Heat disrupt electrostatic bonds but not covalent?

[TheRelevant]

Hey guys, I had a brief question for you all. Thanks in advance for your input!

EK mentions that heat will disrupt the electrostatic bonds that hold the tertiary/quaternary structure of polypeptides together (including acid/base ionic interactions), but doesn't disrupt the covalent bonds. Why is this? Aren't ionic bonds supposed to be stronger than covalent bonds?

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[yestomeds]

Yea this is an interesting question and I'd be interested too.
My guess is that it involves something along the lines of covalent bonds being actual intramolecular forces, meaning electrons b/w atoms within a molecule are actually shared... therefore more 'stable?' We'll let the experts answer.

 

[efle]

EK is just not being clear.

All bonds will be disrupted by sufficient heat. True ionic bonds (NaCl) are stronger than covalent (disulfide bridge), which in turn are stronger than the type of electrostatic bonding seen in protein structure (hydrogen bonding, dipole attractions). In the heat levels relevant to bio, probably only up to boiling, you will have sufficient heat to disrupt the electrostatic bonding seen in proteins but not enough to disrupt covalent or ionic bonds.

 

[TheRelevant]

Charged acidic/basic side chains produce ionic bonds - so the interactions are not limited to H-bonding, vdws etc. I believe the KA videos used the same logic, but never explained it.

 

[efle]

Charged acidic/basic side chains produce ionic bonds - so the interactions are not limited to H-bonding, vdws etc. I believe the KA videos used the same logic, but never explained it.

Salt bridges are weaker than what you typically think of as ionic bonds, in fact they're weaker than the covalent bonding. Real ionic bonding - the kind notably stronger than covalent - requires the charges to have much greater proximity than what you see in side chain interactions.

 

[TheRelevant]

Thanks Efle! That makes tons more sense

 

[Dr. Trenb]

As others said, EK is not clear. However, when you're taking the test, think relative to the subject you're being tested on.
When it comes to biology and proteins, the order of bond strength are as follows: Peptide bond (covalent) > disulfide bond(covalent)> Ionic bond> Hydrogen bond> Hydrophobic rxn.
Also, protein denaturation (with urea, acid, heat) effect secondary, tertiary and quaternary structures ONLY. (primary sturcture not involved)

 

[efle]

I'd summarize as:

Classical ionic bonding (think NaCl) > covalent bonding (peptide > disulfide) > protein electrostatic interactions (salt bridge ionics > hydrogen bonds > dipole-dipole > phobias > vanderwaals)

Temperatures near boiling will disrupt only (and all) protein electrostatics.