Photodissociation 3219996 212827625 2008-05-16T13:22:54Z Scog 5779767 /* See also */ Make the capitalization consistent {{Refimprove|date=May 2007}} {{dablink|This article is about molecular photodissociation. The term "photodissociation" may also refer to nuclear photodissociation.}} '''Photodissociation''', '''photolysis''', or '''photodecomposition''' is a [[chemical reaction]] in which a [[chemical compound]] is broken down by [[photon]]s. Photodissociation is not limited to [[visible light]], but to have enough [[energy]] to break up a molecule, the photon is likely to be an [[electromagnetic wave]] with the energy of visible light or higher, such as [[ultraviolet light]], [[x-ray]]s and [[gamma ray]]s. The direct process is defined as the interaction of one or more photons interacting with one target molecule. ==Photolysis in photosynthesis== Photolysis is a part of the [[light-dependent reaction]]s of [[photosynthesis]]. The general reaction of photosynthetic photolysis can be given as: H<sub>2</sub>A + 2 photons (light) <math>\longrightarrow</math> 2e<sup>-</sup> + 2H<sup>+</sup> + A The chemical nature of "A" depends on the type of organism. For example in [[purple sulfur bacteria]], [[hydrogen sulfide]] (H<sub>2</sub>S) is oxidized to sulfur (S). In oxygenic photosynthesis, water (H<sub>2</sub>O) serves as a substrate for photolysis resulting in the generation of free oxygen (O<sub>2</sub>). This process is responsible for generating the majority of breathable oxygen in earth's atmosphere. Photolysis of water occurs in the [[thylakoid]]s of [[cyanobacterium|cyanobacteria]] and the [[chloroplast]]s of [[green algae]] and [[plant]]s. === Energy transfer models === The conventional, [[First quantization|semi-classical]], model describes the photosynthetic energy transfer process as one in which excitation energy hops from light-capturing pigment molecules to reaction center molecules step-by-step down the molecular energy ladder. The effectiveness of photons of different wavelengths depends on the absorption spectra of the [[photosynthetic pigment]]s in the organism. [[Chlorophyll]]s absorb light in the violet-blue and red parts of the spectrum, while [[accessory pigment]]s capture other wavelengths as well. The [[phycobilin]]s of red algae absorb blue-green light which penetrates deeper into water than red light, enabling them to photosynthesize in deep waters. Each absorbed photon causes the formation of an [[exciton]] (an electron excited to a higher energy state) in the pigment molecule. The energy of the exciton is transferred to a [[chlorophyll]] molecule ([[P680]], where P stands for pigment and 680 for its absorption maximum at 680 nm) in the reaction center of [[photosystem II]] via [[resonance energy transfer]]. P680 can also directly absorb a photon at a suitable wavelength. Photolysis during photosynthesis occurs in a series of light-driven oxidation events. The energized electron (exciton) of P680 is captured by a primary electron acceptor of the photosynthetic [[electron transfer chain]] and thus exits photosystem II. In order to repeat the reaction, the electron in the reaction center needs to be replenished. This occurs by oxidation of water in the case of oxygenic photosynthesis. The electron-deficient reaction center of photosystem II (P680*) is the strongest biological oxidizing agent known on earth, which allows it to break apart molecules as stable as water.<ref name="Campbell">{{cite book | last = [[Neil Campbell (scientist)|Campbell]]| first = Neil A. | coauthors = Reece, Jane B. | title = Biology, 7th Edition | publisher = Pearson - Benjamin Cummings | date = 2005 | location = San Francisco | pages = 186-191 | isbn = 0-8053-7171-0}}</ref> The water-splitting reaction is catalyzed by the [[oxygen evolving complex]] of photosystem II. This protein-bound inorganic complex contains four manganese ions, plus a calcium and chloride ion as cofactors. Two water molecules are complexed by the manganese cluster, which then undergoes a series of four electron removals (oxidations) to replenish the reaction center of photosystem II. At the end of this cycle, free oxygen (O<sub>2</sub>) is generated and the hydrogen of the water molecules has been converted to four protons released into the thylakoid lumen. These protons, as well as additional protons pumped across the thylakoid membrane coupled with the electron transfer chain, form a [[proton gradient]] across the membrane that drives [[photophosphorylation]] and thus the generation of chemical energy in the form of [[adenosine triphosphate]] (ATP). The electrons reach the [[P700]] reaction center of [[photosystem I]] where they are energized again by light. They are passed down another electron transfer chain and finally combine with the [[coenzyme]] NADP<sup>+</sup> and protons outside the thylakoids to [[NADPH]]. Thus, the net oxidation reaction of water photolysis can be written as: 2H<sub>2</sub>O + 2NADP<sup>+</sup> + 8 photons (light) <math>\longrightarrow</math> 2NADPH + 2H<sup>+</sup> + O<sub>2</sub> The free energy change (ΔG) for this reaction is 102 kilocalories per mole. Since the energy of light at 700 nm is about 40 kilocalories per mole of photons, approximately 320 kilocalories of light energy are available for the reaction. Therefore, approximately one-third of the available light energy is captured as NADPH during photolysis and electron transfer. An equal amount of ATP is generated by the resulting proton gradient. Oxygen as a byproduct is of no further use to the reaction and thus released into the atmosphere.<ref name="Raven">{{cite book | last = Raven | first = Peter H. | coauthors = Ray F. Evert, Susan E. Eichhorn | title = Biology of Plants, 7th Edition | publisher = W.H. Freeman and Company Publishers | date = 2005 | location = New York | pages = 115-127 | isbn = 0-7167-1007-2}}</ref> In 2007 a quantum model was proposed by Graham Fleming<ref name="QB">Gregory S. Engel Tessa R. Calhoun, Elizabeth L. Read, Tae-Kyu Ahn, Tomás caron Manc caronal, Yuan-Chung Cheng, Robert E. Blankenship and Graham R. Fleming, "Evidence for wavelike energy transfer through quantum coherence in photosynthetic systems", in Nature 446, 782-786 (12 April 2007)</ref>, which includes the possibility that photosynthetic energy transfer might involve quantum oscillations, explaining its unusually high efficiency. According to Fleming<ref name="QBC">http://www.physorg.com/news95605211.html Quantum secrets of photosynthesis revealed</ref> there is direct evidence that remarkably long-lived wavelike electronic quantum coherence plays an important part in energy transfer processes during photosynthesis, which can explain the extreme efficiency of the energy transfer because it enables the system to sample all the potential energy pathways, with low loss, and choose the most efficient one. ==Photolysis in the atmosphere== Photolysis also occurs in the atmosphere as part of a series of reactions by which primary [[pollutants]] such as [[hydrocarbons]] and [[nitrogen oxides]] react to form secondary pollutants such as [[peroxyacyl nitrates]]. See [[photochemical smog]]. The two most important photodissociaton reactions in the [[troposphere]] are firstly: :O<sub>3</sub> + h&nu; &rarr; O<sub>2</sub> + O(<sup>1</sup>D) &lambda; < 320 nm which generates an excited oxygen atom which can go on to react with water to give the [[hydroxyl radical]]: :O(<sup>1</sup>D) + H<sub>2</sub>O &rarr; 2OH The hydroxyl radical is central to [[atmospheric chemistry]] as it initiates the [[oxidation]] of [[hydrocarbons]] in the atmosphere and so acts like a detergent. Secondly the reaction: :NO<sub>2</sub> + h&nu; &rarr; NO + O is a key reaction in the formation of [[tropospheric ozone]]. The formation of the [[ozone layer]] is also caused by photodissociation. [[Ozone]] in the earth's [[stratosphere]] is created by ultraviolet light striking [[oxygen]] molecules containing two oxygen [[atom]]s (O<sub>2</sub>), splitting them into individual oxygen atoms (atomic oxygen); the atomic oxygen then combines with unbroken O<sub>2</sub> to create [[ozone]], O<sub>3</sub>. In addition, photolysis is the process by which [[CFC]]s are broken down in the upper atmosphere to form ozone-destroying chlorine [[free radicals]]. ==Astrophysics== In [[astrophysics]], photodissociation is one of the major processes through which molecules are broken down (but new molecules are being formed). Because of the [[vacuum]] of the [[interstellar medium]], [[molecule]]s and [[free radical]]s can exist for a long time. Photodissociation is the main path by which molecules are broken down. Photodissociation rates are very important in the study of the composition of [[interstellar clouds]] in which [[star]]s are formed. Typical examples of photodissociation in the interstellar medium are (<math>h\nu</math> is the scientific notation for light, specifically a [[photon]]): :<math>H_2O + h\nu \rightarrow H + OH</math> :<math>CH_4 +h\nu \rightarrow CH_3 + H</math> ==Multiple photon dissociation== In comparison to ultraviolet or other high energy photons, single photons in the [[infrared]] spectral range usually are not energetic enough for direct photodissociation of molecules. However, after absorption of multiple infrared photons a molecule may gain internal energy to overcome its barrier for dissociation. Multiple photon dissociation (MPD) can be achieved by applying high power lasers, e.g. a [[Carbon dioxide laser]], or a [[Free electron laser]], or by long interaction times of the molecule with the radiation field without the possibility for rapid cooling, e.g. by collisions. The latter method allows even for MPD induced by [[black body radiation]]. ==See also== *[[Flash photolysis]] *[[Photocatalysis]] *[[Photohydrogen]] *[[Photochemistry]] ==References== {{reflist}} [[Category:Chemical reactions]] [[Category:Astrophysics]] [[Category:Photosynthesis]] [[ca:Fotòlisi]] [[de:Photolyse]] [[es:Fotólisis]] [[fr:Photolyse]] [[it:Fotolisi]] [[he:פוטוליזה]] [[nl:Fotolyse]] [[pl:Fotodysocjacja]] [[pt:Fotólise]] [[ru:Фотодиссоциация]] [[zh:光解]]