14 DAY TRIAL // In the field of bioengineering, molecules such as DNA, RNA and most proteins count as polymers. This means that scientists have to know a few generally-valid rules in order to be able to design nanoparticles capable of carrying these molecules inside a cell. The creation of these “carriers” is however optional, as polymers can themselves pass through a cellular membrane, given enough time. Researchers at the Rice University have recently taken a huge leap forward in understanding how this phenomenon takes place.
Most cells are surrounded by a membrane, which acts as both a protective coating, and as a means for the larger structure to exchange information and chemicals with its environment. The membrane is riddled with holes that measure only nanometers across, and polymers – compounds made of identical, repeating units – are capable of passing through these “gates,” by employing a wriggling motion. But this takes time, according to a new model developed at Rice. Behind the work is a team led by associate professor of chemistry and of chemical and biomolecular engineering, Anatoly Kolomeisky.
He explains that the information he and his team discovered could come in handy to both bioengineers and experts working in developing high-quality chemical sensors. Designing such tools also requires the presence of a membrane that can be selectively permeated by certain substances, and so the behavior of polymers when they encounter such an obstacle must also be well understood. The new theoretical model is meant to provide an answer to all these questions. Details of the work were published in the July issue of the esteemed Journal of Chemical Physics.
“We assume the polymer is relatively large in comparison with the size of the pore, which is realistic. A typical strand of DNA could be a thousand nanometers long, and the pore could have a length of a few nanometer,” explains Kolomeisky. He adds that polymers pass through pores in a start-stop-repeat sequence. They don't rush through even when they meet a larger opening. Additionally, it was also discovered that the polymers tend to back out of pores at certain time, reconfiguring themselves in the process.
“Previous theorists thought that as soon as the leading end reached the channel, the whole polymer would go through. We're saying it goes back and forth many times before it finally passes,” the team leader adds, saying the electricity can be used to measure what is going on in the pore. “When the current is high, there's no polymer in the channel. When the current is down, it's in the pore and blocking the flux,” he says. The study reveals a polymer can take several milliseconds to pass through a nanoscale pore, which is a lot longer than first thought.