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  • Title: Spectrophotometric and kinetic studies of the binding of Ni2+, Co2+, and Mg2+ to poly(dG-dC).poly(dG-dC). Determination of the stoichiometry of the Ni2+-induced B--->Z transition.
    Author: Schoenknecht T, Diebler H.
    Journal: J Inorg Biochem; 1993 Jun; 50(4):283-98. PubMed ID: 8331344.
    Abstract:
    Data are reported for the binding of Ni2+, Co2+, and Mg2+ to the B-form of double-stranded poly(dG-dC) at ionic strength conditions I = 0.001 M, 0.01 M, and 0.1 M. The apparent binding constants for Ni2+ and Co2+ are about the same and are 2- to 3-fold higher than those for Mg2+. Kinetic studies indicate that Mg2+ binds to the polynucleotide mainly (or solely) as a mobile cloud (electrostatically, outer-sphere), whereas the transition metal ions undergo site binding (inner-sphere coordination) with poly(dG-dC). The kinetic data suggest that an Ni2+ ion coordinates to more than one binding site at the polynucleotide, presumably to G-N7 and a phosphate group. At low ionic strength conditions the addition of Ni2+ induces a B-->Z conformational transition in poly(dG-dC). As demonstrated by UV absorption and DC spectroscopy, the transition occurs at I = 0.001 M already when 3 x 10(-5) - 7 x 10(-5) M of Ni2+ are added to 8 x 10(-5) M (in monomeric units) of poly(dG-dC), and at I = 0.01 M between 2.5 x 10(-4) M and 4.5 x 10(-4) M of Ni2+. Using murexide as an indicator of the concentration of free Ni2+ ions, the amount of Ni2+ which is bound to the polynucleotide could be determined. At I = 0.001 M it was established that the B-->Z transition begins when 1 Ni2+ is bound coordinatively per four base pairs, and the transition is complete when 1 Ni2+ is bound coordinatively per three base pairs. It is this coordinated Ni2+ which induces the B-->Z transition.
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