A new law to accurately measure charged macromolecules

October 24, 2016

AMHERST, Mass. - For biochemists, measuring the size and diffusion properties of large molecules such as proteins and DNA using dynamic light-scattering techniques and the Stokes-Einstein formula has been mostly straightforward for decades, except for one major snag - it doesn't work when these macromolecules carry an electric charge.

Now polymer theorist Murugappan Muthukumar at the University of Massachusetts Amherst has derived a solution to the 40-year dilemma, proposing a new theory that is allowing polymer chemists, engineers and biochemists for the first time to successfully apply the Stokes-Einstein law governing situations that involve charged macromolecules. Details appear in the current early online edition of Proceedings of the National Academy of Sciences.

As Muthukumar explains, "The ability of molecules to diffuse becomes smaller as the molecule's size gets larger, but for charged molecules, it's not true, diffusion doesn't depend on size. This was very surprising to physicists and biochemists 40 years ago when they were trying to measure charged macromolecules using light scattering. They also found that molecules of the same charge were aggregating, or clumping when they should repel each other. It was very surprising and nobody understood why."

Further, experiments showed that when the repulsion between similarly-charged molecules is made weaker by adding salt to the solution, the clumps went away, he says. "People were mystified by not being able to measure the size of these molecules accurately, and by their unusual behavior."

After a long process of eliminating possible explanations, he now understands what is happening. "It turns out that these molecules are not alone, there are small ions all around them, neutralizing the charges of the macromolecules," Muthukumar says. "These small ions are more agile and control the behavior of the macromolecules."

His paper offers formulae and testable predictions of a new theory or law governing charged macromolecules, DNA, proteins and synthetic poly-electrolytes. Experimental polymer scientists are already testing the new ideas in current investigations.

Muthukumar says this solution took him ten years to work out. "I began by simply believing the experimental facts and accepting that there must be an explanation. I started by taking a walk and asking myself, how could this be?"

As the theorist approached experimentalists with his ideas for solving the conundrum over the years, each had an objection that Muthukumar had to overcome. Finally, he reached the ion solution and heard no protest. "They have to be there," he now says. "The whole system has to be electrically neutral, otherwise you'd have an instability, which does not happen. Now we know how much the small ions are contributing. Using my formula, size of charged macromolecules can now be accurately determined using light scattering."
Muthukumar, the Wilmer D. Barrett Professor of polymer science and engineering at UMass Amherst, was selected last month to receive the 2017 American Chemical Society's Award in Polymer Chemistry. He has received many other awards including the Polymer Physics Prize of the American Physical Society and the Humboldt Research Award. Colleagues have called him one of the leading polymer theorists in the world today.

University of Massachusetts at Amherst

Related Molecules Articles from Brightsurf:

Finally, a way to see molecules 'wobble'
Researchers at the University of Rochester and the Fresnel Institute in France have found a way to visualize those molecules in even greater detail, showing their position and orientation in 3D, and even how they wobble and oscillate.

Water molecules are gold for nanocatalysis
Nanocatalysts made of gold nanoparticles dispersed on metal oxides are very promising for the industrial, selective oxidation of compounds, including alcohols, into valuable chemicals.

Water molecules dance in three
An international team of scientists has been able to shed new light on the properties of water at the molecular level.

How molecules self-assemble into superstructures
Most technical functional units are built bit by bit according to a well-designed construction plan.

Breaking down stubborn molecules
Seawater is more than just saltwater. The ocean is a veritable soup of chemicals.

Shaping the rings of molecules
Canadian chemists discover a natural process to control the shape of 'macrocycles,' molecules of large rings of atoms, for use in pharmaceuticals and electronics.

The mysterious movement of water molecules
Water is all around us and essential for life. Nevertheless, research into its behaviour at the atomic level -- above all how it interacts with surfaces -- is thin on the ground.

Spectroscopy: A fine sense for molecules
Scientists at the Laboratory for Attosecond Physics have developed a unique laser technology for the analysis of the molecular composition of biological samples.

Looking at the good vibes of molecules
Label-free dynamic detection of biomolecules is a major challenge in live-cell microscopy.

Colliding molecules and antiparticles
A study by Marcos Barp and Felipe Arretche from Brazil published in EPJ D shows a model of the interaction between positrons and simple molecules that is in good agreement with experimental results.

Read More: Molecules News and Molecules Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.