Evidence for highly cooperative binding between molecular umbrella-spermine conjugates and DNA

Double- and tetrawalled molecular umbrella-spermine conjugates (I and II) have been synthesized, and their binding to calf thymus DNA (CT-DNA), poly[d(AT)], and poly[d(GC)] compared with that of a single-walled analogue (III). At moderate salt concentrations (8 mM NaCl), I and II show significantly greater affinity toward each DNA, relative to III; at high salt concentrations (150 mM NaCl), strong binding of I and II (but not III) was maintained toward poly[d(GC)]. Examination of the influence of I-III on the melting behavior of poly[d(AT)] has provided strong evidence that the binding of I and II reflects highly cooperative interactions among DNA-bound conjugates and that the DNA duplex serves as a nucleation site for umbrella aggregation. The implications of these findings for the rational design of novel drug conjugates that operate at the nuclear level, and also novel transfection agents, are briefly discussed.