Single-Molecule Measurements of Synthetic Molecular Machines at Work

Molecular motors are ubiquitous in nature and are essential in controlling and performing numerous biological functions. They are able to rectify random motion to generate directional force and carry out macroscopic tasks (Schliwa in Molecular motors. Wiley-VCH, Weinheim, 2003 [1]). This ability has inspired attempts to create synthetic machines exhibiting control over rotary or linear motion (Kinbara and Aida in Chem Rev 105:1377-1400, 2005 [2], Kay et al. in Angew Chem Int Ed 46:72-191, 2007 [3]). Whereas some examples of synthetic systems able to use biased Brownian motion to perform work and collectively induce movement of much larger objects have been reported (Kay et al. in Angew Chem Int Ed 46:72-191, 2007 [3], Browne and Feringa in Nat Nanotech 1:25-35, 2006 [4], Berna et al. in Nat Mater 4:704-710, 2005 [5]), seeing such a single molecule at work remains a major challenge. Some elegant experiments on single-molecule machines adsorbed on a surface, imaged and manipulated with the tip of a scanning tunnelling microscope have been realized (Grill et al. in Nat Nanotech 2:95-98, 2007 [6], Manzano et al. in Nat Mater 8:576-579, 2009 [7]). However, in this case, the molecule is adsorbed on the surface, and the experiments are done in ultra-high vacuum at low temperature, which can be a limitation for systems designed to perform work in solution at room temperature or even in physiological conditions. Here, we discuss how a single synthetic small machine at work can be addressed by AFM-based single-molecule force spectroscopy, a tool able to monitor mechanical forces with sub-nanometer resolution and which has been widely used to investigate molecular-level processes in macromolecules and biological machines (Bustamante et al. in Annu Rev Biochem 73:705-748, 2004 [8], Special Issue in Annu Rev Biochem 77:45-228, 2008 [9], Evans in Annu Rev Biophys Biomol Struct 30:105-128, 2001 [10], Liang and Fernandez in ACS Nano 3:1628-1645, 2009 [11], Puchner and Gaub in Curr Opin Struct Biol 19:605-614, 2009 [12]).