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I am told that beta branching interferes with alpha helix formation.

Problem is that I don't see how beta branching has anything to do with alpha-helix formation. Beta-branches are on the outside of alpha helices and internal hydrogen bonding holds the helix together.

My book (Lehninger) also doesn't say anything about beta branching interfering with alpha helix formation.

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    $\begingroup$ Beta branched amino acids are bulky and can sterically clash with neighbouring atoms, forcing the backbone to adopt torsion angles which don't favour helix formation. Does that answer your questions? If it does I can expand on it and post it as an answer. $\endgroup$ – canadianer Feb 3 '15 at 2:54
  • $\begingroup$ Also it shouldn't be said that they stop alpha helix formation outright, it's really contextual. Generally, though, they are favoured in domains where the back bone is more extended than in a helix. $\endgroup$ – canadianer Feb 3 '15 at 2:56
  • $\begingroup$ @canadianer can you expand $\endgroup$ – Dissenter Feb 3 '15 at 3:15
  • $\begingroup$ @canadianer can you post this as an answer. You may also add an picture of helical wheel or Ramachandran plot for these residues. $\endgroup$ – WYSIWYG Feb 3 '15 at 5:28
  • $\begingroup$ @WYSIWYG Yes I will when I have time. Feel free to answer if you wish $\endgroup$ – canadianer Feb 3 '15 at 7:05
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The answer to this, as said by @canadianer in comment, is that beta branched amino acids are bulky and can sterically clash with neighbouring atoms, forcing the backbone to adopt torsion angles which don't favour helix formation.

I couldn't find reference for these exact words, but similar one here:

amino acids such as Valine, Isoleucine, and Threonine all have branching at the beta carbon, this will cause steric clashes in an alpha helix arrangement... valine, threonine, and isoleucine will often destabilize the helix because of branching of the beta carbon. These three amino acid residues are more often found in beta-pleated sheets, where their side chains will lie in a separate plane than the main chain.

Also, for torsion angles, from same source:

The torsion angle is the measure in degrees in bonds between atoms. Folding of proteins are influenced by the degree of rotation amino bonds can hold. There are two different types of torsion angles existing in polypeptide bonds. Psi, ψ is the angle between the α-carbon and the nitrogen atom of a peptide bond. The other bond is called phi, φ which is the angle between the α-carbon and the carbonyl group.

torsion angle ψtorsion angle φ

See this table for details of which amino acid prefers which configuration:

conformational preferences of the amino acids

Source

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