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Some epidemiological studies have suggested that exposure to ambient, low-level 50/60 Hz electric and magnetic fields (EMFs) increases risk of disease. Whether this association has a causal basis depends in part on whether the electrical, chemical and mechanical "signals" induced within living cells by ambient EMFs are detectable in the complex milieu of voltages, currents and forces present within the living organism. Magnetic responsiveness has been found in some animals and bacteria; aquatic animals (e.g. sharks and rays) can sense weak electric fields. We outline the physics of several mechanisms by which EMFs may interact: (1) Energy transfer by acceleration of ions and charged proteins modifies cell membranes and receptor proteins; however, EMF energies are far below those typical of biomolecules in the cell. (2) Electric fields induced inside the body exert force on electric charges and electric moments; however, these forces are considerably smaller than typical biological forces. (3) The magnetic moments of ferromagnetic particles and free radical molecules interact with magnetic fields, but magnetic-moment sensory cells have not been found in humans, and modification of radical recombination rates by EMFs in a biological system is highly problematic. (4) Resonant interactions involve EMFs driving vibrational or orbital transitions in ion-biomolecule complexes; these mechanisms conflict with accepted physics, and many experimental tests have not found the predicted effects. (5) Temporal averaging or spatial summation can improve the ratio of "signal" to "noise" in any system, but this "mechanism" requires biological structures and neural processes having the necessary capabilities of EMF detection and temporal averaging that have not been found in humans. In summary, biological effects in humans due to extremely low-frequency EMFs of the order of those found in residential environments [< or = 2 microT (< or = 20 mG)] are implausible based on current understanding of physics and biology. Biological effects in humans at higher fields [> 10 microT (> 100 mG)] might reach plausibility as a result of time-averaging in combination with a magnetic-moment transduction mechanism; but even here, neither specialized EMF transduction structures nor appropriate averaging networks have been demonstrated. The bypothesis that the epidemiological associations observed between 50/60 Hz EMFs and disease reflect a causal relationship is not supported by what is known about mechanisms.

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Extensive dehydration of air‐dried films of bovine rod outer segment membranes induces fully reversible changes in the absorption spectrum of rhodopsin, indicative of deprotonation of the retinylidene Schiff base in more than 50% of the rhodopsin molecules in the sample. This suggests that water is involved at the site of the Schiff base protonation in rhodopsin. In contrast, the spectrum of metarhodopsin I is resistant to similar dehydrating conditions, implying a significant difference in the mechanism for protonation in metarhodopsin I. The photochemistry of dehydrated membranes was also explored. Photoexcitation of deprotonated rhodopsin (λmax 390 nm) induces a large bathochromic shift of the chromophore. The major photoproduct at room temperature was spectrally similar to metarhodopsin I (λmax, 478 nm). These findings suggest that intramolecular proton transfer involving the Schiff base proton may occur in the earlier stages of the visual cycle, prior to or during the formation of metarhodopsin I.

Light-induced changes in the UV absorption spectrum of bovine rod outer segment membranes were measured by conventional difference spectroscopy and by flash photolysis methods. Different thermal intermediates of rhodopsin (lumirhodopsin, metarhodopsin I, metarhodopsin 11, and metarhodopsin 111) have absorption spectra in the ultraviolet which differ from the rhodopsin spectrum and from each other. The spectra associated with metarhodopsin I, metarhodopsin 11, and metarhodopsin 111 are characteristic of perturbation of a small number of tyr. and/or trp residues, most likely one trp residue. These aromatic residues are in the neighborhood of the retinal Schiff base and undergo coordinated changes of interaction with retinal during the bleaching sequence. At the metarhodopsin I1 stage. the magnitude of the UV spectral changes is consistent with the exposure of a previously shielded trp residue to an aqueous environment. The present results are consistent with previous spectral studies which limit the extent of light-induced conformational changes to regions of the protein in the neighborhood of the retinal Schiff base. An analogous study was made on light-adapted purple membranes of Hatobacterium hafobiurn. The UV absorption spectrum associated with the deprotonated Schiff base intermediate of the trans-bacteriorhodopsin cycle is indicative, in part, of aromatic residue perturbation. However, significant changes in the secondary and tertiary structures of the bacteriorhodopsin protein characteristic of a delocalized conformational change are unlikely at this intermediate stage. 109

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Intense continuous illumination of purified chromatophores from carotenoidless mutant Rhodopseudomonas sphaeroides results in progressive photooxidative loss of the near infrared absorption band near 860 nm assigned to antenna bacteriochlorophyll. The quantum yield of this reaction is low, approximately 1.7 X 10-5. The loss in near infrared absorption is accompanied by a proportional shift in the absorption maximum to shorter wavelengths. The double circular dichroism feature in the near infrared decreases at a faster rate than does the absorbance. These results are explained by a model in which the antenna bacteriochlorophyll, initially associated as dimers (Xmax = 860.2 nm), is progressively converted to the monomeric state (Xmax = 851.9 nm). The wavelength shift is attributed to disruption of exciton coupling in the dimer. Acetone/methanol extraction indicates that the maximum molar extinction coefficients of the dimer and monomer do not differ by more than 4%. The occurrence of an absorption maximum at 852 nm for monomeric bacteriochlorophyll in a protein complex demonstrates that it is not necessary to invoke aggregation of the chromophores as the origin of the shift from 770 nm in typical organic solvents.Carotenoidless mutant strains of the photosynthetic bacteria have long been known to be especially susceptible to photooxidative injury (1, 2). We have exploited this susceptibility to examine the states of association of antenna bacteriochlorophyll in carotenoidless mutant Rhodopseudomonas sphaeroides. The framework of our analysis was suggested by a recent study by Sauer and Austin (3) on antenna pigment-protein complexes derived from various photosynthetic bacteria. They isolated a complex from carotenoidless mutant R. sphaeroides consisting of two bacteriochlorophyll molecules bound noncovalently to a 22-kilodalton lipoprotein. Circular dichroism (CD) spectra of the isolated complex and of chromatophores suggested the presence of exciton coupling characteristic of bacteriochlorophyll dimers. METHODS AND RESULTSPhotooxidation of Antenna Bacteriochlorophyll in Purified Chromatophores. Purified chromatophores from carotenoidless mutant strain R26 of R. sphaeroides were isolated as described (4, 5). They were suspended in 0.01 M Tris-HCI, pH 7.5, and stored at 4°C until use. Chromatophores were illuminated in a cuvette (1-cm path) at 25°C. The actinic light source was a Sylvania 1000-W tungsten/iodine lamp. The actinic light passed through glass lenses and 7.0 cm of water but was otherwise unfiltered. The filament image was focused on the cuvette face. The intensity of light at the cuvette was approximately 520 mW/cm2. Illumination was interrupted periodically and near infrared absorption spectra were recorded with a Cary 14R spectrophotometer (IR1 mode; weak measuring beam). The spectra obtained for one chromatophore suspension are shown in Fig. 1. In the spectrum of the sample before illumination (0 min), the major band centered at 860.2 nm is primarily due to antenna bacteriochlorophyll; about ...