Catalytic Motors

Research on nanoscale robots is opening a completely new trend in the way we look at the world, and has derived interest from various quarters of science and technology.  If the robots at nanoscale do their job as desired, their potential and utility are unbound. One of the major hurdles towards such a design is the interaction of the robots with its local environment. It is well know that when things shrink to the nanoscale, the Brownian motion plays a vital role in the movement of such nanoscale object, and an ‘extra’ amount of force has to be spent by the object to overcome the ‘random walk’. At these scales, Brownian motion makes it all but impossible to keep a steady direction of motion while immersed in a fluid. In fact, all molecular-scale motors in nature—including muscle proteins and the enzymes that produce ATP—are either constrained to run along a track or embedded in a membrane due to this. But this does not discourage us from building motors at nanoscale. In fact, there are now many smart methods to overcome the above disadvantage.

One such method is the Catalytic motor. The idea behind this is very simple: create a chemical reaction at the surface of an object which is coated with a catalyst. When the reaction takes place, the product will escape the surface, which further propels the object in the opposite direction. Let’s see an example of this.

In 2001, Rustem Ismagilov (now at Chicago) and George Whitesides, both then at Harvard University found that centimeter-scale “boats” with catalytic platinum strips on their stern would spontaneously move on the surface of a tank of water and hydrogen peroxide ( H₂O₂). The platinum promotes the breakup of H₂O₂ into oxygen and water, and bubbles of oxygen formed that seemed to push the boats ahead by recoil, the way the exhaust coming out the back of a rocket gives it forward thrust.

Wonderful !.....isn’t it….now the question is how do you bring this down to nanoscale. Ayusman Sen and coworkers at Penn State University came up with a smart idea: Their miniaturized version of the Harvard engine was a gold-platinum rod about as long as a bacterial cell (two microns) and half as wide (350 nanometers). The rods were mixed into the solution, rather than floating on the surface. Like the ATP-powered molecular motors inside the cell, these tiny catalytic cylinders were essentially immersed in their own fuel. And they did indeed move autonomously, at speeds of tens of microns per second ! But the reason why they moved was different from the Harvard engine. The way these nanorods actually work is that they apply a continuous force to prevail over the drag with no need for gliding. At the platinum end, each H₂O₂ molecule is broken down into an oxygen molecule, two electrons and two protons. At the gold end, electrons and protons combine with each H₂O₂ molecule to produce two water molecules. These reactions generate an excess of protons at one end of the rod and a dearth of protons at the other end; consequently, the protons must move from platinum to gold along the surface of the rod. Like all positive ions in water, protons attract the negatively charged regions of water molecules and thus drag water molecules along as they move, propelling the rod in the opposite direction as dictated by Newton’s law of motion that every action has an equal and opposite reaction. And thus a catalytic motor works…..still there are plenty of things to venture, and am sure more progress will be made in this field in coming years.

Every time I come across these nano-excitements in science, a quote by Feynman always resonates in my mind “There is plenty of room at the bottom”, and what a prophetic statement it has turned out to be !

Jellyfish, GFP and Douglas Prasher

Nobel Prize in Chemistry for the year 2008 was awarded "for the discovery and development of the green fluorescent protein, GFP". Three scientists shared this prize: Osamu Shimomura, Martin Chalfie and Roger Tsien, and I was awestruck to learn the stories behind their discoveries.  

Now, coming to the molecule, GFP is a fluorescent protein with it's emission maxima at 509nm, and has been one of the vastly used visual markers in molecular biology today. 

Shimomura was one of the earliest to discover the significance of GFP when he was studying the bioluminescence property of jellyfish. Many of us would have observed a spectacular image of a glowing jellyfish, which is due to the emission of a specific kind of a protein expressed in these organisms. When Shimomura isolated the protein from the jellyfish, to his surprise, he observed that the proteins emitted blue light instead of green. Further studies showed that jellyfish contains another protein which absorbed the blue light and emitted green light, which led to its bioluminescence. This phenomenon is nothing but Forster's Resonance Energy Transfer (FRET), and I was trilled to know that jellyfish too makes use of it !

  In 1988, Chalfie heard about GFP, and realized that it can be harnessed for in vivo bioimaging. He further came up with molecular biology methods to introduce  GFP gene into the DNA of a small worm called C. elegans. His methods showed self expression of GFP by cells, and led to it's usage in imaging various organelles and organisms. 

    The real mechanism of the fluorescence emission was unveiled by Tsien. He showed a one to one correspondence with the structure and emission of GFPs. He further tweaked the structure of GFP to vary the emission maxima of the fluorescence, and thus engineered the emission mechanism. In time, his group also added further fluorescent molecules from other natural sources to the tag collection, which continues to expand.

  I like to mention another key person who was involved in this discovery - Douglas C. Prasher. In fact, Prasher was the first to clone and sequence the gene of GFP, but unfortunately, he lost his tenure as a professor and could not continue his research on GFP. It was sad to know that he is now a courtesy shuttle-bus driver. It highlights that a so-called 'good system' can still err in making the right choice. 

    To conclude, in today's molecular biology, one cannot imagine the absence of fluorescent markers. It has now become integral part of biological research, and has led to deeper insights in understanding biology at the molecular scale. This prize was a celebration for basic science, and signifies the importance of analytical methods, and shows that revealing secrets of nature always leads to enlightenment.

Seeing minus infinity…almost!

On 23^rd April 2009, NASA’s SWIFT satellite recorded the farthest star burst ever in the history of astronomy. The observation with acronym GRB 090423 (GRB is gamma ray burst) recorded a red shift of 8.2, which corresponds to an event in the universe as early as 630 million years since big bang. The accompanying picture is from Gemini North Telescope in Hawaii, USA. On the electromagnetic spectrum, gamma rays are at the blue end, which makes them most energetic radiation. The gamma ray bursts essentially occur due to collapse of a massive star at a distant galaxy to form a black hole. Tremendous amount of energy is released during this event which acts a window to decipher many of the puzzles of early universe. These bursts are also dubbed as novas and supernovas depending on their intensity. One of the spectacular aspects of these bursts is the relativistic jets along the axis of the rotation of the collapsing star. These jets are collimated emission of radiation, which arise due to the frictional collapse of matter towards the center of the black hole. It has been estimated that energy as much as 10⁴⁴ J is released during this process. However, the mechanism of the collimated emission is still under debate in astrophysics. The initial burst of gamma rays is followed by an afterglow of other electromagnetic radiation like x-rays, UV, visible etc. which unveil a wealth of information about the collapsing stars in other galaxies. It is indeed a wonder that what we see in these burst is not only an event which is very, very far from us, but also something which happend when the universe was a mere one-twentieth of its current age. Well, for me, this is almost as good as seeing minus infinity !