The nature of DNA have been intriguing chemists in the past ten years. In the view of chemistry, DNA molecule focuses a number of the most heated concepts: templated synthesis/polymerization, non-covalent interaction, molecular recognition, etc. Molecular motions induced by complementary complexation of DNA strands, in particular, attracts intensive interest among researchers for its potential to develop artificial molecular machines (for example, a molecular tweezers, Nature 2000, 406, 605-608). Now, ball-to-rod transition also induced by coupling of complementary DNA strands has been realized by A. Herrmann et al. (Angew. Chem. Int. Ed. 2007. DOI: 10.1002/anie.200603064.). They synthesized a DNA block copolymer containing a hydrophilic single-stranded (ss) DNA block and a hydrophobic (relatively) polypropyleneoxide (PPO) block. This amphiphilic block copolymer will self-assembly into spherical micelles in water, with the PPO segments converge into the core and the flexing ssDNA segments as the corona. Addition of another ssDNA with complementary sequence and equal length of that in the block copolymer results in the hybridized corona with double-stranded (ds) DNA (see the figure on right). But if the additional ssDNA has several repeating complementary sequences and is several times longer than the ssDNA segment in the block copolymer, the subsequent coupling of the complementary strands leads to the disassembly of the spherical micelles and re-assembly into a rod like structure, in which two long ssDNA serve as templates and complex with a determined number of DNA block copolymer, and the two dsDNA complexes are linked by hydrophobic interaction between their PPO segments. By varying the number of repeated sequence in the templated ssDNA, the length of the rod can be controlled easily, as depicted in the figure on right.
The transition is observable via scanning force microscopy (SFM), where the length, width of the rod as well as the width and depth of the central groove (the PPO protion) of the rod can be measured.
The coupling tendency between two complementary strands of DNA is very high because of multiple sites of hydrogen-bonding between the base pairs. This tendency can surpass the conformational energy, as shown in the DNA tweezer, and the hydrophobic interaction, as in the micelle of DNA block copolymer.
If combined with the concept of 'fuel-DNA' in the DNA machine (please refer to the paper of the DNA tweezer for detail), the ball-to-rod transition can even be performed reversibly. Will this idea be realized in the next paper of this research?
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