Polyglycine a simple polypeptide can form a helix with phi

Polyglycine, a simple polypeptide, can form a helix with phi = -80deg and psi = +150deg. From the ramachandran plot (attached), describe this helix with respect to handedness.

I have l found this is a left handed helix, but can you please explain how to decipher handedness from this kind of plot, and why exactly this one is a left-handed helix.

6.1 Secondary Structure: Regular Antiparallel B strands Parallel B strands 180 A Polypeptide ll helix 120 +60 Left-hand a helix Right-hand 310 helix a helix 120 +180 120 +600 180 120 180

Solution

Most of the proteins and amino acids show right handed alpha helix model. There are very rare exceptions where we can find left handed alpha helix because D-amino acid is the mirror image of L- amino acid.

So in our example we took the psi and phi angles of polyglycine which is a most common exceptional case where we can observe polyglycine residues lack a side chain that will cause a steric hindrance.They simply make all the psi and phi angles corresponding to the left handed helices, often do only for one glycine residue or for a series of glycine residues(polyglycine).

Theoritically , the average phi and psi values of beta sheets or beta strands should be -80 and +150, whereas some experimental structures varies. In the given ramachadran plot above for glycine, we see many dots in the disallowed regions. The torsion angles lack real clustering around secondary structure regions and have a wider distribution.