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Topics of Environmental Sciences
CHAPTER TWENTY THREE
Topics of Environmental Sciences Contents 23.1 Fate of Natural Isoprene Emissions 23.2 Nitrogen Fixing is Affected by Ocean Acidification 23.3 Nitric Oxide (NO) Plays an Important Role in the Global Eco-System 23.4 Methane Formation From CO2 by an Iron Catalyst References Further Reading
255 256 257 258 259 260
23.1 FATE OF NATURAL ISOPRENE EMISSIONS About 500 Tg of 2-methyl-1,3-butadiene (isoprene) is produced by deciduous trees each year. Isoprene oxidation in the atmosphere starts by addition of hydroxyl radicals (OH) to C1 or C4 (see Fig. 23.1.1A) in a ratio of 0.57 to produce two sets of distinct allylic radicals (see Fig. 23.1.1B and C). Oxygen (O2) adds to these allylic radicals either δ (Z or E depending on the radicals of cis or trans) or β to the OH group, forming six distinct peroxy radical isomers. The allylic radicals then undergo bimolecular reactions with NO, O2, or HO2. Teng et al. (2017) used isomer-resolved measurements of the reaction products of the peroxy radicals to diagnose this complex chemistry by laboratory studies using an environmental chamber. They found that the ratio of δ to β hydroxy peroxy radicals depends on their bimolecular lifetime (τbimolecular). At τbimolecular 0.1 s, a transition occurs to thermodynamically controlled distribution at 297 K. As τbimolecular of nature is >10 s, the distribution of isoprene hydroxy peroxy radicals will be controlled by the stability of the peroxy radicals. The β hydroxy peroxy radical isomers comprise 95% of the radicals, which are not initially formed in such large proportion. However, previous studies used the thermodynamically controlled distribution.
Biochemistry for Materials Science https://doi.org/10.1016/B978-0-12-817054-0.00023-0
© 2019 Elsevier Inc. All rights reserved.
255
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Biochemistry for Materials Science
(A)
1-OH system
(B) 4-OH system
O2 OH β 4-OH, 3-OO
(C) Fig. 23.1.1 (A) 2-Methyl-1,3-butadiene (isoprene). (B) and (C). Two separate systems of peroxy radicals by OH addition to isoprene. 1-OH system (B) and 4-OH system (C).
23.2 NITROGEN FIXING IS AFFECTED BY OCEAN ACIDIFICATION An increase of carbon dioxide (CO2) in the surface ocean decreases the seawater pH. As result, nitrogen fixation, where nitrogen in air is converted into NH3, is affected because >50% of the nitrogen fixation is done by the cyanobacteria Trichodesmium. However, studies on how the CO2 increase in seawater affects nitrogen fixing have given contradicting results. The new results by Hong et al. (2017) indicated that higher CO2 levels increase Trichodesmium growth, resulting in higher nitrogen fixation. However, lower pH leads to lower growth of the bacterium, which is overridden
Topics of Environmental Sciences
257
on the net growth. Furthermore, the negative effect is amplified by the lack of nutrients, such as iron.
23.3 NITRIC OXIDE (NO) PLAYS AN IMPORTANT ROLE IN THE GLOBAL ECO-SYSTEM Substantial amounts of nitric oxide (NO) and nitrous oxide (N2O), which cause harmful environmental effects for large-scale agriculture, are emitted by ammonia-oxidizing bacteria (AOB). NO produces the ground-level ozone and acid rain. N2O is an ozone-depleting greenhouse gas with a global warming potential 300 times larger than that of CO2. The presently accepted model for AOB metabolism of Nitrosomonas europaea contains NH3 oxidation to nitrite (NO2 ) via a single obligate intermediate, hydroxylamine (NH2OH). NH3 + O2 + 2e + 2H + ➔NH2 OH + H2 O
(23.3.1)
In this model, the multiheme enzyme hydroxylamine oxidoreductase (HAO) catalyzes the four-electron oxidation of NH2OH to NO2. NH2 OH + H2 O➔NO2 + 4e + 5H +
(23.3.2)
However, Carantoa and Lancastera (2017) demonstrated evidence that HAO oxidizes NH2OH, which is made by (23.3.3), by only three electrons to NO under both anaerobic and aerobic conditions. NH2 OH➔NO + 3H + + 3e
(23.3.3)
NO2 observed in HAO activity analysis is a nonenzymatic product resulting from the oxidation of NO by O2 under aerobic conditions. NO + 1=2 O2 + e ➔NO2
(23.3.4)
The study of Carantoa and Lancastera (2017) suggested that aerobic NH3 oxidation by AOB occurs via two obligate intermediates, NH2OH and NO, necessitating a mediator of the third enzymatic step to NO2 (see Fig. 23.3.1). NO + H2 O➔NO2 + 2H + + e
(23.3.5)
In their model, the following are side reactions. NO + H + + e ➔1=2 N2 O + 1=2 H2 O NO + O2 + e ➔NO3
(23.3.6) (23.3.7)
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Fig. 23.3.1 The new proposed route to oxidize NH3 to NO2 by Carantoa and Lancastera (2017).
23.4 METHANE FORMATION FROM CO2 BY AN IRON CATALYST Converting CO2 into fuel or raw chemical materials could reduce fossil fuel consumption and climate-changing CO2 emissions. One strategy aims for electrochemical conversions powered by electricity from renewable sources, but photochemical approaches driven by sunlight are also possible. A considerable challenge in both approaches is the development of efficient and selective catalysts, ideally based on cheap and Earth-abundant elements rather than expensive precious metals. Of the molecular photo- and electrocatalysts reported, only a few catalysts are stable and selective for CO2 reduction; moreover, these catalysts produce primarily CO or HCOOH, and catalysts capable of generating even low to moderate yields of highly reduced hydrocarbons remain rare. Rao et al. (2017) developed an iron tetraphenylporphyrin complex functionalized with trimethylammonio groups (see Fig. 23.4.1A), which is the most efficient and selective molecular electro-catalyst for converting CO2 to CO known. The iron tetraphenylporphyrin complex could catalyze the eight-electron reduction of CO2 to methane upon visible light irradiation at ambient temperature and pressure. Rao et al. (2017) also found that the catalytic system (see Fig. 23.4.1C), operated in an acetonitrile solution containing a photosensitizer and sacrificial electron donor, remaining stable over several days. CO is the main product of the direct CO2 photoreduction reaction, but a two-pot procedure that first reduces CO2 and then reduces CO generates methane with a selectivity of up to 82% and a quantum yield (light-to-product efficiency) of 0.18%. Rao et al. (2017) thought that the operating principles of their system might aid the development of other molecular catalysts for the production of solar fuels from CO2 under mild conditions.
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Topics of Environmental Sciences
(A)
(B)
(C) Fig. 23.4.1 (A) Fe-p-TMA. (B) Ir(ppy)3. (C) The proposed mechanism for CO2 reduction to CH4 by catalyst (A).
REFERENCES Carantoa, J.D., Lancastera, K.M., 2017. Nitric oxide is an obligate bacterial nitrification intermediate produced by hydroxylamine oxidoreductase. PNAS 114, 8217–8222. Hong, H., Shen, R., Zhang, F., Wen, Z., Chang, S., et al., 2017. The complex effects of ocean acidification on the prominent N2-fixing cyanobacterium Trichodesmium. Science 356, 527–531. Rao, H., Schmidt, L.C., Bonin, J., Robert, M., 2017. Visible-light-driven methane formation from CO2 with a molecular iron catalyst. Nature 548, 74–77.
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Biochemistry for Materials Science
Teng, A.P., Crounse, J.D., Wennberg, P.O., 2017. Isoprene peroxy radical dynamics. J. Am. Chem. Soc. 139, 5367–5377.
FURTHER READING Asara, J.M., Schweitzer, M.H., Freimark, L.M., Phillips, M., Cantley, L.C., 2007. Protein sequences from mastodon and Tyrannosaurus rex revealed by mass spectrometry. Science 316, 280.
Topics of Environmental Sciences Contents 23.1 Fate of Natural Isoprene Emissions 23.2 Nitrogen Fixing is Affected by Ocean Acidification 23.3 Nitric Oxide (NO) Plays an Important Role in the Global Eco-System 23.4 Methane Formation From CO2 by an Iron Catalyst References Further Reading
255 256 257 258 259 260
23.1 FATE OF NATURAL ISOPRENE EMISSIONS About 500 Tg of 2-methyl-1,3-butadiene (isoprene) is produced by deciduous trees each year. Isoprene oxidation in the atmosphere starts by addition of hydroxyl radicals (OH) to C1 or C4 (see Fig. 23.1.1A) in a ratio of 0.57 to produce two sets of distinct allylic radicals (see Fig. 23.1.1B and C). Oxygen (O2) adds to these allylic radicals either δ (Z or E depending on the radicals of cis or trans) or β to the OH group, forming six distinct peroxy radical isomers. The allylic radicals then undergo bimolecular reactions with NO, O2, or HO2. Teng et al. (2017) used isomer-resolved measurements of the reaction products of the peroxy radicals to diagnose this complex chemistry by laboratory studies using an environmental chamber. They found that the ratio of δ to β hydroxy peroxy radicals depends on their bimolecular lifetime (τbimolecular). At τbimolecular 0.1 s, a transition occurs to thermodynamically controlled distribution at 297 K. As τbimolecular of nature is >10 s, the distribution of isoprene hydroxy peroxy radicals will be controlled by the stability of the peroxy radicals. The β hydroxy peroxy radical isomers comprise 95% of the radicals, which are not initially formed in such large proportion. However, previous studies used the thermodynamically controlled distribution.
Biochemistry for Materials Science https://doi.org/10.1016/B978-0-12-817054-0.00023-0
© 2019 Elsevier Inc. All rights reserved.
255
256
Biochemistry for Materials Science
(A)
1-OH system
(B) 4-OH system
O2 OH β 4-OH, 3-OO
(C) Fig. 23.1.1 (A) 2-Methyl-1,3-butadiene (isoprene). (B) and (C). Two separate systems of peroxy radicals by OH addition to isoprene. 1-OH system (B) and 4-OH system (C).
23.2 NITROGEN FIXING IS AFFECTED BY OCEAN ACIDIFICATION An increase of carbon dioxide (CO2) in the surface ocean decreases the seawater pH. As result, nitrogen fixation, where nitrogen in air is converted into NH3, is affected because >50% of the nitrogen fixation is done by the cyanobacteria Trichodesmium. However, studies on how the CO2 increase in seawater affects nitrogen fixing have given contradicting results. The new results by Hong et al. (2017) indicated that higher CO2 levels increase Trichodesmium growth, resulting in higher nitrogen fixation. However, lower pH leads to lower growth of the bacterium, which is overridden
Topics of Environmental Sciences
257
on the net growth. Furthermore, the negative effect is amplified by the lack of nutrients, such as iron.
23.3 NITRIC OXIDE (NO) PLAYS AN IMPORTANT ROLE IN THE GLOBAL ECO-SYSTEM Substantial amounts of nitric oxide (NO) and nitrous oxide (N2O), which cause harmful environmental effects for large-scale agriculture, are emitted by ammonia-oxidizing bacteria (AOB). NO produces the ground-level ozone and acid rain. N2O is an ozone-depleting greenhouse gas with a global warming potential 300 times larger than that of CO2. The presently accepted model for AOB metabolism of Nitrosomonas europaea contains NH3 oxidation to nitrite (NO2 ) via a single obligate intermediate, hydroxylamine (NH2OH). NH3 + O2 + 2e + 2H + ➔NH2 OH + H2 O
(23.3.1)
In this model, the multiheme enzyme hydroxylamine oxidoreductase (HAO) catalyzes the four-electron oxidation of NH2OH to NO2. NH2 OH + H2 O➔NO2 + 4e + 5H +
(23.3.2)
However, Carantoa and Lancastera (2017) demonstrated evidence that HAO oxidizes NH2OH, which is made by (23.3.3), by only three electrons to NO under both anaerobic and aerobic conditions. NH2 OH➔NO + 3H + + 3e
(23.3.3)
NO2 observed in HAO activity analysis is a nonenzymatic product resulting from the oxidation of NO by O2 under aerobic conditions. NO + 1=2 O2 + e ➔NO2
(23.3.4)
The study of Carantoa and Lancastera (2017) suggested that aerobic NH3 oxidation by AOB occurs via two obligate intermediates, NH2OH and NO, necessitating a mediator of the third enzymatic step to NO2 (see Fig. 23.3.1). NO + H2 O➔NO2 + 2H + + e
(23.3.5)
In their model, the following are side reactions. NO + H + + e ➔1=2 N2 O + 1=2 H2 O NO + O2 + e ➔NO3
(23.3.6) (23.3.7)
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Biochemistry for Materials Science
Fig. 23.3.1 The new proposed route to oxidize NH3 to NO2 by Carantoa and Lancastera (2017).
23.4 METHANE FORMATION FROM CO2 BY AN IRON CATALYST Converting CO2 into fuel or raw chemical materials could reduce fossil fuel consumption and climate-changing CO2 emissions. One strategy aims for electrochemical conversions powered by electricity from renewable sources, but photochemical approaches driven by sunlight are also possible. A considerable challenge in both approaches is the development of efficient and selective catalysts, ideally based on cheap and Earth-abundant elements rather than expensive precious metals. Of the molecular photo- and electrocatalysts reported, only a few catalysts are stable and selective for CO2 reduction; moreover, these catalysts produce primarily CO or HCOOH, and catalysts capable of generating even low to moderate yields of highly reduced hydrocarbons remain rare. Rao et al. (2017) developed an iron tetraphenylporphyrin complex functionalized with trimethylammonio groups (see Fig. 23.4.1A), which is the most efficient and selective molecular electro-catalyst for converting CO2 to CO known. The iron tetraphenylporphyrin complex could catalyze the eight-electron reduction of CO2 to methane upon visible light irradiation at ambient temperature and pressure. Rao et al. (2017) also found that the catalytic system (see Fig. 23.4.1C), operated in an acetonitrile solution containing a photosensitizer and sacrificial electron donor, remaining stable over several days. CO is the main product of the direct CO2 photoreduction reaction, but a two-pot procedure that first reduces CO2 and then reduces CO generates methane with a selectivity of up to 82% and a quantum yield (light-to-product efficiency) of 0.18%. Rao et al. (2017) thought that the operating principles of their system might aid the development of other molecular catalysts for the production of solar fuels from CO2 under mild conditions.
259
Topics of Environmental Sciences
(A)
(B)
(C) Fig. 23.4.1 (A) Fe-p-TMA. (B) Ir(ppy)3. (C) The proposed mechanism for CO2 reduction to CH4 by catalyst (A).
REFERENCES Carantoa, J.D., Lancastera, K.M., 2017. Nitric oxide is an obligate bacterial nitrification intermediate produced by hydroxylamine oxidoreductase. PNAS 114, 8217–8222. Hong, H., Shen, R., Zhang, F., Wen, Z., Chang, S., et al., 2017. The complex effects of ocean acidification on the prominent N2-fixing cyanobacterium Trichodesmium. Science 356, 527–531. Rao, H., Schmidt, L.C., Bonin, J., Robert, M., 2017. Visible-light-driven methane formation from CO2 with a molecular iron catalyst. Nature 548, 74–77.
260
Biochemistry for Materials Science
Teng, A.P., Crounse, J.D., Wennberg, P.O., 2017. Isoprene peroxy radical dynamics. J. Am. Chem. Soc. 139, 5367–5377.
FURTHER READING Asara, J.M., Schweitzer, M.H., Freimark, L.M., Phillips, M., Cantley, L.C., 2007. Protein sequences from mastodon and Tyrannosaurus rex revealed by mass spectrometry. Science 316, 280.