Increased generation of reactive oxygen species (ROS) and an altered redox status have long been observed in cancer cells, suggesting that ROS might be involved in the development of these cells. However, recent studies suggest that inducing an excess of ROS in cancer cells can be exploited for therapeutic benefits. Cancer cells in advanced stage tumors frequently exhibit multiple genetic alterations and high oxidative stress, suggesting that it might be possible to preferentially modulate the development of these cells by controlling their ROS production. Low levels of ROS are also important for the development and survival of normal cells. In this manuscript, we present data on the influence of the suppression of the Earth's magnetic field (low level magnetic fields or LLF) which magnitudes range from 0.2 µT to 2 µT on the modulation of hydrogen peroxide (H2O2) in human fibrosarcoma cancer cell line HT1080, pancreatic AsPC-1 cancer cell line, and bovine pulmonary artery endothelial cells (PAEC) exposed to geomagnetic field (control; 45 µT–60 µT). Reduction of the Earth's magnetic field suppressed H2O2 production in cancer cells and PAEC. The addition of catalase and superoxide dismutase (SOD) mimetic MnTBAP inhibited the magnetic field effect. Modulating ROS production by magnetic fields may open new venues of biomedical research and therapeutic strategies.
Citation: Martino CF, Castello PR (2011) Modulation of Hydrogen Peroxide Production in Cellular Systems by Low Level Magnetic Fields. PLoS ONE 6(8): e22753. https://doi.org/10.1371/journal.pone.0022753
Editor: Ganesh Chandra Jagetia, Mizoram University, India
Received: February 22, 2011; Accepted: June 30, 2011; Published: August 26, 2011
Copyright: © 2011 Martino, Castello. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: Funding was provided by The Gordon Prize. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
Reactive oxygen species (ROS) specially de superoxide anion (O2.−) are short-lived species with half-lives of less than a nanosecond to several seconds  and are a consequence of the incomplete reduction of dioxygen by several system including the complex I and III of the mitochondrial respiratory chain, the enzyme Xantine oxidase, etc. They are linked to normal cellular events such as signaling, regulated growth, proliferation and programmed cell death, and have also been associated with certain forms of cell and tissue pathology , . During normal metabolic activity cells remain in a homeostatic state whereby their rate of ROS production is kept in balance through redox regulatory mechanisms that utilize several systems of antioxidant scavenging . ROS are subdivided into O2.−, H2O2, hydroxyl radicals (OH.) and singlet oxygen, molecules, and their source of production include the mitochondria, endoplasmic reticulum, plasma membrane and cytosol. ROS are a necessary biochemical component for the maintenance of healthy cells and tissues, however, in excess concentration they can be highly cytotoxic leading to cell and tissue damage . For example, in comparison to normal healthy cells, ROS have been implicated in the onset and progression of certain cancer types with the observation that cancer cells in particular demonstrate an increased synthesis of O2.− . This can be attributed to the increased oxidative stress load associated with increased metabolic activity, particularly at the level of the mitochondria, to sustain the increased rate of cell division, which is the hallmark of cancer. However, in order to maintain a healthy balance of intracellular ROS, organisms are equipped with the antioxidant enzymes: SOD, glutathione peroxidase (GPx), catalase, and thioredoxin reductase (TPx), which reduce these potentially cytotoxic molecules to a less harmful form .
O2.− is perhaps one of the most important free radicals in biology. This radical comes from the incomplete reduction of oxygen and several known enzymes catalyze it. O2.− contains an unpaired electron that makes it highly reactive and likely to be affected by the magnetic fields . Despite its high reactivity, O2.− is rapidly decomposed by SOD to form H2O2. The H2O2 can then be decomposed by catalase, GPx and TPx. H2O2 can also produce the highly reactive OH. in the presence of the Fe2+cation (Fenton reaction), and both OH. and O2.− can react with other molecules to form new radicals such as peroxides , . Given the fact that both H2O2 and O2.− can react with any kind of biological macromolecule, and their concentration is dependent on both, production and clearance, ROS homeostatic balance is a key factor in the regulation of biological processes that they control .
In our previous papers, we presented results on the inhibition of cancer cell growth rate by reducing the Earth magnetic field  and the response and function of endothelial cells to these LLF . In both experimental models, LLF effects were compared to the geomagnetic field (45 µT–60 µT). A possible mechanism of interaction between the biological systems and the magnetic fields is a process where free radical may be involved –. This mechanism has been suggested to be able to occur even for magnetic fields of environmental intensities , , and the observed low-level magnetic field effects by our group may be predicted by this model .
In this study we have investigated a mechanism by which LLF may modulate reactive oxygen species (ROS) production in cells thereby affecting their growth, proliferation and survival rate . Thus, modulating ROS production by magnetic fields may open new venues of biomedical research and therapeutic strategies including the role of ROS in certain aspects of tumor cell angiogenesis, programmed cell death , , and possible cell regeneration.
The purpose of this study was to examine the role that LLF plays on ROS production in cancer cells lines, primary endothelial cells, and pancreatic cells. This was achieved by measuring rates of H2O2 production using fluorometric methods. H2O2 can freely pass through cell membranes and can be detected outside the cell. The extracellular quantification of the production of H2O2 may be a direct measure of the production and decomposition of O2.− , . For the quantification of H2O2 we will use a method that has been successfully applied before . This method involves the oxidation of Amplex Red by H2O2 in the presence of horseradish peroxidase (HRP) to produce the fluorescent product resorufin.
Materials and Methods
Fibrosarcoma HT1080 and pancreatic AsPC-1 cancer cells (ATCC #CCL-121, #CCL-1682, Manassas, VA) were grown and maintained in EMEM (Eagle's Minimum Essential Medium) and RPMI 1640, respectively, supplemented by 10% Fetal Bovine Serum (ATCC, Manassas, VA). Bovine PAECs were maintained in a growth medium (D-Valine MEM medium) containing 10% Fetal Bovine Serum, 2% L-glutamine (Invitrogen). The cells were cultured in 75 cm2 flasks to expand cell number (Life Science Products, CO). After reaching confluence, the cells were seeded in 35 mm Petri dishes (BD Falcon). The volume of medium totaled 2 ml. Medium was then changed every two days. The cultures were incubated in a 5% CO2 atmosphere at 37°C in the same incubator (Fisher Scientific, Model 5). The temperature and CO2 levels were monitored daily and maintained at 37°C and 5%, respectively. The difference in temperature between the upper and lower areas of the incubator was 0.1°C (0.003°C/cm). All experiments were conducted in the same incubator. To control for location in the incubator and any associated electromagnetic noise or other spatial variation, the orientation of experimental and control cultures were periodically reversed. Cells were seeded and allowed to rest for twenty-four hours under the same magnetic background conditions, at which timed magnetic exposures began. This time is denoted as t0.
Magnetic Stimulating System
The background static magnetic field intensity inside the incubator used in these studies varied considerably as measured by a gauss meter (Walker Scientific, Model FGM 4D2N), which depends on the relative position in the room and the relative position of surrounding objects. The control group in this report is subjected to 45 µT static fields as follows. A tri-axial square Helmholtz coil consisting of 25 turns and 12.5 cm on each side separated by 6.25 cm implemented a uniform 45 µT static magnetic field perpendicular to the plane of growth of the cells (z-direction; x–y fields were canceled). The coils were driven by a power supply (HP 6205C Dual, Hewlett Packard, Palo Alto, CA) and resistive circuitry.
Static LLF were implemented by shielding the fields with μ-metal cylinder. In the μ-metal cylinder, the static fields inside registered between 0.5 µT–2 µT. Temperature measurements were made inside the cylinder and in the coils; the temperature difference was 0.1°C, which is consistent with the variation inside the incubator.
The background time-varying magnetic field was measured by induction with a sensor comprised of a set of square mutually perpendicular coils (Diameter 1 = 9.92×10−04 m, Diameter 2 = 9.70×10−04 m, Diameter 3 = 7.56×10−04 m, each with a thickness of 2×10−03 m and 200 turns, fed in twisted pair). The signal from the coils was amplified and filtered (10 KHz low pass filter) by a high impedance differential amplifier (OSP-1 Oscilloscope Preamplifier, Advanced Research instruments, Golden, CO, USA) and the spectrum up to 600 Hz was analyzed. The complete time-varying sensing system inherent noise was below 2.41×10−06 T*Hz. The field measurements obtained for each experimental environment are presented in Table 1.
Fluorometric Detection of H2O2 Production
H2O2 was measured using the horseradish peroxidase-linked Amplex Ultra Red™ (Invitrogen) fluorometric assay. Fibrosarcoma cells and endothelial cells were seeded at a concentration of 2.0×105 cells per 35 mm Petri dish and allowed to rest for 24 hours prior to experiment. Medium was aspirated and 2 ml of new medium was added containing 0.2 units/ml horseradish peroxidase and 100 µM Amplex UltraRed (AUR). By using these measurement conditions we were able to keep the linear response of the AUR system for incubation times longer than 36 hours. Resorufin fluorescence was followed by a Gemini fluorescence microplate reader (Molecular Devices, Sunnyvale, CA). H2O2 calibration curves with HRP-AUR in LLF do not show any difference compared to control, thus demonstrating that LLF do not interact with the detection system.
Statistical analysis was performed using 1-way ANOVA with a minimal confidence level of 0.05 for statistical significance. Each experiment in vitro was performed at least 3 times with a minimum of 3 samples per termination point, resulting in a total number of 6 samples for each experiment. The data shown constitutes a representative sample of the experiments performed.
LLF suppress H2O2 production in fibrosarcoma cancer cells
H2O2 was measured using the horseradish peroxidase-linked Amplex Ultra Red™ fluorometric assay (Invitrogen). LLF decrease ROS activity in fibrosarcoma cancer cells (Fig. 1; p<0.05). Levels of ROS activity decreased 30%, 25%, and 25% after 6 hr, 12 hr, and 24 hr exposure to LLF respectively. Catalase was added as control at a concentration of 40 units/ml. Addition of external catalase suppressed the geomagnetic field effects on H2O2 production; catalase brought GMF levels of H2O2 production to LLF levels (data not shown).
Effect of SOD mimetic on H2O2 production in fibrosarcoma cells
In order to verify that the H2O2 production in this particular cell line is dependent on the dismutation of O2.− anion by SOD, in a separate control, we assayed the effect of the magnetic fields in the presence of the ROS scavenger Manganese (III) tetrakis (4-benzoic acid) porphyrin chloride (MnTBAP) (Fig. 2, right panel). Adding a 200 nM concentration of MnTBAP to the cell medium did not alter the magnetic field's final effect on decreasing the H2O2 production. However, when 2 mM of MnTBAP was added to the cell medium, the level of H2O2 in the unexposed sample increased to a level identical to the levels observed as a result of magnetic field exposure (data not shown). In other words, production of H2O2 was identical between LLF and control by the addition of 2 mM MnTBAP. Also, the difference of H2O2 production of HT1080 cells exposed to 100 µT (Fig. 2, left panel) is greater than HT1080 cells exposed to 45 µT when normalized to each proper LLF group. These results combined suggest a dose dependence of H2O2 production on magnetic field intensity and MnTBAP concentration.
Left panel: production of H2O2 in HT1080 cancer cells (p<0.05) in the presence (100 µT) or in the absence (low level) of magnetic field. Right panel: Effect of the SOD mimetic MnTBAP on the H2O2 production induced by magnetic fields.
H2O2 production in pancreatic cancer cell become indistinguishable between LLF and geomagnetic field
Next we proceeded to investigate if H2O2 production differs between distinct cancer cells subjected to LLF. The same protocol as the previous experiment for HT1080 was used. Differences in H2O2 production become indistinguishable after 12 hr and 24 hr between the LLF and the control for pancreatic cancer line AsPC-1 (data not shown).
LLF also inhibits H2O2 production in endothelial cells
In view of the last results, production of H2O2 may differ between cancer cells and primary cells. Previous results showed that LLF inhibit growth rates of endothelial cells , which may be related to the suppression of H2O2. Specifically, LLF suppressed hydrogen peroxide production in endothelial cells after 8 hours and 24 hours of exposure (Fig. 3; p<0.05).
Targets for oxidative processes are molecular complexes that readily give up or acquire a single electron. The redox reactions, with simple transfer of electrons affect almost all complex biological processes and have profound effects on cell growth, proliferation, survival and the propagation of various pathological processes, to include cancer.
We have shown that exposure of fibrosarcoma cancer cells and primary endothelial cells to low level magnetic fields modulate H2O2 production and that the addition of free radical scavengers suppressed this magnetic field effect. Previous results on inhibition of growth rates of cancer cells and endothelial cells by reducing the Earth's magnetic field suggest that changes in H2O2 production may be a consequence of the magnetic field effect , . However, it is unknown whether the inhibition of growth observed is a result of biologic events that may involve cell cycle modulation, cell signaling or possibly the induction of apoptosis .
We suggest here that the magnetic sensitivity of cellular systems may involve superoxide. In recent reports, it has been hypothesized that superoxide radical may be involved in the magnetoreception of birds , . Low level fields may modulate the interconversion of singlet to triplet product yields involving the superoxide free radical , . Inhibition of growth rates by low level fields  together with suppression of H2O2 production provide evidence supporting the hypothesis presented in the introduction.
Singlet (S; antiparallel electron spins) and triplet (T; parallel electron spins) lie at the heart of the radical pair mechanism. There are a number of conditions to be met for a radical pair to respond to an external magnetic field , : a) the singlet and triplet states must have different chemical fates; b) the reactivity of the pair should be spin dependent; c) and magnetic hyperfine interactions should be present in one or both radicals, which drives the oscillations . The interconversion is also influenced by the interaction of the electron's spin and the magnetic field, which is known as the Zeeman interaction. Both radical partners contribute to the hyperfine interaction of the radical pair. Most organic radicals have individual coupling constants ranging from 1–10 G , . A free radical pair involving superoxide, with nuclear spin state zero, decreases the hyperfine interaction. In order to observe changes in reaction yields in the presence of the Earth's magnetic field, the hyperfine interaction between radical pairs should be in the same order of magnitude as the Zeeman interaction , , , thus making the possibility for the magnetic field effect.
Reducing the Earth's magnetic field inhibits H2O2 production in cellular systems. Suppression of growth rates of cancer and primary cells by LLF, as we previously reported , , may be linked to the modulation of H2O2 by magnetic fields. The addition of catalase suppress the increment of ROS production by its direct role in the decomposition of H2O2. Nevertheless, the effect of MnTBAP suppressing the effect of the normal magnetic field suggests that O2.− dimutation by SOD would be a possible point of intervention of the magnetic field with this particular system. The biological system and the relative change of H2O2 production at distinct static magnetic field intensity points out to a free radical mechanism. In this study we show that not all the cells types may share the same mechanism of ROS control by magnetic fields. Pancreatic cell are a clear example of that. Interestingly, most cancer cells undergo increased oxidative stress and ROS production due to excessive metabolic requirements to support an increased rate of cell division. It is reasonable to consider investigating a therapeutic that might target cancer cells through a free radical-mediated mechanism. Whether radio frequency exposure can influence growth rates and H2O2 production remains open and we propose to investigate it in future studies.
We would like to thank Distinguished Professor Frank Barnes for the useful discussions. We also thank Dr. Frank Sivo and Mr. Lucas Portelli for assisting in reviewing the manuscript and exposure systems, respectively.
Conceived and designed the experiments: CFM PC. Performed the experiments: CFM. Analyzed the data: CFM PC. Contributed reagents/materials/analysis tools: CFM PC. Wrote the paper: CFM.
- 1. Simko M (2007) Cell Type Specific Redox Status is Responsible for Diverse Electromagnetic Field Effects. Current Medicinal Chemistry 14: 1141–1152.M. Simko2007Cell Type Specific Redox Status is Responsible for Diverse Electromagnetic Field Effects.Current Medicinal Chemistry1411411152
- 2. Sheng KC, Pietersz GA, Tang CK, Ramsland PA, Apostolopoulos V (2010) Reactive oxygen species level defines two functionally distinctive stages of inflammatory dendritic cell development from mouse bone marrow. J Immunol 184: 2863–2872.KC ShengGA PieterszCK TangPA RamslandV. Apostolopoulos2010Reactive oxygen species level defines two functionally distinctive stages of inflammatory dendritic cell development from mouse bone marrow.J Immunol18428632872
- 3. Guyton KZ, Kensler TW (1993) Oxidative mechanisms in carcinogenesis. Br Med Bull 49: 523–544.KZ GuytonTW Kensler1993Oxidative mechanisms in carcinogenesis.Br Med Bull49523544
- 4. Fruehauf JP, Meyskens FL (2007) Reactive Oxygen Species: A Breath of Life or Death? Clinical Cancer Research 13: 789–794.JP FruehaufFL Meyskens2007Reactive Oxygen Species: A Breath of Life or Death?Clinical Cancer Research13789794
- 5. D'Autreaux B, Toledano MB (2007) ROS as signalling molecules: mechanisms that generate specificity in ROS homeostasis. Nat Rev Mol Cell Biol 8: 813–824.B. D'AutreauxMB Toledano2007ROS as signalling molecules: mechanisms that generate specificity in ROS homeostasis.Nat Rev Mol Cell Biol8813824
- 6. Trachootham D, Alexandre J, Huang P (2009) Targeting cancer cells by ROS-mediated mechanisms: a radical therapeutic approach? Nat Rev Drug Discov 8: 579–591.D. TrachoothamJ. AlexandreP. Huang2009Targeting cancer cells by ROS-mediated mechanisms: a radical therapeutic approach?Nat Rev Drug Discov8579591
- 7. Mannerling AC, Simko M, Mild KH, Mattsson MO (2010) Effects of 50-Hz magnetic field exposure on superoxide radical anion formation and HSP70 induction in human K562 cells. Radiat Environ Biophys. AC MannerlingM. SimkoKH MildMO Mattsson2010Effects of 50-Hz magnetic field exposure on superoxide radical anion formation and HSP70 induction in human K562 cells.Radiat Environ Biophys
- 8. Kellogg EW 3rd, Fridovich I (1975) Superoxide, hydrogen peroxide, and singlet oxygen in lipid peroxidation by a xanthine oxidase system. J Biol Chem 250: 8812–8817.EW Kellogg 3rdI. Fridovich1975Superoxide, hydrogen peroxide, and singlet oxygen in lipid peroxidation by a xanthine oxidase system.J Biol Chem25088128817
- 9. Kellogg EW 3rd, Fridovich I (1977) Liposome oxidation and erythrocyte lysis by enzymically generated superoxide and hydrogen peroxide. J Biol Chem 252: 6721–6728.EW Kellogg 3rdI. Fridovich1977Liposome oxidation and erythrocyte lysis by enzymically generated superoxide and hydrogen peroxide.J Biol Chem25267216728
- 10. Martino CF, Portelli L, McCabe K, Hernandez M, Barnes F (2010) Reduction of the Earth's magnetic field inhibits growth rates of model cancer cells. Bioelectromagnetic Society. CF MartinoL. PortelliK. McCabeM. HernandezF. Barnes2010Reduction of the Earth's magnetic field inhibits growth rates of model cancer cells.Bioelectromagnetic SocietyIn Press. In Press.
- 11. Martino CF, Perea H, Hopfner U, Ferguson VL, Wintermantel E (2010) Effects of weak static magnetic fields on endothelial cells. Bioelectromagnetics 31: 296–301.CF MartinoH. PereaU. HopfnerVL FergusonE. Wintermantel2010Effects of weak static magnetic fields on endothelial cells.Bioelectromagnetics31296301
- 12. Steiner UE, Ulrich T (1989) Magnetic field effects in chemical kinetics and related phenomena. Chemical Reviews 89: 51–147.UE SteinerT. Ulrich1989Magnetic field effects in chemical kinetics and related phenomena.Chemical Reviews8951147
- 13. Salikhov KM (1983) On the largest possible contribution from hyperfine interactions to the recombination probability of radical pairs. Chemical Physics 82: 163–169.KM Salikhov1983On the largest possible contribution from hyperfine interactions to the recombination probability of radical pairs.Chemical Physics82163169
- 14. Brocklehurst B (1976) Spin Correlation in Geminate Recombination of Radical Ions in Hydrocarbons .1. Theory of Magnetic-Field Effect. Journal of the Chemical Society-Faraday Transactions Ii 72: 1869–1884.B. Brocklehurst1976Spin Correlation in Geminate Recombination of Radical Ions in Hydrocarbons .1. Theory of Magnetic-Field Effect.Journal of the Chemical Society-Faraday Transactions Ii7218691884
- 15. Brocklehurst B, McLauchlan KA (1996) Free radical mechanism for the effects of environmental electromagnetic fields on biological systems. International Journal of Radiation Biology 69: 3–24.B. BrocklehurstKA McLauchlan1996Free radical mechanism for the effects of environmental electromagnetic fields on biological systems.International Journal of Radiation Biology69324
- 16. Eveson RW, Timmel CR, Brocklehurst B, Hore PJ, McLauchlan KA (2000) The effects of weak magnetic fields on radical recombination reactions in micelles. International Journal of Radiation Biology 76: 1509–1522.RW EvesonCR TimmelB. BrocklehurstPJ HoreKA McLauchlan2000The effects of weak magnetic fields on radical recombination reactions in micelles.International Journal of Radiation Biology7615091522
- 17. Timmel CR, Henbest KB (2004) A study of spin chemistry in weak magnetic fields. Philosophical Transactions of the Royal Society of London Series a-Mathematical Physical and Engineering Sciences 362: 2573–2589.CR TimmelKB Henbest2004A study of spin chemistry in weak magnetic fields.Philosophical Transactions of the Royal Society of London Series a-Mathematical Physical and Engineering Sciences36225732589
- 18. Fang J, Seki T, Maeda H (2009) Therapeutic strategies by modulating oxygen stress in cancer and inflammation. Adv Drug Deliv Rev 61: 290–302.J. FangT. SekiH. Maeda2009Therapeutic strategies by modulating oxygen stress in cancer and inflammation.Adv Drug Deliv Rev61290302
- 19. Na AR, Chung YM, Lee SB, Park SH, Lee M-S, et al. (2008) A critical role for Romo1-derived ROS in cell proliferation. Biochem Biophys Res Commun 369: 672–678.AR NaYM ChungSB LeeSH ParkM-S Lee2008A critical role for Romo1-derived ROS in cell proliferation.Biochem Biophys Res Commun369672678
- 20. Xia C, Meng Q, Liu LZ, Rojanasakul Y, Wang XR, et al. (2007) Reactive oxygen species regulate angiogenesis and tumor growth through vascular endothelial growth factor. Cancer Res 67: 10823–10830.C. XiaQ. MengLZ LiuY. RojanasakulXR Wang2007Reactive oxygen species regulate angiogenesis and tumor growth through vascular endothelial growth factor.Cancer Res671082310830
- 21. Castello PR, Drechsel DA, Day BJ, Patel M (2008) Inhibition of mitochondrial hydrogen peroxide production by lipophilic metalloporphyrins. J Pharmacol Exp Ther 324: 970–976.PR CastelloDA DrechselBJ DayM. Patel2008Inhibition of mitochondrial hydrogen peroxide production by lipophilic metalloporphyrins.J Pharmacol Exp Ther324970976
- 22. Castello PR, Drechsel DA, Patel M (2007) Mitochondria are a major source of paraquat-induced reactive oxygen species production in the brain. J Biol Chem 282: 14186–14193.PR CastelloDA DrechselM. Patel2007Mitochondria are a major source of paraquat-induced reactive oxygen species production in the brain.J Biol Chem2821418614193
- 23. Castello PR, David PS, McClure T, Crook Z, Poyton RO (2006) Mitochondrial cytochrome oxidase produces nitric oxide under hypoxic conditions: implications for oxygen sensing and hypoxic signaling in eukaryotes. Cell Metab 3: 277–287.PR CastelloPS DavidT. McClureZ. CrookRO Poyton2006Mitochondrial cytochrome oxidase produces nitric oxide under hypoxic conditions: implications for oxygen sensing and hypoxic signaling in eukaryotes.Cell Metab3277287
- 24. Ritz T, Wiltschko R, Hore PJ, Rodgers CT, Stapput K, et al. (2009) Magnetic Compass of Birds Is Based on a Molecule with Optimal Directional Sensitivity. Biophysical journal 96: 3451–3457.T. RitzR. WiltschkoPJ HoreCT RodgersK. Stapput2009Magnetic Compass of Birds Is Based on a Molecule with Optimal Directional Sensitivity.Biophysical journal9634513457
- 25. Solov'yov IA, Schulten K (2009) Magnetoreception through Cryptochrome May Involve Superoxide. Biophysical journal 96: 4804–4813.IA Solov'yovK. Schulten2009Magnetoreception through Cryptochrome May Involve Superoxide.Biophysical journal9648044813
- 26. Ritz T, Adem S, Schulten K (2000) A model for photoreceptor-based magnetoreception in birds. Biophys J 78: 707–718.T. RitzS. AdemK. Schulten2000A model for photoreceptor-based magnetoreception in birds.Biophys J78707718
- 27. Timmel CR, Till U, Brocklehurst B, McLauchlan KA, Hore PJ (1998) Effects of weak magnetic fields on free radical recombination reactions. Molecular Physics 95: 71–89.CR TimmelU. TillB. BrocklehurstKA McLauchlanPJ Hore1998Effects of weak magnetic fields on free radical recombination reactions.Molecular Physics957189
- 28. Cintolesi F, Ritz T, Kay CWM, Timmel CR, Hore PJ (2003) Anisotropic recombination of an immobilized photoinduced radical pair in a 50-mu T magnetic field: a model avian photomagnetoreceptor. Chemical Physics 294: 385–399.F. CintolesiT. RitzCWM KayCR TimmelPJ Hore2003Anisotropic recombination of an immobilized photoinduced radical pair in a 50-mu T magnetic field: a model avian photomagnetoreceptor.Chemical Physics294385399