Differential scattering of circularly polarized light by chromatin modeled as a helical array of dielectric ellipsoids within the born approximation.

Abstract

AbstractTheoretical predictions within the Born approximation of the expected differential light scattering of circularly polarized light (CIDS) were made for the 300‐Å chromatin fiber, modeled as a helical array of dielectric ellipsoids. Computed CIDS values were strongly dependent on the exact geometry of the solenoid model, depending particularly on parameters relaed to the chiral nature of the fiber and the orientation of the nucleosomes within the helix, in contrast to the values of the total light scattering, which mainly probed size and shape. In particular, both a superbead model and a strict linear 110‐Å nucleosome filament would be predicted as giving rise to zero CIDS (in disagreement with the finite values observed). At the same time, helical models in which the normal vectors to the nucleosome faces were exactly parallel to the helical axis also yielded zero CIDS. Confirming earlier expectations, CIDS values were significantly less dependent on helical length than total light scattering. Finally, comparison of these calculated results from those extrapolated from available experimental data indicates that predicted CIDS values, based on currently accepted models of solenoid structure, are within an order of magnitude of those experimentally observed. Together, these results indicate the potential of differential light scattering measurements as a probe of chromatin higher‐order structure, complementary to existing scattering measurements.