Accumulation of fluorophores into DNA duplexes to mimic the properties of quantum dots.

By using perylene and pyrene as fluorophores, we have designed various fluorophore assemblies that mimic inorganic quantum dots in showing a high emission intensity, a large Stokes' shift, and a modulated emission maximum. For this purpose, we utilized two kinds of duplex motifs with D-threoninols as scaffolds: cluster and interstrand-wedged motifs. In the cluster motif, fluorophores are introduced into both strands to produce tentative pseudo-"base-pairs", in which the dyes strongly interact with each other and form dimers, trimers, or hexamers. In the interstrand-wedged motif, a base-pair is inserted between the fluorophores to suppress their direct interaction. These two motifs were applied to accumulate dyes within a DNA duplex, depending on their emission properties. Since pyrene exhibits strong excimer emission, the emission at 500 nm of a pyrene cluster motif strongly increased as the number of accumulated dyes increased, whereas the interstrand-wedged motif quenched pyrene monomer emission. In contrast, assembled perylenes, which are mostly quenched by dimerization, showed intense monomer emission in the interstrand-wedged motif, whereas perylene cluster motifs strongly suppressed perylene emission. These two motifs were then applied to the heteroassembly of pyrenes and perylenes. Both a large Stokes' shift and a modulation of the emission maximum, which are also characteristics of inorganic quantum dots, were successfully realized using fluorescent resonance energy transfer (FRET) and exciplex formation. These fluorophore assemblies thus obtained could be enzymatically ligated to longer DNA, demonstrating that this technique has the potential to be a versatile labeling agent for biomolecules.