Terminal Sequence-Specific Interparticle Attraction between DNA Duplex-Carrying Polystyrene Microparticles in Aqueous Salt Solution Assessed by Optical Tweezers.

The dispersion behavior of DNA duplex-carrying colloidal particles in aqueous high-salt solutions shows extraordinary selectivity against the duplex terminal sequence. We investigated the interparticle force between DNA duplex-carrying polystyrene (dsDNA-PS) microparticles in aqueous salt solutions and examined their behavior in relation to the duplex terminal sequences. Force-distance () curves for a pair of dsDNA-PS particles were recorded with a dual-beam optical tweezers system with the two optically trapped particles closely approaching each other. Interestingly, only 3-5% of the oligo-DNA strands on the dsDNA-PS particles formed a duplex with complementary DNAs, and the curves showed a distinct specificity to the duplex terminal sequences in the interparticle force at a high-NaCl concentration; a clear attraction peak was observed in curves only when the duplex terminal was a complementary base pair. The attractive strength reached 2.6 ± 0.5 pN at 500 mM NaCl and 4.3 ± 1.0 pN at 750 mM NaCl. By sharp contrast, no significant attraction occurred for the particles with mismatched duplex terminals even at 750 mM NaCl. Similar duplex terminal-specificity in the interparticle force was also confirmed for dsDNA-PS particles in divalent MgCl solutions. Considering that the duplex terminal sequences on the dsDNA-PS particles showed only a negligible difference in their surface charges under identical salt conditions, we concluded that the interparticle attraction observed only for the dsDNA-PS particles with complementary duplex terminals is attributable to the salt-facilitated stacking interaction between the paired terminal nucleobases (i.e., blunt-end stacking) on the dsDNA-PS surfaces. Our results thus demonstrate the occurrence of a duplex terminal-specific interparticle force between dsDNA-PS particles under high-salt conditions.