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Influence of ionic strength on the dichroism properties of polynucleosomal fibers
Author(s) -
Lee Keun Su,
Crothers Donald M.
Publication year - 1982
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.360210109
Subject(s) - chemistry , dichroism , absorbance , anisotropy , circular dichroism , ionic strength , polarizability , fiber , fluorescence anisotropy , analytical chemistry (journal) , linear dichroism , crystallography , salt (chemistry) , electric field , optics , aqueous solution , chromatography , molecule , membrane , organic chemistry , physics , biochemistry , quantum mechanics
We report electric‐dichroism and electron‐microscopic studies of chromatin fibers fixed by protein–protein crosslinking at salt concentrations ranging from 10 to 100 m M . 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 m M 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 m M . 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 m M 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 Mg 2+ to the measurement buffer affected the dichroism amplitude of samples fixed below 100 m M salt but not those fixed at 100 m M salt. The results reinforce the need for caution in interpreting electric‐dichroism measurements on chromatin fibers because of possible field‐induced distortion effects.