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DNA strands are built like chains, with each link comprising one of four molecular bases whose sequence codes the genetic instructions for our cells. Like metal chains, DNA strands can easily change shape, bending, curling and wiggling around as they collide with other molecules around them.
All of this bending and wiggling can disrupt the ability of the electrons to travel like waves. Previously, it was believed that the electrons could only be shared over at most three bases.
Using computer simulations, the Beratan team found that certain sequences of bases could enhance the electron sharing, leading to wave-like behavior over long distances. In particular, they found that stacking alternating series of five guanine (G) bases created the best electrical conductivity.
The team theorizes that creating these blocks of G bases causes them to all “lock” together so the wave-like behavior of the electrons is less likely to be disrupted by the random motions of the DNA strand.
“We can think of the bases being effectively linked together so they all move as one. This helps the electron be shared within the blocks,” Liu said.
Next, the Tao group carried out conductivity experiments on short, six to 16 base strands of DNA, carrying alternating blocks of three to eight guanine bases. By tethering their test DNA between a pair of two gold electrodes, the team could flip on and control a small current to measure the amount of electrical charge flowing through the molecule.
They found that by varying a simple repeating “CxGx” pattern of DNA letters (x is the odd- or even-numbered G or C letters), there was an odd-even pattern in the ability of DNA to transport electrons. With an odd number, there was less resistance, and the electrons flowed faster and more freely (more wave-like) to blaze a path across the DNA.
They were able to exert precise molecular-level control and make the electrons hop (known as incoherent transport, the type found in most semiconductors) or flow faster (coherent transport, the type found in metals) based on variations in the DNA sequence pattern.
The experimental work confirmed the predictions of the theory.
Information charge
The results shed light on a long-standing controversy over the exact nature of electron transport in DNA and might provide insight into the design of DNA nanoelectrics and the role of electron transport in biological systems, Beratan said.
In addition to practical DNA-based electronic applications (for which the group has filed several patents), one of the more intriguing aspects is relating their work — done with short simple stretches of DNA — back to the complex biology of DNA thriving inside of every cell.
Of upmost importance to survival is maintaining the fidelity of DNA to pass along an exact copy of the DNA sequence every time a cell divides. Despite many redundant protection mechanisms in the cell, sometimes things go awry, causing disease. For example, absorbing too much UV light can mutate DNA and trigger skin cancer.
One of the DNA chemical letters, “G,” is the most susceptible to oxidative damage by losing an electron (think of rusting iron — a result of a similar oxidation process). Xiang points out that long stretches of Gs are also found on the ends of every chromosome, maintained by a special enzyme known as telomerase. Shortening of these G stretches has been associated with aging.
But for now, the research team has solved the riddle of how the DNA information influences the electrical charge.
“This theoretical framework shows us that the exact sequence of the DNA helps dictate whether electrons might travel like particles, and when they might travel like waves,” Beratan said. “You could say we are engineering the wave-like personality of the electron.”
The research was funded by a multimillion-dollar, multi-institute project under the support of the Department of Defense’s Multidisciplinary University Research Initiative (MURI) program, aimed at aiding high-priority basic science that could lead to innovative advancements.
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