An attractive force appears between particles suspended in solutions of macromolecules when there is neither direct interaction between two particles nor energetic interaction between particles and solute macromolecules. The magnitude of this force is of the order of the osmotic pressure of the solution of macromolecules and the range is of the order of the diameter of macromolecules. This force is calculated as a function of concentration, shape, and charge of macromolecules, and it is shown that it becomes stronger in solutions of chain macromolecules or of macromolecules of dissymmetrical shape than in solutions of rigid spherical macromolecules at the same net concentration. If macromolecules have charge, the force can be greatly intensified. In every case numerical estimation is made, and it is found that actually this kind of force can have a remarkable influence on the state of suspended particles. Numerical examples of the critical concentration of particles at their macroscopic aggregation are given. Finally a short description is added on the effect of energetic interaction between particles and macromolecules.

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SynopsisThe thermal fluctuation in the concentration of counterions bound to a rodlike polyion was analyzed by expanding the fluctuation in a Fourier series along the rod. The amplitude and the relaxation time of fluctuations of various wave lengths were obtained as functions of the charge density and the length of the polyion. From these results the real and imaginary parts of the dielectric constant of the polyelectrolyte solution were derived as the sum of contributions of fluctuations of different modes. The dielectric dispersion curve or the Cole-Cole plot obtained was found to be in good agreement with experimental data.

Actin is found in almost all kinds of non-muscle cells where it is thought to have an important role in cell motility. A proper understanding of that role will only be possible when reliable in vitro systems are available for investigating the interaction of cellular actin and myosin. A start has been made on several systems, most recently by Sheetz and Spudich who demonstrated unidirectional movement of HMM-coated beads along F-actin cables on arrays of chloroplasts exposed by dissection of a Nitella cell. As an alternative approach, we report here the direct observation by fluorescence microscopy of the movements of single F-actin filaments interacting with soluble myosin fragments energized by Mg2+-ATP.

SynopsisThis paper gives a very simple method based on the characteristic property of the electric free energy to calculate the repulsive force between parallel rodlike macroions in a solution as a function of the charge density on rods. The total extensive force (dj/dX) of an assembly of m rods of length 2 and charge number n (charge density -neo/2) at small extension X in the absence of low molecular salts is given bywhere z is the valency of counterions and Q ( = neo2/EkTl) is a dimensionless quantity representing the charge density. The repulsion between two parallel rods is given by putting m = 2. A t large charge densities the repulsion is very much smaller than the direct coulomb force between charged rods, even a t small distances. The addition of low molecular salts does not depress the repulsion appreciably, as long as the average concentration of salt ions is much smaller than the concentration of counterions accumulated in the space between rods. The effect of fluctuation of the counterion distribution is also analyzed, and it is found that the attractive force due to the ion fluctuation may predominate over the above repulsive force in the case of polyvalent counterions and rods of high charge densities a t small distances.

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Muscle contraction results from a sliding movement of actin filaments induced by myosin crossbridges on hydrolysis of ATP, and many non-muscle cells are thought to move using a similar mechanism. The molecular mechanism of muscle contraction, however, is not completely understood. One of the major problems is the mechanochemical coupling at high velocity under near-zero load. Here, we report measurements of the sliding distance of an actin filament induced by a myosin crossbridge during one ATP hydrolysis cycle in an unloaded condition. We used single sarcomeres from which the Z-lines, structures which anchor the thin filaments in the sarcomere, had been completely removed by calcium-activated neutral protease (CANP) and trypsin, and measured both the sliding velocity of single actin filaments along myosin filaments and the ATPase activity during sliding. Our results show that the average sliding distance of the actin filament is less than or equal to 600 A during one ATP cycle, much longer than the length of power stroke of myosin crossbridges deduced from mechanical studies of muscle, which is of the order of 80 A (for example, ref. 15).