Beta Bulge

Motif Glossary

Beta Bulge


A motif of two consecutive residues (doubleton), a residue separate from these (singleton), and two H-bonds in which there is:

Show 3D view


In February 2019 we revised the sub-categories to include the conformation at residue ‘0’ — N-terminal to residue 1. This was because the conformation at this residue determines whether the Beta Bulge is of the ‘classic’ type or is a ‘G1α’ or ‘G1β’ Beta Bulge.

The base of the sub-categorization is between 1,2-αRβ (which include those designated ‘classic’) Beta Bulges and 1,2-αLβR (which include those designated ‘G1’).

The full classification also distinguishes between the possibilities that the singleton is N-terminal to (N-singleton) or C-terminal to (C-singleton) the doubleton for each of the above.

Alternative visualization

It is often convenient to present the β-bulge and its composites in a view that is flipped both vertically and horizontally from that above, with the singleton designated ‘X’ and the residue at position 0 included:


Richardson et. al, Proc. Natl. Acad. Sci. 75:2574-2578 (1978)

Leader and Milner-White, Acta Cryst. D77 217–223 (2021)


1. This motif is also termed the ‘Narrow’ Beta Bulge.

2. A third hydrogen bond — from the singleton CO to the NH of residue-2 — is also frequently found.


1. The Beta Bulge is most often found at the edges of β-sheets, but its general location differs for the three main subcategories.

2. 0,1,2-αRαLβR (G1α) Beta Bulges are components of Beta Bulge Loops; 0,1,2-βRαLβR (G1β) Beta Bulges are components of Beta Links.