Selective
Binding of Zn<sup>2+</sup> Complexes to
Human Telomeric G‑Quadruplex DNA
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Abstract
The Zn<sup>2+</sup> complex of 5-(1,4,7,10-tetraazacyclododecan-1-ylsulfonyl)-<i>N</i>,<i>N</i>-dimethylnaphthalen-1-amine, Zn(DSC),
binds selectively to the biologically relevant human telomeric (H-Telo)
G-quadruplex. An increase in the Zn(DSC) dansyl group fluorescence
with a simultaneous shift in emission is consistent with the complex
binding to H-Telo. The H-Telo G-quadruplex has two binding sites for
Zn(DSC) with binding constants in the low micromolar range (2.5 μM).
Isothermal calorimetric titrations confirm low micromolar dissociation
constants with a 2:1 stoichiometry. The interaction between H-Telo
and Zn(DSC) is highly pH-dependent, consistent with binding to the
unpaired thymines in the G-quadruplex loops. As a result, Zn(DSC)
selectively binds to H-Telo over duplex DNA. In contrast to Zn<sup>2+</sup>, Fe<sup>2+</sup> and Co<sup>2+</sup> do not complex to the
DSC macrocycle appreciably under the conditions of the experiment.
The Cu<sup>2+</sup> complex of DSC does not interact measurably with
the H-Telo G-quadruplex. Interestingly, the H-Telo-Zn(DSC) adduct
self-assembles from its individual components at physiological pH
and 100 mM KCl. The self-assembly feature, which is specific for the
Zn<sup>2+</sup> ion, suggests that this system may be viable as a
Zn<sup>2+</sup> sensor. Pentanucleotides were studied in order to
better describe the binding of Zn(DSC) to thymine sequences. NMR studies
were consistent with the binding of Zn(DSC) to thymine-containing
oligonucleotides including CCTCC, CTTCC, and CTCTC. Studies showed
that the dansyl group of Zn(DSC) interacts with thymines in CTTCC.
Fluorescence spectroscopy and ITC data indicate that Zn(DSC) forms
2:1 adducts with thymines that are spaced (CTCTC) but not tandem thymines
(CTTCC). These data are consistent with one Zn(DSC) complex binding
to two separate loops in the G-quadruplex. A second Zn<sup>2+</sup> complex containing an acridine pendent, Zn(ACR), binds tightly to
pentanucleotides with both tandem and spaced thymines. Zn(ACR) indiscriminately
binds to both H-Telo and duplex DNA