A structure of a biological molecule characterized by the local folding within the biopolymer as a result of hydrogen bonding (within the biopolymer).
In proteins, there are four recognized levels of protein structure: primary structure, secondary structure, tertiary structure, and quaternary structure.
A secondary structure of a protein pertains to the folding of a polypeptide chain, resulting in an alpha helix, beta sheet or a random coil structure.
Another example of a secondary structure is that of a nucleic acid such as the clover leaf structure of tRNA.
See also: protein structure