Complexes of the antitumoral drugs Doxorubicin and Sabarubicin with telomeric G-quadruplex in basket conformation: ground and excited state properties

Abstract

We studied the binding of two anthracycline drugs, Doxorubicin and Sabarubicin, to a model telomeric sequence 5'-d[GGG(TTAGGG)(3)]-3' (21-mer), assuming the basket G-quadruplex (G4) conformation in Na(+)-rich aqueous solution. We used an approach that combines spectroscopic and microcalorimetric techniques to obtain information about ground and excited state properties of the most stable complexes. Both drugs bind to the 21-mer in basket conformation and complexes of 1:1 and 2:1 drug : 21-mer stoichiometry coexist in solution. Binding constants were determined from fluorescence and isothermal titration calorimetry experiments. For both drugs association is driven by enthalpy and disfavoured by entropy in the case of two sequential binding events to different sites. The drug fluorescence is completely quenched in the 1:1 complex, most likely by electron transfer from the guanine system to the anthraquinone moiety, while part of the emission survives in the 2:1 complex. Circular dichroism (CD) of the individual complexes is dominated by the G-quadruplex signal in the UV and by the anthracycline signal in the near-UV and Vis region. The experimental CD spectra combined with conformational calculations at MM level and quantum mechanical calculation of the rotational strength of the electronic transitions afforded information on the binding geometries.

Ultrafast Electron Transfer in Complexes of Doxorubicin with Human Telomeric G‐Quadruplexes and GC Duplexes Probed by Femtosecond Fluorescence Spectroscopy

“…CD spectroscopy has shown that DOX molecules bind Tel21 in their monomeric form. Typically, fluorescence titration yields two binding constants, log K 1 =5.6±0.3 and log K 21 =11.2±0.7; these values correspond to the formation of 1:1 and 2:1 DOX–Tel21 complexes, respectively . For comparison, we estimated the binding constant of DOX to d(GC) 10 duplexes by steady‐state absorption spectroscopy, under the same buffer conditions.…”

“…A previous study of DOX–Tel21 complexes by steady‐state spectroscopy combined with micro‐calorimetric techniques and molecular mechanics calculations has suggested the existence of two independent binding sites (BSs; see Scheme S1 in the Supporting Information). The first one (BS1), which is found in the two stoichiometries, results from the interaction of DOX with the bottom of the G4 structure; the presence of the diagonal loop precludes direct π–π stacking on the external guanine tetrads . Such an arrangement is expected to be favourable to electron transfer due to the quasi‐parallel orientation of DOX relative to the guanine tetrads.…”

“…However, it has been observed that doxorubicin (DOX; Scheme ), which is a natural anthracycline compound, can also cause telomere dysfunction for some cellular lines; this effect might arise from the ability of the drug to interact with telomeric G4 . In this respect, it was shown that DOX can form complexes with the human telomeric G4 sequence, 5′‐d[GGG(TTAGGG) 3 ]‐3′ (Tel21), with a comparable affinity to that for B‐DNA ,…”

“…In this context, we undertook a comparative study of photoinduced electron‐transfer reactions in the complexes of DOX with the G4 sequence, Tel21, and the duplex sequence, 5′‐d(GC) 10 ‐3′, in the presence of 50 mol L −1 NaCl. Under these conditions, Tel21 is known to adopt a well‐defined anti‐parallel G4 basket conformation, as illustrated in Scheme . Photoinduced electron transfer in DNA is known to induce DNA damage and has potential use for the development of new anti‐tumour therapies.…”

“… Schematic representation of the folding topology of Tel21 in aqueous solution with Na + (red circles) . Blue squares indicate guanines in the syn conformation, and dark blue squares denote guanines in the anti conformation.…”