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Analysis of cooperativity and ion effects in the interaction of quinacrine with DNA
Author(s) -
Wilson W. David,
Lopp Irene G.
Publication year - 1979
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.1979.360181210
Subject(s) - cooperativity , chemistry , ionic strength , ligand (biochemistry) , cooperative binding , counterion condensation , binding constant , ionic bonding , crystallography , counterion , ion , equilibrium constant , binding site , computational chemistry , inorganic chemistry , aqueous solution , organic chemistry , receptor , biochemistry
The interaction of quinacrine with calf thymus DNA was monitored at several different ionic strengths using spectrophotometric and equilibrium dialysis techniques. The binding results can be explained, assuming each base pair is a potential binding site, using a model containing two negative cooperative effects: (1) ligand exclusion at binding sites adjacent to a filled binding site and (2) ligand–ligand negative cooperativity at adjacent filled binding sites. The logarithm of the observed equilibrium constant ( K obs ) determined by this model varies linearily with log[Na + ], as predicted by the ion condensation theory for polyelectrolytes. When the log K obs plot is correlated for sodium release by DNA in the intercalation conformational change, the predicted number of ion pairs between the ligand and DNA is approximately two, as expected for the quinacrine dication. Even though K obs depends strongly on ionic strength, the ligand negative cooperativity parameter ω was found to be indpendent of ionic strength within experimental error. This finding is also in agreement with the ion condensation theory, which predicts a relatively constant amount of condensed counterion on the DNA double helix over this ionic strength range. Drugs would, therefore, experience a relatively constant ionic environment when complexed to DNA even though the ionic conditions of the solvent could change considerably.