Protein fragment library

Proteins can adopt an exponential number of states when modeled discretely. Typically a protein’s conformations are represented as sets of dihedral angles, bond lengths, and bond angles between all bonded atoms. The most common simplification is to assume ideal bond lengths and bond angles. However this still leaves the phi-psi angles of the backbone, and up to four dihedral angles for each side chain, leading to a worst case complexity of k^6*n possible states of the protein, where n is the number of residues and k is the number of discrete states modeled for each dihedral angle.

In order to reduce the conformational space, one can use protein fragment libraries rather than explicitly model every dihedral angle.