Evidence for magnetite receptors

Numerous techniques, including superconducting quantum interference device magnetometry, x-ray fluorescence, and atomic force microscopy, have been used in efforts to localize magnetite-based receptors. So far, the best evidence has come from trout and homing pigeons. In trout, confocal and atomic force microscopy have found single-domain magnetite crystals in cells near a nerve that responds to magnetic stimuli. In pigeons, a complex array of magnetic minerals has been found in a part of the beak coupled to a nerve that responds to magnetic field changes. Six clusters of such minerals have been found, three on each side of the beak (figure 4). The apparent functional unit, found in the branches of nerve cells, consists of a vesicle 3–5 µm in diameter that is coated with a noncrystalline iron compound and surrounded by about 10 to 15 1-µm-diameter spherical clusters, each containing approximately 8 million 5-nm-diameter crystals of magnetite that alternate with chains of about 10 plates, each roughly 1 × 1 × 0.1 µm, of maghemite. The functional units are regularly spaced at roughly 100-µm intervals in each of the six locations. Interestingly, the orientation of the units in each of three pairs of magnetic regions is perpendicular to the other two pairs, which suggests a triaxial system.

Gerta Fleissner, Gunther Fleissner, and their colleagues have proposed that the three different elements of the functional unit have different functions (figure 4d–f). The maghemite platelets, which are large enough to have approximately four magnetic domains, are thought to act as soft magnets that locally amplify Earth's field in the same way that a soft iron core increases the strength of an electromagnet. The amplified field then interacts with the clusters of tiny magnetite crystals. Those crystals are too small to have a stable magnetic moment at body temperature. An applied field will align the moments to a degree that depends on the field's strength and the temperature, but it will not rotate the particles themselves like compass needles. Termed superparamagnetic, such small particles of ferrimagnetic minerals have magnetic moments that are weak compared with those of single-domain particles. Nevertheless, Earth's field, concentrated by the platelets, may be able to move or deform a large enough cluster of the particles. Calculations based on the morphology of the system suggest that when aligned with Earth's field, the maghemite platelets increase the local field strength 20-fold, producing a force of about 0.2 piconewtons on the 2.6-picogram magnetite clusters. The resulting movement of the clusters might then open membrane channels either through direct physical connections or by deforming the nerve cell membrane. The function of the coated vesicle is uncertain, though iron storage and additional field concentration have been suggested.

Because finding magnetic minerals in tissue is hard and proving that they function in magnetoreception is harder, some researchers have tested the hypothesis indirectly using strong pulsed magnetic fields (about 500 µT for 5 ms) to alter the direction of magnetization in single-domain magnetite particles. After the pulses were applied, the magnetic orientation of certain birds and sea turtles either vanished or was slightly altered. However, given the high strength of the field and the even larger induced electric field, it is impossible to rule out effects on other compass mechanisms or even general physiology.

Radical pairs

The third proposed magnetoreception mechanism involves biochemical reactions. Although magnetic-field-dependent chemical reactions are known, a magnetoreception system based on chemistry must clear some high hurdles. First, in Earth's 50 µT field, energy shifts of molecular states due to Zeeman splitting are only one five-millionth of kT at body temperature (10–27 versus 5 × 10–21 joules); thus product yields and rates of most chemical reactions will not be sensitive to weak magnetic fields. But a class of chemical reactions involving pairs of radicals shows an unusual sensitivity to the strength and orientation of magnetic fields. For example, the rates of certain redox reactions involving horseradish peroxidase are slightly increased in fields of 1 mT. However, no room-temperature reaction of any kind has shown a measurable effect at geomagnetic field strengths. Second, any such reaction used for a compass requires immobilization of at least one of the reactants, so that a constant orientation relative to the field is maintained. With the exception of structural components, biological molecules continually rotate and move. Even proteins bound in cell membranes are in constant motion.

Assuming that spins are relatively isolated from thermal effects, researchers interested in the possibility of chemically mediated magnetoreception have focused on the correlated spin states of paired radical ions. The reaction, first proposed by Klaus Schulten in 1982 and then developed by Thorsten Ritz, begins with an electron transfer between two molecules, leaving two unpaired electrons in a pure singlet state. Over what is assumed to be a relatively long period (about 100 ns), the spins interact with the nuclear spins and precess at different rates that depend on the local magnetic neighborhood and the orientation and strength of the geomagnetic field. Back-transfer of the electron can only occur if the spins are oppositely aligned, and their alignment depends on the length of the reaction and the difference in precession rates. Because the geomagnetic field can influence the precession rate, it may be able, under the right set of conditions, to influence reaction rates or products.

In quantum mechanical terms, the initial singlet state is coupled to a nearly degenerate triplet state via the hyperfine interactions between the electron spins and the nuclear spins, the coupling strength depends on the magnetic field, and the rate at which the state acquires triplet character is thus field dependent. If one assumes that the radical pair in the triplet state forms a chemical product that differs from that of singlet pairs, one has a potentially viable detector for weak magnetic fields. It's important to note that the radical-pair mechanism can detect only the field's axis, not its polarity. However, few animals appear to be able to detect the polarity of Earth's magnetic field (exceptions are lobsters, salamanders, and mole rats). Instead, they define "poleward" as the direction along Earth's surface in which the angle formed between the magnetic-field vector and the gravity vector is smallest.

Because the influence of the geomagnetic field on singlet-to-triplet conversion is very weak, the lifetime of the singlet state due to other decay modes—such as fluorescence, decoherence of the quantum state, and intramolecular conversion—must be quite long for any appreciable magnetic effects to develop. Quantum mechanical calculations of model systems, using plausible parameters, have shown that the conditions can be met. In addition, the relationship between the reaction time and the internal magnetic interactions must be precise, and the molecules must contain few hydrogen or nitrogen atoms, whose relatively strong magnetic moments will overwhelm any effects due to Earth's field. Furthermore, the formation of the initial state must not randomize the spin relationship of the two unpaired electrons. In general, that requirement is met only in reactions begun by photoexcitation.

Date: 2016-01-14; view: 358