Shivprasad Patil and Surya Deopa with special, home-built interferometer based Atomic Force Microscope. Credit: Suyambulingam

An interferometer-based atomic force microscope has revealed the viscosity and elasticity of a single molecule of a folded giant muscle protein called titin1.

The study could help understand and monitor small changes in protein structure and understand at the molecular level how some proteins manage multiple functions, say researchers at the Indian Institute of Science Education and Research in Pune.

Direct measurement of viscoelasticity of a single protein molecule has not been possible so far. So scientists developed a special fibre interferometer-based atomic force microscope and used it to measure the viscous and elastic components of a titin molecule. They subjected it to an oscillatory load which was about 10 billion times smaller than one gram. It produced an oscillatory deformation of one billionth of a centimetre which is much smaller than the diameter of a hydrogen atom.

The team, which included physicists, Shivprasad Patil and Surya Pratap S. Deopa, applied force on a single molecule of the protein to measure its stiffness and internal friction. Beyond a threshold force, it moves from an elastic solid-like native state to a viscoelastic mechanical intermediate, which dissipates energy. Such a change of state provides first direct evidence of internal friction and indicates how dynamic the protein is.

This technique could also be used to produce a data repository of other proteins by making a direct connection between their flexibility and activity, the researchers say.