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New Rule of Physics?
August 2, 2010   
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Researchers at the Polish Academy of Sciences’ Institute of Physical Chemistry in Warsaw have discovered what they say is a new rule of physics while seeking to explain why proteins move faster inside living cells than anyone expected.

Given that the viscosity of cytoplasm is almost a million times higher than that of water, one would expect proteins to move inside living cells as slow as molasses; however, the process is only slightly slower than in water, the Warsaw researchers say.

Since humans do not possess the sensory abilities to precisely determine changes in viscosity, rapeseed oil seems more or less as viscous as water, but in reality the viscosity of water is 400 times lower, according to the Warsaw researchers. Consequently, people often confuse viscosity with thickness. For example, a typical shampoo is 95 percent water and so shampoos are basically as thick as water, but due to their high viscosity, they spill over the palm very slowly.

Viscosity under scrutiny

Viscosity was first used as a physical parameter in the Navier-Stokes equations from the 19th century. The equations provide an accurate description of the flow of rivers or airstreams along aircraft wings. According to this description, viscosity does not depend on the size-scale and should be identical for an airplane in motion and a protein molecule traveling inside a cell nucleus. Meanwhile, measurements indicate otherwise. Back in the 1950s, experiments that employed ultracentrifuges to study sedimentation of small particles at high load factors revealed a surprising fact. It turned out that tiny particles could experience viscosity rates which were tens, if not hundreds of thousands of times lower than those experienced by macroscopic objects that were billions of times larger. The question of what caused that dramatic change in viscosity would remain unanswered for a long time.

Even though viscosity began to be studied by the likes of Newton, it remains a mysterious natural property to this day. Physicists understand what causes viscosity in gases. When two layers of gas are in motion in relation to each other, single molecules jump from one layer to the other resulting in collisions that slow down the movement of the gas. However, when gas thickens and turns into a fluid, molecules start interacting with one another and it is hard to identify the phenomena which are responsible for viscosity. As a result, for the time being researchers are still studying viscosity in the simplest real fluids made up of atoms of argon or other noble gases.

Size matters

The research team at the Institute, led by Prof. Robert Ho造st, showed recently that every hydrodynamic system has a fundamental length-scale at which a macroviscosity-to-nanoviscosity crossover occurs. The scale depends on the size of objects suspended in the fluid. For polymers, it is the size of a single polymer lump while in a suspension of viruses it is the length of a rod-shaped virus.

“When a polymer molecule is 10 nanometers in size, then each object larger than that and suspended in the polymer will experience macroscopic viscosity, while for all smaller objects this will be nanoviscosity,” says Ho造st. What is particularly interesting is that viscosity changes exponentially and so the change is abrupt around the fundamental length-scale. When the size of the traveling object gets reduced by 10 nanometers, the viscosity can change by up to five or six orders of magnitude, Ho造st says.

The discovery means that the currently used hydrodynamic equations with constant viscosity parameters will have to be reformulated.

The research project at the Institute of Physical Chemistry employed state-of-the-art measurement methods and devices such as a confocal microscope using fluorescence correlation spectroscopy (FCS - pictured below). This new technique enables researchers to use a laser focus to see how single proteins molecules behave in cubic micrometers of fluid.

The institute conducted the experiments for five years. For two years, the experiments were sponsored by the Unilever corporation of Britain, which was interested in using the findings to design shampoos and conditioners.

The research on nanoviscosity is of fundamental significance to science, because nanoviscosity affects the rate of diffusion and limits the speed of biochemical reactions inside living cells, the researchers say.

“It is no accident that proteins in cells, small proteins in particular, start aggregating into larger complexes only where a biochemical reaction is about to take place,” says Ho造st. “They have to, because a large complex would travel a million times slower than each of the proteins individually.”

The researchers hope their discovery will find application in industries where viscosity plays a key role in many biotechnological reactions. The findings are expected to be embraced by producers of shampoos and other cosmetics and the new rule of physics will also play an important role in nanodevice design.

“Scientists still do not fully understand phenomena that occur at such small scales,” Ho造st said. “But if we want to build nanomachines, we should gather as much information as possible about phenomena characteristic of the environment in which such machines are supposed to work.”
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