Origin of low-frequency motions in biological macromolecules. A view of recent progress in the quasi-continuity model

The recent progress in the quasi-continuity model and its applications in studying the low-frequency internal motions of biological macromolecules have been surveyed. Emphasis is placed on revealing the origin of this kind of internal collective motion, which involves many atoms and has significant biological functions. In light of such a line, the low-frequency motions in alpha-helix structure, beta-structure (including beta-sheet and beta-barrel), and DNA double-helix structure, the three most fundamental component elements in biological macromolecules, are discussed, and the corresponding physical pictures described. It turns out that the low-frequency motion in biological macromolecules originates from their two common intrinsic characteristics, i.e., they possess a series of weak bonds, such as hydrogen bonds and salt bridges, and a substantial mass distributed over the region containing those weak bonds.