Single-stranded nucleic acids can adopt structures more complex than
simple stem-loops through the interaction of more widely separated bases.
Often, three or more bases may interact to stabilize these structures. In such
cases, hydrogen-bond donors and acceptors that do not participate in
Watson-Crick base pairs may participate in hydrogen bonds of nonstandard
pairings. Metal ions such as magnesium ion (Mg2) often assist in the stabilization of these more elaborate structures.