Single-molecule force spectroscopy has emerged as a powerful tool to investigate the forces and motions associated with biological molecules and enzymatic activity. The most common force spectroscopy techniques are optical tweezers, magnetic tweezers and atomic force microscopy. Here we describe these techniques and illustrate them with examples highlighting current capabilities and limitations.

Single-molecule force spectroscopy: optical tweezers, magnetic tweezers and atomic force microscopy

The shapes of eukaryotic cells and ultimately the organisms that they form are defined by cycles of mechanosensing, mechanotransduction and mechanoresponse Local sensing of force or geometry is transduced into biochemical signals that result in cell responses even for complex mechanical parameters such as substrate rigidity and cell-level form These responses regulate cell growth, differentiation, shape changes and cell death Recent tissue scaffolds that have been engineered at the micro- and nanoscale level now enable better dissection of the mechanosensing, transduction and response mechanisms

https://faculty.washington.edu/nsniadec/ME599/S09/private/25Vogel.pdf

Local force and geometry sensing regulate cell functions.

Recent advances in single-molecule detection and single-molecule spectroscopy at room temperature by laser-induced fluorescence offer new tools for the study of individual macromolecules under physiological conditions. These tools relay conformational states, conformational dynamics, and activity of single biological molecules to physical observables, unmasked by ensemble averaging. Distributions and time trajectories of these observables can therefore be measured during a reaction without the impossible need to synchronize all the molecules in the ensemble. The progress in applying these tools to biological studies with the use of fluorophores that are site-specifically attached to macromolecules is reviewed.

https://science.sciencemag.org/content/sci/283/5408/1676.full.pdf

Fluorescence spectroscopy of single biomolecules.

The field of magnetoelectronics has been growing in practical importance in recent years1 For example, devices that harness electronic spin—such as giant-magnetoresistive sensors and magnetoresistive memory cells—are now appearing on the market2 In contrast, magnetoelectronic devices based on spin-polarized transport in semiconductors are at a much earlier stage of development, largely because of the lack of an efficient means of injecting spin-polarized charge Much work has focused on the use of ferromagnetic metallic contacts3,4, but it has proved exceedingly difficult to demonstrate polarized spin injection More recently, two groups5,6 have reported successful spin injection from an NiFe contact, but the observed effects of the spin-polarized transport were quite small (resistance changes of less than 1%) Here we describe a different approach, in which the magnetic semiconductor BexMnyZn1-x-ySe is used as a spin aligner We achieve injection efficiencies of 90% spin-polarized current into a non-magnetic semiconductor device The device used in this case is a GaAs/AlGaAs light-emitting diode, and spin polarization is confirmed by the circular polarization state of the emitted light

Injection and detection of a spin-polarized current in a light-emitting diode

https://nanomechanics.mit.edu/sites/default/files/documents/2007_Acta_Bio_Cancer_S.Suresh.pdf

Biomechanics and biophysics of cancer cells.

The giant muscle protein titin, also called connectin, is responsible for the elasticity of relaxed striated muscle, as well as acting as the molecular scaffold for thick-filament formation. The titin molecule consists largely of tandem domains of the immunoglobulin and fibronectin-III types, together with specialized binding regions and a putative elastic region, the PEVK domain. We have done mechanical experiments on single molecules of titin to determine their visco-elastic properties, using an optical-tweezers technique. On a fast (0.1s) timescale titin is elastic and force-extension data can be fitted with standard random-coil polymer models, showing that there are two main sources of elasticity: one deriving from the entropy of straightening the molecule; the other consistent with extension of the polypeptide chain in the PEVK region. On a slower timescale and above a certain force threshold, the molecule displays stress-relaxation, which occurs in rapid steps of a few piconewtons, corresponding to yielding of internal structures by about 20 nm. This stress-relaxation probably derives from unfolding of immunoglobulin and fibronectin domains.

L. Tskhovrebova

Papers

Elasticity and unfolding of single molecules of the giant muscle protein titin.