Random-phase calculation of poly-L-proline II circular dichroism

Recent experiments have shown that the circular dichroism of poly-L -proline II is negative from 130 to 220 nm. Therefore, calculations based on exciton interactions alone cannot give the observed spectrum. The present calculations were carried out in the framework of the random-phase approximation. It includes exciton coupling of π-π* transitions. It also couples the π-π* transitions to the higher energy density of states (1) of the polymer chain via atomic polarizabilities, and (2) of bound solvent molecules. The relative importance of (1) and (2) is discussed in light of available experimental evidence. Qualitatively, (1) and (2) modify the excition-only results in the same way: the negative band near 200 nm shifts to longer wavelengths and increases in intensity; the crossover near 215 nm shifts to longer wavelengths; and the positive band near 225 nm shifts to longer wavelengths and decreases in intensity.