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A unique binding cavity for divalent cations in the DNA–metal–chromomycin A 3 complex
Author(s) -
Itzhaki Laura,
Weinberger Sarah,
Livnah Nurit,
Berman Elisha
Publication year - 1990
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.360290303
Subject(s) - chemistry , ionic radius , divalent , metal , binding constant , inorganic chemistry , ionic bonding , stoichiometry , ionic strength , dna , binding site , crystallography , aqueous solution , ion , organic chemistry , biochemistry
Binding of chromomycin A 3 (CRA) to calf thymas DNA was investigated in the presence of divalent cations using visible absorption and 1 H‐nmr spectroscopies. An apparent equilibrium binding constant (∼ 10 11 M −1 ) was obtained from metal competition experiments using EDTA to remove the metal cation from the DNA–M–CRA (M: metal) complex. The large binding constant of the drug to DNA enabled us to obtain essentially complete complexation of CRA to the short homogeneous d(ATGCAT) 2 duplex using stoichiometric amounts of the metal cation. Large induced chemical shifts were observed in the 1 H‐nmr spectrum of the above complex using the paramagnetic Co 2+ cation indicating that the metal occupies a unique binding site. Since no induced 1 H‐nmr chemical shifts were observed for the drug–Co 2+ mixture, it was concluded that no metal–drug complex is formed. In addition, it was found that bound CRA is negatively charged at physiological pH and binding to the DNA could be affected only by using metal cations whose ionic radius size (< 0.85 Å) and charge (2+) were simultaneously satisfied. Stringent metal cation selectivity for the DNA–M–CRA complex may be intimately connected with the antitumor selectivity of CRA, since different types of cells generally possess widely differing molar concentrations of metal cations.