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Individual molecules of the giant protein titin span the A-bands and I-bands that make up striated muscle. The I-band region of titin is responsible for passive elasticity in such muscle, and contains tandem arrays of immunoglobulin domains. One such domain (I27) has been investigated extensively, using dynamic force spectroscopy and simulation. However, the relevance of these studies to the behaviour of the protein under physiological conditions was not established. Force studies reveal a lengthening of I27 without complete unfolding, forming a stable intermediate that has been suggested to be an important component of titin elasticity. To develop a more complete picture of the forced unfolding pathway, we use mutant titins--certain mutations allow the role of the partly unfolded intermediate to be investigated in more depth. Here we show that, under physiological forces, the partly unfolded intermediate does not contribute to mechanical strength. We also propose a unified forced unfolding model of all I27 analogues studied, and conclude that I27 can withstand higher forces in muscle than was predicted previously.

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In this work we present a unified method to study the mechanical properties of cells using the atomic force microscope. Stress relaxation and creep compliance measurements permitted us to determine, the relaxation times, the Young moduli and the viscosity of breast cancer cells (MCF-7). The results show that the mechanical behaviour of MCF-7 cells responds to a two-layered model of similar elasticity but differing viscosity. Treatment of MCF-7 cells with an actin-depolymerising agent results in an overall decrease in both cell elasticity and viscosity, however to a different extent for each layer. The layer that undergoes the smaller decrease (36–38%) is assigned to the cell membrane/cortex while the layer that experiences the larger decrease (70–80%) is attributed to the cell cytoplasm. The combination of the method presented in this work, together with the approach based on stress relaxation microscopy (Moreno-Flores et al 2010 J. Biomech. 43 349–54), constitutes a unique AFM-based experimental framework to study cell mechanics. This methodology can also be extended to study the mechanical properties of biomaterials in general.

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Crystalline S(urface)-layers are the most commonly observed cell surface structures in prokaryotic organisms (bacteria and archaea). S-layers are highly porous protein meshworks with unit cell sizes in the range of 3 to 30 nm, and thicknesses of ~10 nm. One of the key features of S-layer proteins is their intrinsic capability to form self-assembled mono- or double layers in solution, and at interfaces. Basic research on S-layer proteins laid foundation to make use of the unique self-assembly properties of native and, in particular, genetically functionalized S-layer protein lattices, in a broad range of applications in the life and non-life sciences. This contribution briefly summarizes the knowledge about structure, genetics, chemistry, morphogenesis, and function of S-layer proteins and pays particular attention to the self-assembly in solution, and at differently functionalized solid supports.

Commercial poly(acrylic acid) (PAA) samples with M v = 150 000 and 450 000 were labeled randomly with small amounts (2−3 mol %) of 1-pyrenylmethyl (MePy) groups. A sample of PAA (M v = 450 000) labeled with 2 mol % MePy and modified with 2 mol % n-dodecyl (C12) groups was also prepared. The photophysical properties of the polymers have been investigated by steady-state fluorescence spectroscopy. The ratio (I E /I M) of excimer-to-monomer emission intensities was used to determine the effect of changes in pH and addition of sodium dodecyl sulfate (SDS) on the solution properties of the labeled polymers in the absence and presence of salt (NaCl). Changes in polymer conformation in aqueous solution upon addition of base were revealed by the curve I E /I M vs pH that had an inflection point at pH 4.7, the pK a value of labeled PAA. The ratio I 1/I 3 of the emission of MePy linked to PAA was monitored to obtain information on the micropolarity sensed by the pyrene label and how it is affected by external stimuli such as changes in pH and addition of surfactant. SDS interacts with labeled PAA in solution of pH 3 to form a polymer/SDS complex with a critical aggregation concentration (CAC) lower than the critical micelle concentration of SDS. The CAC values decrease further in the presence of NaCl but are not affected significantly by the molecular weight of the parent PAA or the grafted dodecyl along the PAA chain.