Labs researchers observe molecular shuttling that mimics cellular behavior
Labs researchers recently created and then examined molecular movements that could evolve into some of the first useful tools at future nanoconstruction sites, where proteins might be shuttled from place to place in tiny chemical wheelbarrows or built upon molecular scaffolding.
Using improved observational methods, the Sandia team watched as huddled receptor — or grabber — molecules on a man-made cell membrane rapidly dispersed across the membrane when they latched onto free-floating ligands (chemical particles), then rehuddled when the ligands were removed.
The behavior mimics biological reactions at the cell level, such as immune system response to viral particles, says Darryl Sasaki of Biomolecular Materials & Interfaces Dept. 1140. The work is based on previous research at Sandia to create metal-detecting sensors based on chemical recognition events (www.sandia.gov/media/metal.htm).
The team’s observations are published as the cover story in the April 30 issue of the biweekly chemical and biophysics journal Langmuir, and work on a related system recently appeared in Biophysical Journal (November 2001).
For the experiment, the researchers created an artificial cell membrane made of "phospholipid bilayers" — rows of long molecules that, like empty pop bottles bobbing on water, self-organize into an orderly heads-up/tails-down formation.
They implanted this flat lipid film with specialized lipids carrying tall receptor headgroups — pincher- or lasso-shaped structures that chemically grab onto free-floating ligands. (See "Receptors team up to signal cellular response" on page 4.)
Then they watched as the receptors reacted to the addition of metal ions, not only for insights into cellular behavior but also for possible nanoscience advances such insights might offer.
At rest in solution the receptor-lipids pooled into aggregate zones between islands of receptor-less lipids.
But when metal ions (lead or copper) were added, the headgroups latched onto the ions, and ZIP!, the receptor-lipids dispersed evenly across the membrane surface as their newly acquired electrostatic charges caused them to become mutually repulsed.
When the metal ions were removed, the wayward receptor-lipids retraced their steps and regrouped into the same aggregated pools.
"When they bind to the metal, they each race away from their nearest neighbor," says Darryl. "When the ions are removed, they race back to where they were."
The process was performed repeatedly on the same membranes with the same result — reversible reorganization.
Darryl believes the trails the receptor-lipids follow and the pools they return to correspond, quite literally, to the paths of least resistance on the membrane’s surface — areas where the lipid film is more liquid than solid, allowing the traveling lipids to flow like water.
Tracking tiny travelers
Although producing such chemical recognition events on an artificial membrane is not an achievement in itself, examining them with such fidelity is, says Darryl. The Sandia team used novel microscopy and spectroscopic techniques to make the first documented observations of receptor-lipids repeatedly stepping out and then returning home.
Fluorescent pyrene tags were attached to the tails of the receptor-lipids to aid in tracking their travels on the membrane. When the receptors were aggregated — as seen using fluorescence spectroscopy — the huddles appeared bright. When the receptors were dispersed, their fluorescent signals were dim.
In addition, the team used an atomic force microscope to map the surface texture, or topography, of the lipid membrane, identifying locations of the tall receptor headgroups that towered 8 angstroms (about one billionth of a meter) higher than the tops of the membrane lipids.
These observations provided unprecedented clarity about the locations of the receptors in both the dispersed and aggregated states, says Darryl.
"We’ve been able to characterize films as they change their properties at both the macroscale and nanoscale," he says. "It’s the first time such a dynamic molecular system has been imaged this way."
As a result of the team’s work, he says, scientists will have a better understanding of chemical recognition on cell-like membrane systems.
Perhaps more tantalizing, he says, are the possibilities the new understandings might bring to the nanotechnology community’s growing toolbox.
"The idea of using chemical recognition to form specific structures in the membrane may be a potent tool to aid in the development of controllable nanoscale architectures," says Darryl.
If receptor headgroups propelled to and fro by chemical recognition events can be enlisted to hoist molecules and proteins and deposit them in planned locations, he says, designing and building nanosized structures, such as single-molecule-wide wires, might be possible.
And the receptor-lipids’ tendencies to follow preferred pathways offer promise for engineered construction of nano-railroad tracks along which a variety of molecular cargo could be recurringly moved, perhaps aboard motor-protein railcars, he says.
If nano-engineers can control these routes, two- or three-dimensional lipid scaffolds might be designed upon which proteins could be laid down to build nanoscale electronic or photonic circuits.
Nano-switching structures might be designed that self-construct and self-destruct based on chemical recognition events.
In addition, researchers have long sought to build cell-like pods that, when injected into a person’s blood stream, would recognize diseased cells and release a drug to destroy those cells selectively. Such a capability could revolutionize medical approaches to treating a variety of illnesses.
"By harnessing even a fraction of the capability of cellular membrane recognition systems, it may be possible to build unique sensor systems that are not only rapid and specific in response but also are innately biocompatible," adds Darryl.
Sandia team members include Tina Waggoner, Julie Last (both 1140), and Todd Alam (1811).