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Helix–coil transition in nucleoprotein. Effect of ionic strength on thermal denaturation of polylysine–DNA complexes
Author(s) -
Li Hsueh Jei,
Brand Benjamin,
Rotter Arnold,
Chang Catherine,
Weiskope Manuel
Publication year - 1974
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.1974.360130813
Subject(s) - chemistry , polylysine , ionic strength , circular dichroism , crystallography , ionic bonding , intermolecular force , denaturation (fissile materials) , biophysics , molecule , aqueous solution , ion , organic chemistry , biology , nuclear chemistry
Thermal denaturation of direct‐mixed and reconstituted polylysine–DNA complexes in 2.5 × 10 −4 M EDTA, pH 8.0 and various concentrations of NaCl has been studied. For both complexes, increasing ionic strength of the solution raises T m , the melting temperature of free base pairs. The linear dependence of T m on log Na + indicates that the concept of electrostatic shielding on phosphate lattice of an infinitely long pure DNA by Na + can be applied to short free DNA segments in a nucleoprotein. For a direct‐mixed polylysine–DNA complex, the melting temperature of bound base pairs T m ′ remains constant at various ionic strengths. On the other hand, the T m ′ in a reconstituted polylysine–DNA complex is shifted to lower temperature at higher ionic strength. This phenomenon occurs for reconstituted complex with long polylysine of one thousand residues or short polylysine of one hundred residues. It is shown that such a decrease of T m ′ is not due to a reduction of coupling melting between free and bound regions in a complex when the ionic strength is raised. It is also not due to intermolecular or intramolecular change from a reconstituted to a direct‐mixed complex. It is suggested that this phenomenon is due to structural change on polylysine‐bound regions by ionic strength. It is suggested further that Na + may replace water molecules and bind polylysine‐bound regions in a reconstituted complex. Such a dehydration effect destabilizes these regions and lowers T m ′. This explanation is supported by circular dichroism (CD) results.