Streptavidin-biotin (receptor-ligand) interaction forces were measured directly as a function of their intermolecular separation in various salt solutions and at various temperatures with a surface forces apparatus. Electrostatic and van der Waals forces were found to dominate the long-range streptavidin-biotin interaction at > 20 A. At intermediate separations, down to approximately 10 A, the interaction is governed by repulsive steric and attractive van der Waals and hydrophobic forces. A much stronger short-range attraction giving rise to the strong, specific adhesive binding was measured at molecular separations of less than 5 A. A decrease in the pH from 7.2 to 6.0 resulted in complete charge reversal on the binding surface of streptavidin (pK approximately 6) from net negative to net positive, while leaving the negatively charged biotin surface (pK approximately 3.0) unchanged, and the long-range interaction switched from repulsive to attractive. This observed behavior can be attributed to the titration of two histidines on the biotin binding surface of streptavidin. These results reveal a strong sensitivity of the long-range interaction forces to the detailed amino acid composition of the biotin binding surface. They also demonstrate the powerful regulatory potential conferred by small changes in local surface ionic conditions on protein interaction forces over different distance regimes. The effects of temperature on receptor-ligand dynamics and on the strength of intermembrane adhesion forces were studied by measuring the long-range force profiles and short-range adhesion forces above and below the chain melting temperature (Tc approximately 30 degrees C) of the lipids in the supporting bilayers. Increased bilayer fluidity due to a temperature increase to 33 degrees C (T > Tc) increased short-range adhesion by 7-fold relative to bilayers in the gel state at 25 degrees C (T< Tc). This effect was attributed to the enhanced rates of lateral diffusion and molecular rearrangements on the more fluid bilayer surfaces, which resulted in greater and more rapid intermembrane bond formation. A change in the rates of molecular rearrangements was also found to affect the repulsive part of the interaction potential at intermediate separations (10-20 A) via modulation of the steric repulsion between streptavidin and the highly flexible, polymer-like biotin molecules. This is expected to have a large effect on the association rates of receptor-ligand binding, even if it does not change the equilibrium binding energy.(ABSTRACT TRUNCATED AT 250 WORDS)

The association of lipid molecules into spherical vesicles in solution as a result of non-specific intermolecular forces constitutes a primary self-assembly process. Such vesicles can undergo a secondary self-assembly into higher order structures in a controlled and reversible manner by means of site-specific ligand-receptor (biotin-streptavidin) coupling. Cryoelectron microscopy shows these structures to be composed of tethered, rather than adhering, vesicles in their original, unstressed state. In contrast, vesicles aggregated by nonspecific, such as van der Waals, forces are deformed and stressed, producing unstable structures. Vesicle association by site-specific binding provides a practical mechanism for the production of stable, yet controllable, microstructured biomaterials.

This review provides an overview about recent developments and current knowledge about monitoring, generation and the functional role of reactive oxygen species (ROS) - H2O2, HO2, HO, OH(-), (1)O2 and O2(-) - in both oxidative degradation and signal transduction in photosynthetic organisms including microscopic techniques for ROS detection and controlled generation. Reaction schemes elucidating formation, decay and signaling of ROS in cyanobacteria as well as from chloroplasts to the nuclear genome in eukaryotes during exposure of oxygen-evolving photosynthetic organisms to oxidative stress are discussed that target the rapidly growing field of regulatory effects of ROS on nuclear gene expression.

A surface force apparatus was used to measure a long-range attractive protein-ligand force at separations D less than 85 angstroms. This force may effectively "steer" ligand trajectories, resulting in a greater than 27-fold enhancement of the association rate. A much stronger specific attraction is measured at contact (D less than 4 angstroms). A sevenfold increase in intermembrane adhesion resulted from increased lateral mobility of the receptors and molecular rearrangements in membranes above the solid-fluid transition temperature.

The set up described in Steffen et al. (Biochemistry 40:173-180, 2001) was used to monitor in the time domain from 100 ns to 10 s single turnover flash-induced transients of the normalized fluorescence yield (SFITFY) on dark-adapted cells of the thermophilic algae Chlorella pyrenoidosa Chick. Perfect data fit was achieved within the framework of a previously proposed model for the PS II reaction pattern (Lebedeva et al., Biophysics 47:968-980, 2002; Belyaeva et al., Biophysics 51:860-872, 2006) after its modification by taking into account nonradiative decay processes including nonphotochemical quenching due to time-dependent populations of P680(+*) and (3)Car. On the basis of data reported in the literature, a consistent set of rate constants was obtained for electron transfer at the donor and acceptor sides of PS II, pH in lumen and stroma, the initial redox state of plastoquinone pool and the rate of plastoquinone oxidation. The evaluation of the rate constant values of dissipative processes due to quenching by carotenoid triplets in antennae and P680(+*)Q(A)(-*) recombination as well as the initial state populations after excitation with a single laser flash are close to that outlined in (Steffen et al., Biochemistry 44:3123-3133, 2005a). The simulations based on the model of the PS II reaction pattern provide information on the time courses of population probabilities of different PS II states. We analyzed the maximum (F(m)(STF)) and minimum (F(0)) of the normalized FL yield dependence on the rate of the recombination processes (radiative and dissipative nonradiative) and of P680(+*) reduction. The developed PS II model provides a basis for theoretical comparative analyses of time-dependent fluorescence signals, observed at different photosynthetic samples under various conditions (e.g. presence of herbicides, other stress conditions, excitation with actinic pulses of different intensity, and duration).