Lateral diffusion of photopigments in photoreceptor disk membrane vesicles by the dynamic Kerr effect

Electro-optic Kerr effect in the study of mixtures of oppositely charged colloids. The case of polymer-surfactant mixtures in aqueous solutions

In this review I highlight a very sensitive experimental technique for the study of polymer-surfactant complexation: The electro-optic Kerr effect. This review does not intend to be exhaustive in covering the Kerr Effect nor polymer-surfactant systems, instead it aims to call attention to an experimental technique that, even if applied in a qualitative manner, could give very rich and unique information about the structures and aggregation processes occurring in mixtures of oppositely charged colloids. The usefulness of electric birefringence experiments in the study of such systems is illustrated by selected results from literature in hope of stimulating the realization of more birefringence experiments on similar systems. This review is mainly aimed at, but not restricted to, researchers working in polyelectrolyte-surfactant mixtures in aqueous solutions, Kerr effect is a powerful experimental tool that could be used in the study of many systems in diverse areas of colloidal physics.

Explicit spatiotemporal simulation of receptor-G protein coupling in rod cell disk membranes

Dim-light vision is mediated by retinal rod cells. Rhodopsin (R), a G-protein-coupled receptor, switches to its active form (R(∗)) in response to absorbing a single photon and activates multiple copies of the G-protein transducin (G) that trigger further downstream reactions of the phototransduction cascade. The classical assumption is that R and G are uniformly distributed and freely diffusing on disk membranes. Recent experimental findings have challenged this view by showing specific R architectures, including RG precomplexes, nonuniform R density, specific R arrangements, and immobile fractions of R. Here, we derive a physical model that describes the first steps of the photoactivation cascade in spatiotemporal detail and single-molecule resolution. The model was implemented in the ReaDDy software for particle-based reaction-diffusion simulations. Detailed kinetic in vitro experiments are used to parametrize the reaction rates and diffusion constants of R and G. Particle diffusion and G activation are then studied under different conditions of R-R interaction. It is found that the classical free-diffusion model is consistent with the available kinetic data. The existence of precomplexes between inactive R and G is only consistent with the data if these precomplexes are weak, with much larger dissociation rates than suggested elsewhere. Microarchitectures of R, such as dimer racks, would effectively immobilize R but have little impact on the diffusivity of G and on the overall amplification of the cascade at the level of the G protein.

Nonlinear response of the electric birefringence of polyelectrolyte solutions

A theoretical approach for the nonlinear ac electro-optical response in polyelectrolytes is developed in the case where a weak ac electric field superimposed on a strong dc bias electric field is applied to these electrically charged systems. By restricting ourselves to an assembly of noninteracting and nonpolar rodlike macroions, we use a perturbation procedure and establish expressions for the first two harmonic components of the electric birefringence up to the second order in the electric field strength. An attempt is also made to extend this theory to the (non-Markovian) subdiffusive regime based on a fractional kinetic equation written in a configuration space where angular and linear displacements are taken into account. The results obtained are illustrated by three-dimensional dispersion and absorption plots together with Cole-Cole-like diagrams to show the importance of the coupling effect between translation and rotation. Besides considering the stationary ac response, we have also derived, in the context of subdiffusion, new expressions for the transient electric birefringence in the presence of a constant electric field, both for the buildup and the reversing pulse. All these results are illustrated by plots demonstrating the effect of the coupling (rotation-translation) parameter a and the critical exponent α (subdiffusion). A comparison of our theoretical model with experimental measurements of the ac Kerr effect response of a polyelectrolyte solution of NaCMC appears to be quite satisfactory.

Nonlinear harmonic components in dielectric relaxation of polyelectrolytes

The problem of the nonlinear ac dielectric response in polyelectrolytes is carried out by considering these electrically charged systems acted on by a strong dc bias electric field superimposed on a weak ac field. An infinite set of coupled (translation-rotation) kinetic equations is derived, and by using a perturbation procedure, we establish a new expression for the complex dielectric increment of an assembly of nonpolar rodlike macroions surrounded by small counterions. By considering the stationary regime, we show that the electric polarization of the nonpolar macroion is given by calculating the expectation value of P(1)(cos theta)H(1)(x), where P(1) and H(1) stand for the first Legendre and Hermite polynomials, respectively. The results so obtained provide new relaxation features, which are presented in the form of analytic expressions and illustrated by Cole-Cole-like diagrams and three-dimensional dispersion and absorption plots in order to show the influence of the coupling parameter a measuring the importance of the rotational motion over the translational one.

Transient electric birefringence of two small DNA restriction fragments of the same molecular weight

The transient electric birefringence of two small DNA restriction fragments of the same molecular weight, one of which migrates anomalously slowly on polyacrylamide gels, has been investigated. Both fragments exhibit negative birefringence. The decay of the birefringence of the anomalously slowly migrating fragment is 8-9% faster than that of the normally migrating fragment. The faster birefringence decay of the anomalous fragment 12A persists under a variety of buffer conditions, suggesting that it is due primarily to static bending and/or curvature of fragment 12A. In reversing electric fields the absolute amplitude of the birefringence of fragments 12A and 12B decreased about 26% before returning to the steady state value. The minimum in the birefringence occurred faster than expected from the birefringence decay times and decreased with increasing electric field strength, suggesting that the minimum is due to a slow polarization of the ion atmosphere. For both fragments, the rise of the birefringence in the Kerr region is about 10% slower than the field-free decay. The buildup of the negative birefringence is preceded either by an interval when no birefringence is observed or by a small positively birefringent transient, suggesting that a small transverse ionic polarizability is also present. Both DNA fragments exhibit Kerr law behavior over most of the range of electric field strengths investigated. Analysis of the shapes of the saturation curves suggests that differences may exist in the polarization mechanisms of the two fragments.

Contribution of asymmetric ligand binding to the apparent permanent dipole moment of DNA

Despite its antiparallel symmetry, DNA often appears to possess a permanent electric dipole moment in transient electro-optical experiments. We propose that this may be due to the asymmetric binding of charged ligands to the DNA. We have used the fluctuating dipole theory of Kirkwood and Shumaker to calculate the contribution of asymmetric ligand binding to the electro-optic orientation function, and Monte Carlo computer simulation to calculate the reversing pulse behavior, as a function of ligand binding density. The results indicate that the effect should be observable even against the background of the sizable induced dipole moment produced by polarization of the counterion atmosphere.

Models for transmitter activation process in retinal rod outer segments to flash stimuli

It has been generally accepted that transmitter molecules, which participated in the visual transduction process in retinal rod outer segments, are activated by bleached rhodopsin molecules serially one by one (serial activation model). This serial activation model accounts for some experimental observations, but not the quick and amplified activation of transmitter molecules. In this study, two models are proposed and solutions from the differential equations of the models are compared quantitatively with experimental observations. The two models proposed in this study are the serial-parallel activation model and the parallel-parallel activation model. In the serial-parallel activation model, rhodopsin molecules are activated by bleached rhodopsin molecules. Transmitter molecules are activated both by bleached and activated rhodopsin molecules. In the parallel-parallel activation model, both rhodopsin and transmitter molecules are activated by bleached and activated rhodopsin molecules. The time taken to activate 90% of transmitter molecules after flash stimuli, which bleaches 10(-4)% of rhodopsin, is 5.94 s, 3.91 ms and 23.7 microseconds for the serial, serial-parallel and parallel-parallel activation models, respectively. As contrasted to the serial activation model in which transmitter activation proceeds too slow, the present serial-parallel and parallel-parallel activation models can reproduce transmitter activation quick enough to account for the experimental observations.

A barrier to lateral diffusion of porphyropsin in Necturus rod outer segment disks

Microspectrophotometry was used to study lateral diffusion of the visual pigment, porphyropsin , in the disk membrane in intact mudpuppy (Necturus maculosus) rod outer segments (ROS), isolated in frog Ringer's solution. A concentration gradient of unbleached visual pigment was produced on the disks by rapidly photobleaching 40% of the pigment in an area spanning 1/4 or 1/2 of the cell's width. The change in optical density of the cells at 580 nm was then followed with time on either the bleached or unbleached side. The temperature dependence of porphyropsin diffusion yielded a Q10 of 2.5 between 10 and 20 degrees C with an activation energy of 12 +/- 2 kcal. At completion of pigment diffusion, the center and edge of the disk had, respectively, attained only 90 and 55% of the concentration expected. Computed diffusion coefficients (5.4 X 10(-9) cm2/s) were similar at the center and periphery of the disk immediately after the flash, however, an additional slow component for diffusion was detected at the periphery. A comparison of optical density at 525 nm along the diameter of ROS before and after the flash showed a persistent (20 min) postbleach concentration gradient of unbleached porphyropsin . This suggests that 15% of the prophyropsins may be sequestered into distinct areas on a mudpuppy disk and are not free to diffuse over the whole surface. This argument is supported by the observation that mudpuppy disks are separated into petal -shaped regions by incisures, some of which penetrate nearly to the disk center.

The chemistry of vision

The visual response is initiated by light reception and transduction into chemical and electrical energy in the outer-segment membranes of rod and cone cells. Recent research on the molecular events controlled by light has clarified the roles of some of the rod outer-segment biomolecules. These developments and the current unresolved questions are described.

Electric and optical anisotropy and their osmotically induced changes of photoreceptor disk membrane vesicles

Electro-optical characterization of the photoreceptor disk membrane vesicle is performed by examining the electric field and concentration dependence of the study-state birefringence of aqueous suspensions of the vesicles. The electric polarizability anisotropy is found to be negative and of large magnitude: alpha 1 - alpha 2 = -(1-3) X 10 cm3. The optical anisotropy is determined to be also negative but of small magnitude: g 1 - g 2 = -1 X 10(-7). The specific Kerr constant deduced from the concentration dependence of the Kerr constant is found to be very large: Ksp = 7 X 10(-4) e.s.u. Upon deforming the vesicles osmotically from the spherical shell to the disk structure, the steady-state birefringence increases by an order of magnitude which is attributed solely to the increase in optical anisotropy attending the corresponding change in the geometric eccentricity of the vesicle. A plausible birefringence mechanism based on the known structural features of the vesicles is proposed, which would account for these findings.