Coralyne Binds Tightly to Both T·A·T- and C·G·C+-Containing DNA Triplexes

Abstract

Coralyne is a DNA-binding antitumor antibiotic whose structure contains four fused aromatic rings. The interaction of coralyne with the DNA triplexes poly(dT)·poly(dA)·poly(dT) and poly [d(TC)]·poly[d(GA)]-poly[d(C+T)] was investigated by using three techniques. First, Tm values were measured by thermal denaturation analysis. Upon binding coralyne, both triplexes showed Tm values that were increased more than those of the corresponding duplexes. A related drug, berberinium, in which one of the aromatic rings is partially saturated, gave much smaller changes in Tm. Second, the fluorescence of coralyne is quenched in the presence of DNA, allowing the measurement of binding parameters by Scatchard analysis. The binding isotherms were biphasic, which was interpreted in terms of strong intercalative binding and much weaker stacking interactions. In the presence of 2 mM Mg2+, the binding constants to poly(dT)·poly-(dA)-poly(dT) and poly[d(TC)]·poly[d(GA)]·poly[d(C+T)] were 3.5 × 106 M−1 and 1.5 × 106 M−1, respectively, while the affinity to the parent duplexes was at least 2 orders of magnitude lower. In the absence of 2 mM Mg2+, the binding constants to poly[d(TC)]·poly[d(GA)]·poly[d(C+T)] and poly-[d(TC)]·poly[d(GA)] were 40 · 106 M−1 and 15 × 106 M−1, respectively. Thus coralyne shows considerable preference for the triplex structure but little sequence specificity, unlike ethidium, which will only bind to poly(dT)·poly(dA)·poly(dT). Further evidence for intercalation of coralyne was provided by an increase in the relative fluorescence quantum yield at 260 nm upon binding of coralyne to triplexes as well as an absence of quenching of fluorescence in the presence of Fe[(CN)6]4−. Third, coralyne promoted intermolecular triplex formation between plasmids containing a pyr·pur tract and a single-stranded polypyrimidine. The ability of coralyne to bind to triplexes may relate to its in vivo activity and also suggests ways of designing triplex-binding drugs with sequence specificity. © 1993, American Chemical Society. All rights reserved.