. As said earlier, i think, it can not be generalized.

[Keith Callenberg]

I think these are good points, but I disagree with your conclusion that no generalization is possible. I have edited my question to specify non-terminal residues. I don't need an exact value, within 1 or 2 kcal/mol would be fine.