Methylation of H3K9 demarcates heterochromatin while H3K4 methylation demarcates euchromatin.

TLS Online TPP Program

#Id: 7713

#Unit 1. Molecules and their Interaction Relevant to Biology

                                                        𝝅-Turn

Consists of 6 amino acids, generally termed as i, i + 1 , i + 2 i + 3 i + 4 and i + 5

H-bond is seen between i and i + 5 (First and sixth residue)

TLS Online TPP Program

#Id: 8443

#Unit 1. Molecules and their Interaction Relevant to Biology

The 3¹⁰ helix has 3 residues per tum and contains 10 atoms between the hydrogen bond donor and acceptor, hence its name.

TLS Online TPP Program

#Id: 8444

#Unit 1. Molecules and their Interaction Relevant to Biology

Both the pi helix and the 310 helix occur rarely and usually only at the ends of a helices or as single-tum helices. They are not energetically favorable, since the backbone atoms are too tightly packed in the 310 helix and so loosely packed in the n helix that there is a hole through the middle. 

Only in the a helix are the backbone atoms properly packed to provide a stable structure.

TLS Online TPP Program

#Id: 8445

#Unit 1. Molecules and their Interaction Relevant to Biology

Right-handed helical conformation has 3.6 residues per turn and 1.5A/aa.

TLS Online TPP Program

#Id: 8446

#Unit 1. Molecules and their Interaction Relevant to Biology

In the α helix, the backbone hydrogen bonds are arranged such that the peptide CO bond of the nth residue points along the helix axis toward the peptide NH group of the (n + 4)th residue. 

TLS Online TPP Program

#Id: 8447

#Unit 1. Molecules and their Interaction Relevant to Biology

This results in a strong hydrogen bond that has the nearly optimum N…O distance of 2.8 A.