D. Macromolecules Optical rotation in helical polymers

Our knowledge of the circular dichroism of helical molecules like polypeptides and nucleic acids has advanced enormously in the last ten years, yet the theoretical problem of calculating the optical rotation of molecules with several coupled chromophores has a long history, going back to the beginning of this century. It starts when Oseen (1915) and Born (1933) developed the concept of coupled elec­tronic oscillators in great detail to explain the propagation of light in crystals. After the development of the quantum theory by Rosenfeld (1928) and Kuhn (1932), chemists tended to stress the role of single electrons making transitions in an asymmetric environment as the main source of optical activity (Condon, Altar & Eyring 1937; Condon 1937). But the subject continued to develop (Boys 1934) and the next important landmark is Kirkwood’s famous quantum mechanical theory of coupled chromophores (Kirkwood 1937). He showed how the Coulomb interaction between electronic transitions in neighbouring, but non-overlapping, parts of a molecule gave rise to new terms in the rotatory dispersion, which were proportional to the electric polarizabilities of the groups. Kirkwood’s final formulae were remarkably like those of the old classical theory. Now there is a pause of nearly twenty years until Moffitt (1956 a, b ) and Kirk­wood’s (Fitts & Kirkwood 1956, 1957) brilliant work revived interest in the optical rotation of polymers. His theory laid a sure foundation for all the advances which have been made since his time and came at a most opportune moment.