H-boding in alpha helices

[donkeykong1]

the h-bonding in the secondary structure of proteins, specifically alpha helix is considered by kaplan to be INTRAmolecular. I thought h-bonds were intermolecular like dipole/ion dipole and vanderwalls...

also, they mention that h-bonds aren't covalent, and that besides for peptide bonds, the only other covalent interaction in initial protein structures are the disulfide bridges. but if h-bonds are intra, shouldn't they be covalent?

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[Rabolisk]

H-bonding, dipole-dipole, van der Waals interactions are all intermolecular forces, not intra. I mean, if they were intramolecular, and covalent bonds were intramolecular.. What would be an example of an intermolecular force?

 

[ilovemcat]

the h-bonding in the secondary structure of proteins, specifically alpha helix is considered by kaplan to be INTERmolecular. I thought h-bonds were intramolecular like dipole/ion dipole and vanderwalls...

also, they mention that h-bonds aren't covalent, and that besides for peptide bonds, the only other covalent interaction in initial protein structures are the disulfide bridges. but if h-bonds are intra, shouldn't they be covalent?

This is an important concept you should definetly understand for the MCAT. First of all, a Hydrogen-Bond is definetly not a covalent bond. It's much much weaker than a covalent bond.

An INTRAmolecular bond is a covalent bond. An INTERmolecular bond includes: Hydrogen Bonding, (Polar) Dipole-Dipole interactions, and Van der Waals Forces.

When you're boiling water for example, it's the INTERmolecular forces you're breaking (ie. Hydrogen Bonds, Polar Interactions, etc.) NOT the covalent nature of the molecule itself (ie. the bonds in H2O; H-O-H).

 

[Rabolisk]

I post first, so I win!! Regardless of how much better yours was compared to mine!

 

[ilovemcat]

I post first, so I win!! Regardless of how much better yours was compared to mine!

Lol, damn it!

 

[donkeykong1]

sorry guys, i know this. i just edited it. i meant to ask the other way around. but the question still stands see below:
p.291 kaplan orgo review notes:

alpha helices are secondary structures characterized by:

a-INTRAmolecular h-bonds
b-disulfide bonds
c-rippled effect
d-INTERmolecular h-bonds

the correct answer is said to be A. but i think and it seems like u guys think as well think the answer to be d.

 

[ilovemcat]

sorry guys, i know this. i just edited it. i meant to ask the other way around. but the question still stands see below:
p.291 kaplan orgo review notes:

alpha helices are secondary structures characterized by:

a-INTRAmolecular h-bonds
b-disulfide bonds
c-rippled effect
d-INTERmolecular h-bonds

the correct answer is said to be A. but i think and it seems like u guys think as well think the answer to be d.

yEAH, BOOK ERROR. HAppens sometimes.

asdflasdgl beat u rabo

 

[Rabolisk]

You did beat me. Unfortunately I disagree with you. This is sort of gray but I can justify the answer being A. The H-bonds in an alpha helix is between the NH of a residue and CO of another residue 4 units down. This is intramolecular because the H-bonding is within the protein molecule.

 

[ilovemcat]

You did beat me. Unfortunately I disagree with you. This is sort of gray but I can justify the answer being A. The H-bonds in an alpha helix is between the NH of a residue and CO of another residue 4 units down. This is intramolecular because the H-bonding is within the protein molecule.

Ah you're right. That's what I get for being so hasty, lol. You learn something new everyday.

OP: http://en.wikipedia.org/wiki/Hydrogen_bond

 

[donkeykong1]

Nice. thanks you two

A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond. These bonds can occur between molecules (intermolecularly), or within different parts of a single molecule (intramolecularly).[2]