2.1 Brownian motion  (Page 5/5)

Within the cells of the body, intracellular microtubule-based movement is directed by the proteins kinesin anddynein. The long-accepted explanation for this transport action is that the kinesins, fueled by energy provided by ATP, use their twoappendage-like globular heads to “walk” deliberately along the lengths of the microtubule paths to which they are attached.Kinesin, as a motor protein, has conventionally been understood to allow for the movement of objects within cells by harnessing theenergy released from either the breaking of chemical bonds or the energy released across a membrane in an electrochemical gradient.The kinesin proteins thus were believed to function as cellular “tow trucks” that pull chemicals or enzymes along these microtubulepathways.

New research, however, posits that what appeared to be a deliberate towing action along the microtubules isactually a result of random motion controlled by ATP-directed chemical switching commands. It is now argued that kinesins utilizerectified Brownian motion (converting this random motion into a purposeful unidirectional one).

We begin with a kinesin protein with both of its globular heads chemically bound to a microtubule path.According to the traditional power stroke model for motor proteins, the energy from ATP hydrolysis provides the impetus to trigger achemo-mechanical energy conversion, but according to the rectified Brownian motion model, the energy released by ATP hydrolysis causesan irreversible conformational switch in the ATP binding protein, which in turn results in the release of one of the motor proteinheads from its microtubule track. Microtubules are composed of fibrous proteins and include sites approximately 8 nm apart wherekinesin heads can bind chemically. This new model suggests that the unbound kinesin head, which is usually 5-7 nm in diameter, is movedabout randomly because of Brownian motion in the cellular fluid until it by chance encounters a new site to which it can bind.Because of the structural limits in the kinesin and because of the spacing of the binding sites on the microtubules, the moving headcan only reach one possible binding site – that which is located 8 nm beyond the bound head that is still attached to the microtubule.Thus, rectified Brownian motion can only result in moving the kinesin and its cargo 8 nm in one direction along the length of themicrotubule. Once the floating head binds to the new site, the process begins again with the original two heads in interchangedpositions. The mechanism by which random Brownian motion results in movement in only one pre-determined direction is commonly referredto a Brownian ratchet.

Ordinarily, Brownian motion is not considered to be purposeful or directional on account of its sheer randomness.Randomness is generally inefficient, and though in this case only one binding site is possible, the kinesin head can be likened toencounter that binding site by “trial and error.” For this reason, Brownian motion is normally thought of as a fairly slow process;however, on the nanometer scale, Brownian motion appears to be carried out at a very rapid rate. In spite of its randomness,Brownian motion at the nanometer scale allows for rapid exploration of all possible outcomes.

Brownian motion and nanotechnology

As things turn out, Brownian motion is one of the main controlling factors in the realm of nanotechnology. When one hears about the concept ofnanotechnology, tiny robots resembling scaled down R2D2-style miniatures of the larger ones most likely come to mind.Unfortunately, creating nano-scale machines will not be this easy. The nano-ships that are shrunk down to carry passengers through thehuman bloodstream in Asimov’s Fantastic Voyage, for example, would due to Brownian motion be tumultuously bumped around and flexed bythe molecules in the liquid component of blood. If, miraculously, the forces of Brownian motion did not break the Van der Waals bondsmaintaining the structure of the vessel to begin with, they would certainly make for a bumpy voyage, at the least.

Eric Drexler’s vision of rigid nano-factories creating nano-scale machines atom by atom seems amazing. While itmay eventually be possible, these rigid, scaled-down versions of macro factories are currently up against two problems: surfaceforces, which cause the individual parts to bind up and stick together, and Brownian motion, which causes the machines to bejostled randomly and uncontrollably like the nano-ships of science fiction.

As a consequence, it would seem that a basic scaling down of the machines and robots of the macro world will notsuffice in the nano world. Does this spell the end for nanotechnology? Of course not. Nature has already proven that thisrealm can be conquered. Many organisms rely on some of the properties of the nano world to perform necessary tasks, as manyscientists now believe that motor proteins such as kinesins in cells rely on rectified Brownian motion for propulsion by means ofa Brownian ratchet. The Brownian ratchet model proves that there are ways of using Brownian motion to our advantage.

Brownian motion is not only be used for productive motion; it can also be harnessed to aid biomolecularself-assembly, also referred to as Brownian assembly. The fundamental advantage of Brownian assembly is that motion isprovided in essence for free. No motors or external conveyance are required to move parts because they are moved spontaneously bythermal agitation. Ribosomes are an example of a self-assembling entity in the natural biological world. Another example of Brownianassembly occurs when two single strands of DNA self-assemble into their characteristic double helix. Provided simply that therequired molecular building blocks such as nucleic acids, proteins, and phospholipids are present in a given environment, Brownianassembly will eventually take care of the rest. All of the components fit together like a lock and key, so with Brownianmotion, each piece will randomly but predictably match up with another until self-assembly is complete.

Brownian assembly is already being used to create nano-particles, such as buckyballs. Most scientists viewthis type of assembly to be the most promising for future nano-scale creations.