Supramolecular chemistry at the single-molecule level

The characterisation of rough particle contacts by atomic force microscopy

An Atomic Force Microscopy (AFM) reverse imaging technique has been used to determine the contact zone topography of glass and UO3 particles in contact with flat mica substrates. A method is proposed that uses this topography to determine an effective asperity radius of curvature for the contacting particle. Application of the method has been found to be consistent with established contact mechanics models, for both glass and UO3 particle probes that present significantly different surface roughness. The method proposed is straightforward to apply and offers a greater insight into the influence of particle micro- and nano-roughness on adhesion. This is important for applications that rely on the control of granular flow such as pellet or tablet manufacture.

Theoretical analysis of single-molecule force spectroscopy experiments: heterogeneity of chemical bonds

We show that the standard theoretical framework in single-molecule force spectroscopy has to be extended to consistently describe the experimental findings. The basic amendment is to take into account heterogeneity of the chemical bonds via random variations of the force-dependent dissociation rates. This results in a very good agreement between theory and rupture data from several different experiments.

Probing DNA–peptide interaction forces at the single-molecule level

The versatility of chemical peptide synthesis combined with the high sensitivity of AFM single-molecule force spectroscopy allows us to investigate, quantify, and control molecular recognition processes (molecular nanotechnology), offering a tremendous potential in chemical biology.Single-molecule force spectroscopy experiments are able to detect fast intermediate transition states, details of the energy landscape, and structural changes, while avoiding multiple binding events that can occur under ensemble conditions. Dynamic force spectroscopy (DFS) is even able to provide data on the complex lifetime. This minireview outlines the biophysical methodology, discusses different experimental set-ups, and presents representative results in the form of two case studies, both dealing with DNA-binding peptides. They may serve as model systems, e.g., for transcription factors or gene transfection agents.

Heteropolymetallic Supramolecular Solid-State Architectures Constructed from Dicyanoaurate Ion, Phen, and 3d Metals

Two new supramolecular polymeric coordination complexes, {Zn(phen)2[Au2(CN)4](H2O)}·1.5H2O 1 and {Mn(phen)[Au2(CN)4](H2O)}·PriOH 2, have been obtained through a self-assembly process. The three-dimensional supramolecular architectures of compounds 1 and 2 are sustained by aurophilic, hydrogen-bonding, and/or π–π interactions. Gold atoms linked with aurophilic bonds arrange in a pattern not frequently observed.

Single molecule recognition between cytochrome C 551 and gold-immobilized azurin by force spectroscopy

Recent developments in single molecule force spectroscopy have allowed investigating the interaction between two redox partners, Azurin and Cytochrome C 551. Azurin has been directly chemisorbed on a gold electrode whereas cytochrome c has been linked to the atomic force microscopy tip by means of a heterobifunctional flexible cross-linker. When recording force-distance cycles, molecular recognition events could be observed, displaying unbinding forces of approximately 95 pN for an applied loading rate of 10 nN/s. The specificity of molecular recognition was confirmed by the significant decrease of unbinding probability observed in control block experiments performed adding free azurin solution in the fluid cell. In addition, the complex dissociation kinetics has been here investigated by monitoring the unbinding forces as a function of the loading rate: the thermal off-rate was estimated to be approximately 14 s(-1), much higher than values commonly estimated for complexes more stable than electron transfer complexes. Results here discussed represent the first studies on molecular recognition between two redox partners by atomic force microscopy.