I'd say no, because the other peptide bond atoms possess a dipole as well, and thus exert an additional polarization upon the original peptide bond.

Dear Martin and Anselm,

I have heard that when amino acids are hydrogen bonding, the peptide bond vibration energy is lower (IR measurements). Furthermore, if the an α-helix is long it can gain an overall dipole, which requires neutralization with a charged amino acid. Could these two distant ideas be related to this question?

Ali

thx anselm and muhammad for your answers.

I don´t know enough about the vibration energy.

the peptide bond dipole consists out of the C = O part with the negative pole at the oxygen and the the N - H moiety with the positiv pole at the hydrogen. If the oxygen participates at a hydrogen bonding, then the oxygen looses electron density, with this MAYBE the negative dipole also gets weaker. equal to this at the hydrogen from the N-H part. when the hydrogen takes part at hydrogen bonding it will move further away from the nitrogen and then more electron density will acumulate there and with this MAYBE the positive dipole part gets more negative and with this weaker.

so the positive pol gets more negative and the negative pole gets more positiv, which should weaken the overall dipole ????

this is what I think. but I don´t know enough about electronegativity and dipoles, and if this really happens so.

The hydrogen bond induces the dipole to be stronger. In your question you have hit upon one of the key aspects of the hydrogen bond that influences molecular interactions from protein structure to the structure of water and its high boiling point. Networks of hydrogen bonds tend to increase the dipole and lead to stronger hydrogen bonds. In essence, the hydrogen bond takes induced dipole interactions to the next level by partially sharing the electron on the hydrogen (a partially covalent interaction). The hydrogen bond shifts electron density, such that the charge on the donor’s oxygen atom increases whereas the charge on the acceptor’s hydrogen atom decreases so they are better hydrogen bonding partners for the next molecule. The alignment of peptide bond dipoles in an alpha helix results in a net helix dipole (~1/2 of a charge), favoring the incorporation of additional residues into the helix. Within the protein environment (low dielectric contact compared to water) the hydrogen bond energy is ~2kcal/mol.

thank you very much john for your detailed answer,

I hope I can get it.

The hydrogen bond shifts the electron density form the oxygen. So less electron density remains at the oxygen. At the nitrogen the electron density increases, because the proton moves further away to the acceptor.

The value of electronegativity can´t increase as it is a characteristic of the atom core.

But by loosing of electron density, the oxygen gets more favored to attract electrons and with this the negative pol of the dipole increases.

This sounds confusing, electron density at the negative pole decrease and at the same time the negative pole gets stronger.

Yes - neither the electronegativity nor the net charge change. It is the induced dipole that gets stronger.

Thank you a lot john