Divalent Metal Cations upon Coordination to the N7 of Purines Differentially Stabilize the PyPuPu DNA Triplex due to Unequal Hoogsteen-type Hydrogen Bond Enhancement

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Abstract
The PyPuPu triplexes consisting of CG*G triads are stabilized by alkaline earth cations (Ca2+, Mg2+) and transition metal cations (Mn2+, Co2+, Ni2+, Zn2+, Cd2+), while similar triplexes including TA*A triads are stabilized only by transition metal cations. We hypothesize that such a differential triplex stabilization by divalent metal cations can be the consequence of their coordination to the N7 of the third strand purines with concomitant polarization effects on the bases resulting in unequal Hoogsteen-type hydrogen bond enhancement.