Influence of ionic strength on the dichroism properties of polynucleosomal fibers.

Abstract

AbstractWe report electric‐dichroism and electron‐microscopic studies of chromatin fibers fixed by protein–protein crosslinking at salt concentrations ranging from 10 to 100 mM. The results confirm a progressive disorganization of the fiber as the salt concentration is lowered. The positive dichroism and large polarizability anisotropy characteristic of the 300‐Å diameter fiber found in 100 mM salt are replaced by negative dichroism and smaller effective polarizability anisotropy or dipole moment for samples fixed at lower salt concentration. We interpret the results in terms of segmental, field‐induced orientation of the disorganized structure which is present in low salt concentrations. We also observed a field‐induced absorbance decrease in chromatin fibers fixed at salt concentration at and below 100 mM. All three optical effects, namely overall orientation of the high‐salt fixed fiber, segmental orientation of the low‐salt fixed fiber, and field‐induced absorbance decrease, occur on roughly the same time scale, 20–100 μs for 50 nucleosome polynucleosomes. The polarizability anisotropy of fibers fixed in 100 mM salt was found to be proportional to the length of the fragment and to the reciprocal square root of the conductivity of the solution used for electric‐dichroism measurements. Addition of Mg2+ to the measurement buffer affected the dichroism amplitude of samples fixed below 100 mM salt but not those fixed at 100 mM salt. The results reinforce the need for caution in interpreting electric‐dichroism measurements on chromatin fibers because of possible field‐induced distortion effects.