Secondary structure and backbone conformation:part 5: Beta Sheet Reverse turns:

The b-sheet Reverse turns: 

A reverse turn is region of the polypeptide having a hydrogen bond from one main chain carbonyl oxygen to the main chain N-H group 3 residues along the chain (i.e. O_i to N_i+3). Helical regions are excluded from this definition and turns between b-strands form a special class of turn known as the b-hairpin (see later). Reverse turns are very abundant in globular proteins and generally occur at the surface of the molecule. It has been suggested that turn regions act as nucleation centres during protein folding.

Reverse turns are divided into classes based on the F and Y angles of the residues at positions i+1 and i+2. Types I and II shown in the figure below are the most common reverse turns, the essential difference between them being the orientation of the peptide bond between residues at (i+1) and (i+2).

The torsion angles for the residues (i+1) and (i+2) in the two types of turn lie in distinct regions of the Ramachandran plot.

Note that the (i+2) residue of the type II turn lies in a region of the Ramachandran plot which can only be occupied by glycine. From the diagram of this turn it can be seen that were the (i+2) residue to have a side chain, there would be steric hindrance with the carbonyl oxygen of the preceding residue. Hence, the (i+2) residue of type II reverse turns is nearly always glycine.