06 April 2016

A nucleotide rectifier

The smallest diode in the world was made from DNA

Vladimir Korolev, N+1

The smallest molecular rectifier in the artist's view
U. Georgia/Ben-Gurion U 

An Israeli-American group of chemists has created the smallest diode rectifier of electric current. The device is a short fragment of DNA into which two foreign molecules are introduced (intercalated). Despite the symmetry of the DNA used, the conductivity for electric current differs in different directions by 15 times. The study was published in the journal Nature Chemistry (Guo et al., Molecular rectifier composed of DNA with high rectification ratio enabled by intercalation), a press release from the University of Georgia UGA researchers use a single molecule of DNA to create the world's smallest diode briefly reports about it.

The authors used an oligonucleotide consisting of 11 bases as the basis of the rectifier. At the same time, the end sections – four nucleotides each – are palindromes, in other words, complementary to themselves. The central part of this DNA consists of three adenine fragments, so in solution two such molecules can connect "tails", but not the middle part.

However, this situation changes if a small organic molecule is intercalated – introduced into a DNA molecule without forming strong chemical bonds. Chemists used coraline in their study, which is able to bind adenine-adenine pairs together. According to the authors, two molecules of the substance were embedded in the DNA. 

The scheme of the experiment. Oligonucleotide sequence: (5'-CGCGAAACGCG-3'-SH).
Cunlan Guo et al. / Nature Chemistry, 2016

After the formation of the complex, the scientists deposited the molecules on a gold substrate and carried out measurements using a scanning tunneling microscope. At the same time, chemists ensured that the molecules of the complexes were connected at one end to the substrate, and at the other end to the thin conductive needle of the microscope. Then the volt-ampere characteristic was measured. The scientists found that the current values at voltages of about 1.1 volts differ by 15 times, depending on which direction the voltage was applied. No such observation was observed for free DNA. 

Volt-ampere characteristic of DNA (blue) and DNA-coraline complex (red)
Cunlan Guo et al. / Nature Chemistry, 2016

In order to find out the cause of the observed phenomenon, the authors theoretically calculated the electronic structure of the molecule. It turned out that the main "culprits" were coraline molecules, causing a strong spatial asymmetry in the oligonucleotide. According to chemists, the study offers a new way to create molecular electronic components based on pre-modeled DNA complexes.

Earlier, a similar measurement technique allowed chemists to catalyze the reaction with an electric field for the first time. Instead of DNA, simpler organic molecules were used in the work, the rate of interaction between which was determined by the sign of the applied potential difference to the substrate and the needle.

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