二級結構
Secondary structure is the general three-dimensional form of local segments of biopolymers such as proteins and nucleic acids (DNA, RNA). It does not, however, describe specific atomic positions in three-dimensional space, which are considered to be the tertiary structure. Secondary structure can be formally defined by the hydrogen bonds of the biopolymer, as observed in an atomic-resolution structure. In proteins, the secondary structure is defined by the patterns of hydrogen bond between backbone amino and carboxyl groups. Conversely, for nucleic acids, the secondary structure consists of the hydrogen bonding between the nitrogenous bases. The hydrogen bonding patterns may be significantly distorted, which makes automatic determination of secondary structure difficult. Efforts to use computers to predict protein secondary structures, based only on their given primary structure sequences, have been ongoing since the 1970s.