Felix T. Hong

Magnetic field effects on biomolecules, cells, and living organisms

This article surveys three major areas of biomagnetic research: (a) the magneto-orientation effect; (b) the role of the geomagnetic field in bird orientation and navigation; and (c) the biological effects of extremely low-frequency magnetic fields. The magneto-orientation effect is caused by diamagnetic anisotropy of highly ordered biological structures, such as visual photoreceptor and chloroplast membranes, in a homogeneous magnetic field of about 10 kG. While it is not possible to orient the individual constituent molecules with such a field because of thermal fluctuation, these ordered structures can be oriented as a whole by virtue of summing the anisotropy over a large number of mutually oriented molecules. While the magneto-orientation effect seems to require the use of unphysiologically strong magnetic fields, certain birds apparently have highly sensitive sensors to detect the geomagnetic field for the purpose of orientation and navigation. However, the advances in this latter field were made mainly in the behavioral studies; the magneto-sensors the the neural mechanisms remain elusive. A number of candidates of the sensors are evaluated. We suggest that pecten oculi, which is unique to avian eyes, should not be overlooked for its possible role as a magneto-sensor based on the magneto-orientation effect. Birds primarily use a static (DC) magnetic field for orientation, but recent investigations indicate that weak alternating (AC) magnetic fields with extremely low frequency (ELF) may have hazardous health effects. Such reports are often received with skepticism, because the effects usually involve magnetic energies that are less than the kT energy. However, some of the in vitro studies yield experimental results that are too significant to be ignored. Here, we propose an argument to explain why low-level magnetic fields can be detected without being overshadowed by thermal noises. Relevance of biomagnetic research to the development of biosensors and novel computational paradigms is also discussed.

Bacteriorhodopsin in model membranes. A new component of the displacement photocurrent in the microsecond time scale.

A quasi-short-circuit (tunable voltage clamp) measurement method with microsecond time resolution was applied to a bacteriorhodopsin model membrane formed by a novel interfacial technique. A new component (B1) of the displacement photocurrent was recorded: it has no detectable latency at an instrumental time constant of 1.5 museconds, and persists at 5 degrees C. In addition, a slower component (B2) of opposite polarity inhibited by low temperature (5 degrees C) and low pH (pH = 3.0) was recorded. The technique is very sensitive for the study of fast capacitative photoresponses in model membranes, and allows the detection of charge displacements in bacteriorhodopsin associated with distinct stages of the photochemical transformation.

Interfacial photochemistry of retinal proteins

Retinal proteins are membrane-bound protein pigments that contain vitamin A aldehyde (retinal) as the chromophore. They include the visual pigment rhodopsin and four additional ones in the plasma membrane of Halobacterium salinarium (formerly Halobacterium halobium). These proteins maintain a fixed and asymmetric orientation in the membranes, and respond to a light stimulus by generating vectorial charge movement, which can be detected as an electric potential across the membrane or an electric current through the membrane. These phenomena are collectively called the photoelectric effects, which defy a rigorous quantitative treatment by means of either conventional (solution phase) photochemistry or conventional electrophysiology. As an alternative to the mainstream approach, we utilize the analytic tools of electrochemical surface science and electrophysiology to analyze two molecular models of light-induced charge separation and recombination. Being tutorial in nature, this article demands no prior knowledge about the subject. A parsimonious equivalent circuit model is developed. Data obtained from reconstituted bacteriorhodopsin membranes are used to validate the theoretical model and the analytical approach. Data generated and used by critics to refute our approach is shown to actually support it. The present analysis is sufficiently general to be applicable to other pigment-containing membranes, such as the visual photoreceptor membrane and the chlorophyll-based photosynthetic membranes. It provides a coherent description of a wide range of light-induced phenomena associated with various pigment-containing membranes. In contrast, the mainstream approach has been plagued with self-contradictions and paradoxes. Last, but not least, the alternative bioelectrochemical approach also exhibits a predictive power that has hitherto been generally lacking. Comparison of the photoelectric effects is made with regard to bacteriorhodopsin, rhodopsin, and the chlorophyll-based photosynthetic apparatus — in the spirit of reverse engineering (biomimetic science). The technological applications of bacteriorhodopsin as an advanced material for the construction of molecular devices and the implication of the photoelectric behavior of bacteriorhodopsin for solar energy conversion are also discussed.

The bacteriorhodopsin model membrane system as a prototype molecular computing element.

The quest for more sophisticated integrated circuits to overcome the limitation of currently available silicon integrated circuits has led to the proposal of using biological molecules as computational elements by computer scientists and engineers. While the theoretical aspect of this possibility has been pursued by computer scientists, the research and development of experimental prototypes have not been pursued with an equal intensity. In this survey, we make an attempt to examine model membrane systems that incorporate the protein pigment bacteriorhodopsin which is found in Halobacterium halobium. This system was chosen for several reasons. The pigment/membrane system is sufficiently simple and stable for rigorous quantitative study, yet at the same time sufficiently complex in molecular structure to permit alteration of this structure in an attempt to manipulate the photosignal. Several methods of forming the pigment/membrane assembly are described and the potential application to biochip design is discussed. Experimental data using these membranes and measured by a tunable voltage clamp method are presented along with a theoretical analysis based on the Gouy-Chapman diffuse double layer theory to illustrate the usefulness of this approach. It is shown that detailed layouts of the pigment/membrane assembly as well as external loading conditions can modify the time course of the photosignal in a predictable manner. Some problems that may arise in the actual implementation and manufacturing, as well as the use of existing technology in protein chemistry, immunology, and recombinant DNA technology are discussed.

Charge transfer across pigmented bilayer lipid membrane and its interfaces.

Abstract— The technique of forming bilayer lipid membranes (BLM) has made it possible to study photoreactions of pigments in an environment that is much closer to those in photosynthetic and visual membranes. A pigmented BLM system with Mg2+‐porphyrins as membrane‐bound pigments and with ferricyanide and ferrocyanide as the aqueous electron acceptor and donor, respectively, was used to illustrate the photoelectric effects due to coupled interfacial charge transfer reactions.

Kinetic analysis of displacement photocurrents elicited in two types of bacteriorhodopsin model membranes.

Fast displacement photocurrents have been reported in bacteriorhodopsin model membranes by several groups of investigators since 1977. A fast component (B1) is associated with positive charge displacement in the direction opposite to that of a physiological proton translocation. A slower component (B2) of opposite polarity is associated with positive charge displacement in the same direction as the proton translocation. Using two slightly different methods for model membrane formation, we observed photosignals with or without a significant B2 component under appropriate conditions. By means of the tunable voltage clamp method of measurement (Hong, F.T., and D. Mauzerall, 1974, Proc. Natl. Acad. Sci. USA, 71:1564-1568) we demonstrated that the time course of the B1 signal is completely predictable by an equivalent circuit containing a chemical capacitance. From the equivalent circuit analysis, we obtained a first-order relaxation time constant of 12.3 +/- 0.7 microseconds at room temperature. We also found a slight temperature dependence of the B1 relaxation with an activation energy of 2.54 +/- 0.24 kcal/mol. We found no pH dependence of the B1 component in the range of 0 to 11, whereas the B2 component is diminishing in a graded manner when the pH is varied from 0 to 10. These results are diametrically different from what reported previously (Drachev, L.A., A.D. Kaulen, L.V. Khitrina, and V.P. Skulachev, 1981, Eur. J. Biochem., 117:461-470). Our results support the interpretation that the B1 component is generated by an intramolecular charge displacement accompanying the light-induced reactions of bacteriorhodopsin and that the B2 component is generated by a process of proton uptake from the intracellular aqueous phase and subsequent release into the same aqueous phase. The impact of the present results on the conventional practice of identifying photointermediates of bacteriorhodopsin by spectroscopic means is discussed.

Photoelectric and Magneto-Orientation Effects in Pigmented Biological Membranes

Abstract Photoelectric and magneto-orientation effects in pigmented biological membranes are analyzed and discussed with special reference to three different levels of order in the membrane structures. The kinetic results of photoelectric responses from an artificial pigmented bilayer lipid membrane/aqueous redox system, measured by a tunable voltage clamp method are analyzed in terms of an equivalent circuit model which contains a novel chemical capacitance. The molecular basis of the equivalent circuit and the concept of chemical capacitance is established by a combined kinetic and electrostatic calculation, using Gouy-Chapman diffuse double layer theory. The generality of the concept of chemical capacitance is demonstrated by the ability of the model to explain previously reported data on a variety of pigmented membranes. The theory is further applied to photoelectric effects in three important photobiological systems: photosynthetic membranes of chloroplasts, visual disc membranes of rod outer segments, and purple membranes of Halobacterium halobium. Structure-function correlation is also stressed. Magneto-orientation effects in isolated rod outer segments and in Chlorella cells are discussed on the basis of summed diamagnetic anisotropy. The relation of this effect to the mechanism of bird orientation and navigation in the terrestrial magnetic field is speculated upon.

The separation of voltage-dependent photoemfs and conductances in Rudin-Mueller membranes containing magnesium porphyrins.

Abstract It is shown that the photoemf in a pigmented membrane is specific to the magnesium-porphyrin conductance channel. A null current method was devised to measure directly the voltage dependence of the photoemf and the magnesium-porphyrin conductance. Their voltage dependence is in agreement with the hypothesis that the magnesium-porphyrin cation is the majority carrier

Molecular sensors based on the photoelectric effect of bacteriorhodopsin: Origin of differential responsivity

Abstract Bacteriorhodopsin is a popular advanced material. When oriented in a membrane or a thin film deposited on a transparent electrode, it responds to sudden changes of the illumination level with spikes of fast photoelectric signal, of which the amplitude and polarity reflect the extent and sense of changes (a phenomenon known as differential responsivity). The present article explain how light-induced rapid charge separation and recombination in the membrane leads to differential responsivity. This article also evaluates several alternative proposed mechanisms appearing in the literature.

234 - Mechanisms of generation of the early receptor potential revisited☆

Abstract The early receptor potential (ERP) is a fast photoelectric response from visual photoreceptor membranes when light is absorbed. It is a capacitive response with no detectable latency. The ERP is quite distinct from many other bioelectric phenomena, in its characteristics, and possibly also in the mechanism of generation. It has generally been concluded that the ERP is not directly involved in visual excitation, but is merely an epi-phenomenon due to internal charge redistribution accompanying the light-induced conformational change of the visual pigment. This conclusion might be premature at this stage. In this paper, possible mechanisms of generation of the ERP are cast into two fundamentally different models: an oriented dipole model, which seems widely accepted, and an interfacial charge transfer model, which can be demonstrated in an artificial system that is capable of generating a fast photoelectric response with all the major ERP characteristics. These two models are analyzed in terms of the Gouy-Chapman diffuse double layer theory. Pertinent literature is reviewed and the above conclusion is re-evaluated in the light of our conceptual and technical insight acquired from model system studies. An explanation of apparent discrepancy is suggested, whenever possible. Possible roles of the ERP in visual excitation are also discussed.