SynopsisA theory of the electrophoresis of DNA through gels with large interfiber spacing, such as dilute agarose, is presented. We assume that the DNA molecule moves along its axis through a "tube" in a neutral gel under the influence of the electric field. The tube is random except for possible bias due to the effects of the field. When the field is small, we easily recover the inverse-length dependence of the mobility found previously by de Gennes and by Doi and Edwards. At higher fields, a new effect appears; the tube becomes oriented because the field biases the direction of the leading end of the chain as it moves to form an extension of the tube. This leads to an increase of the mobility with increasing field by adding a field-dependent but length-independent term to the mobility expression. In agreement with experiment, we find that the field effect can be important a t fields as low as 1 V/cm and that the effect can seriously decrease the sensitivity of the mobility to chain length. We also examine the fluctuation of the migration distance, the degree of orientation induced by the field, and the transient effects occurring when the field direction is rotated by a right angle.

In this paper, we show that the technique of reflectometry, applied to the reflection of a "p" wave around the Brewster angle, provides information similar to ellipsometry to characterize a layer of adsorbed macromolecules. Application of this method to study adsorption from solutions of human fibrinogen at the surface of pure silica leads to the following observations: (1) When the external equilibrium concentration is raised, but remains low, a layer of almost constant optical thickness builds up, while the concentration within the layer increases. ( 2) At an external crossover concentration close to 10~1 2 34% (w/w), this first layer appears to be saturated.(3) Above this concentration threshold, an increase in the equilibrium concentration induces structural changes within the adsorbed layer, which remain to be completely explained. These observations correlate with former results obtained by using a hydrodynamic flow technique.

We report the formation of a hybrid biological/artificial nanopore by the direct insertion of non-modified α-hemolysin at the entrance of a high aspect ratio (length/diameter) biomimetic nanopore. In this robust hybrid system, the protein exhibits the same polynucleotide discrimination properties as in the biological membrane and the polynucleotide dwell time is strongly increased. This nanopore is very promising for DNA sequencing applications where the high DNA translocation velocity and the fragility of the support are the main bottlenecks.

We examined the adsorption kinetics of alpha-chymotrypsin (pH 8.6, 10(-2) to 0.5 M Tris buffer) on muscovite mica in conditions of laminar flow through a slit. The range of buffer concentrations is between two limits: (i) no adsorption in 1 M Tris and (ii) no desorption in 10(-3) M Tris. Studying the dependence of adsorption kinetics on the wall shear rate leads to the determination of the interfacial adsorption kinetic constant ka and the diffusion coefficient. The obtained value for the diffusion coefficient is close to the one expected from the molecular size of alpha-chymotrypsin. The interfacial adsorption kinetic constant of alpha-chymotrypsin decreases when ionic strength increases, while the initial desorption constant (over a part of all the adsorbed population) shows the contrary. Although alpha-chymotrypsin is almost at its isoelectric point, the effect of ionic strength on the adsorption kinetics suggests the importance of electrostatic interactions between the protein and mica. We observed an increase in the adsorption rate, at a surface coverage near 0.14 microg cm(-2), for adsorption in 10(-2) M Tris and the low wall shear rates (<300 s(-1)). This change in the adsorption rate suggests a structural transition, that we assume again to be due to electrostatic interactions, but between proteins. The large dipole moment of the protein may induce this transition, illustrated here by the ferroelectric/antiferroelectric pattern. The variation of the zeta potential with interfacial concentration seems to be in agreement with such a model.