Spectrin is a vital and abundant protein of the cytoskeleton. It has an elongated structure that is made by a chain of so-called spectrin repeats. Each repeat contains three antiparallel K K-helices that form a coiled-coil structure. Spectrin forms an oligomeric structure that is able to cross-link actin filaments. In red cells, the spectrin/actin meshwork underlying cell membrane is thought to be responsible for special elastic properties of the cell. In order to determine mechanical unfolding properties of the spectrin repeat, we have used single molecule force spectroscopy to study the states of unfolding of an engineered polymeric protein consisting of identical spectrin domains. We demonstrate that the unfolding of spectrin domains can occur in a stepwise fashion during stretching. The forceê xtension patterns exhibit features that are compatible with the existence of at least one intermediate between the folded and the completely unfolded conformation. Only those polypeptides that still contain multiple intact repeats display intermediates, indicating a stabilisation effect. Precise force spectroscopy measurements on single molecules using engineered protein constructs reveal states and transitions during the mechanical unfolding of spectrin. Single molecule force spectroscopy appears to open a new window for the analysis of transition probabilities between different conformational states. ß

Spectrin repeats are triple-helical coiled-coil domains found in many proteins that are regularly subjected to mechanical stress. We used atomic force microscopy technique and steered molecular dynamics simulations to study the behavior of a wild-type spectrin repeat and two mutants. The experiments indicate that spectrin repeats can form stable unfolding intermediates when subjected to external forces. In the simulations the unfolding proceeded via a variety of pathways. Stable intermediates were associated to kinking of the central helix close to a proline residue. A mutant stabilizing the central helix showed no intermediates in experiments, in agreement with simulation. Spectrin repeats may thus function as elastic elements, extendable to intermediate states at various lengths.

A BSTR ACTAtomic force microscopy in a forcespectroscopy mode has been used to investigate the kinetics of the adsorption process of fibrinogen molecules on a silica surface. An original ''approach͞retraction'' cycle of the tip͞ surface was used for this purpose. Fibrinogen molecules were adsorbed on the atomic force microscopy tip and were brought into contact with the silica surface for different interaction times varying from 5 to 2,000 ms. Multiple consecutive ruptures were observed. The mean number of ruptures n r per cycle increases steadily with the interaction time as well as the mean strength f r which varies from 300 pN for 5 ms to 1,400 pN for 2,000 ms. The minimal interaction time for a fibrinogen molecule to bind strongly to a silica surface during an adsorption process appears to lie between 50 and 200 ms. The histograms of the distances between two consecutive ruptures in one cycle exhibit maxima around 20-25 nm. This length is comparable to the characteristic distance between D and E globules of one fibrinogen molecule and suggests that fibrinogen molecules mainly adsorb through their D and E globules.

We present a scanning probe microscope based on optical tweezers for three-dimensional imaging of the topology of transparent material in the nanometer range. A spherical nanoparticle serves as a probe. An optical trap moves it through the sample ͑e.g., a polymer network͒, while the position of the particle center is recorded by three-dimensional interferometry. Accessible volumes are reconstructed from the histogram of thermal position fluctuations of the particle. The resolution in determining the position of surfaces in three dimensions is about 20 nm.

Elastic colloidal crystals, even without a full photonic band gap, hold promise for fascinating applications and for easy large-scale fabrication by self-assembly. However, high mechanical robustness is required for optical, decorative, or security applications, such as tunable optical modulators/filters or optical tension indicators. Here, we present brilliantly colored filled-pore colloidal crystals that withstand elongation by 100%, i.e., one optical octave. We employ a variety of vertical deposition techniques to self-assemble monodisperse core-shell polymer beads with a film-forming shell and flexible core. We find a good theoretical description of crystal thickness for all techniques. The crystals have centimeter-sized macroscopic order, and their orientation is fully controlled by the substrate plane and meniscus line.