Editing Methylation

{{Short description|Chemical process in which a methyl (CH3) group is covalently attached to a molecule}}
{{Use dmy dates|date=March 2023}}
'''Methylation''', in the [[chemistry|chemical sciences]], is the addition of a [[methyl group]] on a [[substrate (chemistry)|substrate]], or the substitution of an atom (or group) by a methyl group. Methylation is a form of [[alkylation]], with a methyl group replacing a [[hydrogen#Compounds|hydrogen]] atom.  These terms are commonly used in [[chemistry]], [[biochemistry]], [[soil science]], and [[biology]].

In [[biological systems]], methylation is [[Catalysis|catalyzed]] by [[enzyme]]s; such methylation can be involved in modification of [[heavy metals]], regulation of [[gene expression]], regulation of [[Protein#Functions|protein function]], and [[RNA processing]]. ''In vitro'' methylation of tissue samples is also a way to reduce some [[histology#Histological Artifacts|histological staining artifacts]]. The reverse of methylation is [[demethylation]].

==In biology==
In biological systems, methylation is accomplished by enzymes.  Methylation can modify heavy metals and can regulate gene expression, RNA processing, and protein function. It is a key process underlying [[epigenetics]]. Sources of methyl groups include S-methylmethionine, methyl folate, methyl B12.<ref>{{cite book |doi=10.1016/S0083-6729(08)00410-X |chapter=Catalysis of Methyl Group Transfers Involving Tetrahydrofolate and B12 |title=Folic Acid and Folates |series=Vitamins & Hormones |date=2008 |last1=Ragsdale |first1=Stephen W. |volume=79 |pages=293–324 |pmid=18804699 |pmc=3037834 |isbn=978-0-12-374232-2 }}</ref>

===Methanogenesis===
[[Methanogenesis]], the process that generates methane from CO<sub>2</sub>, involves a series of methylation reactions. These reactions are caused by a set of enzymes harbored by a family of anaerobic microbes.<ref>Thauer, R. K., "Biochemistry of Methanogenesis: a Tribute to Marjory Stephenson", Microbiology, 1998, volume 144, pages 2377-2406.</ref>
[[File:Methanogenesis cycle.png|thumb|320px|Cycle for methanogenesis, showing intermediates]]
In reverse methanogenesis, methane is the methylating agent.<ref>{{cite journal |doi=10.1155/2017/1654237 |title=Reverse Methanogenesis and Respiration in Methanotrophic Archaea |date=2017 |last1=Timmers |first1=Peer H. A. |last2=Welte |first2=Cornelia U. |last3=Koehorst |first3=Jasper J. |last4=Plugge |first4=Caroline M. |last5=Jetten |first5=Mike S. M. |last6=Stams |first6=Alfons J. M. |journal=Archaea |volume=2017 |pages=1–22 |doi-access=free |pmid=28154498 |pmc=5244752 |hdl=1822/47121 |hdl-access=free }}</ref>

===O-methyltransferases===
{{Main|O-methyltransferase}}
A wide variety of [[phenols]] undergo O-methylation to give [[anisole]] derivatives.  This process, catalyzed by such enzymes as [[caffeoyl-CoA O-methyltransferase]], is a key reaction in the biosynthesis of [[Monolignol|lignols]], [[precursor (chemistry)|percursors]] to [[lignin]], a major structural component of plants.

Plants produce flavonoids and isoflavones with methylations on hydroxyl groups, i.e. [[methoxy group|methoxy bonds]]. This 5-O-methylation affects the flavonoid's water solubility. Examples are [[5-O-Methylgenistein|5-O-methylgenistein]], [[5-O-Methylmyricetin|5-O-methylmyricetin]], and [[5-O-Methylquercetin|5-O-methylquercetin]] (azaleatin).

===Proteins===
Along with [[Ubiquitin|ubiquitination]] and [[phosphorylation]], methylation is a major biochemical process for modifying protein function.  The most prevalent protein methylations affect arginine and lysine residue of specific histones. Otherwise histidine, glutamate, asparagine, cysteine are susceptible to methylation.  Some of these products include [[S-Methylcysteine|''S''-methylcysteine]], two isomers of ''N''-methylhistidine, and two isomers of ''N''-methylarginine.<ref>{{cite journal|doi=10.1074/jbc.AW118.003235|pmid=29743234|pmc=6036201|title=The ribosome: A hot spot for the identification of new types of protein methyltransferases|journal=Journal of Biological Chemistry|volume=293|issue=27|pages=10438–10446|year=2018|last1=Clarke|first1=Steven G.|doi-access=free}}</ref>

====Methionine synthase====
[[File:VitaminB12 2.png|thumb|right|The methylation reaction catalyzed by [[methionine synthase]]]]

[[Methionine synthase]] regenerates [[methionine]] (Met) from [[homocysteine]] (Hcy). The overall reaction transforms [[5-methyltetrahydrofolate]] (N<sup>5</sup>-MeTHF) into [[tetrahydrofolate]] (THF) while transferring a methyl group to Hcy to form Met. Methionine Syntheses can be cobalamin-dependent and cobalamin-independent: Plants have both, animals depend on the methylcobalamin-dependent form.

In methylcobalamin-dependent forms of the enzyme, the reaction proceeds by two steps in a ping-pong reaction. The enzyme is initially primed into a reactive state by the transfer of a methyl group from N<sup>5</sup>-MeTHF to Co(I) in enzyme-bound [[cobalamin]] ((Cob), also known as vitamine B12)) ,
, forming methyl-cobalamin(Me-Cob) that now contains Me-Co(III) and activating the enzyme. Then, a Hcy that has coordinated to an enzyme-bound [[zinc]] to form a reactive thiolate reacts with the Me-Cob. The activated methyl group is transferred from Me-Cob to the Hcy thiolate, which regenerates Co(I) in Cob, and Met is released from the enzyme.<ref name = Zinc>{{Cite journal | last1 = Matthews | first1 = R. G. | last2 = Smith | first2 = A. E. | last3 = Zhou | first3 = Z. S. | last4 = Taurog | first4 = R. E. | last5 = Bandarian | first5 = V. | last6 = Evans | first6 = J. C. | last7 = Ludwig | first7 = M. | doi = 10.1002/hlca.200390329 | title = Cobalamin-Dependent and Cobalamin-Independent Methionine Synthases: Are There Two Solutions to the Same Chemical Problem? | journal = Helvetica Chimica Acta | volume = 86 | issue = 12 | pages = 3939–3954 | year = 2003 }}</ref>

===Heavy metals: arsenic, mercury, cadmium===
Biomethylation is the pathway for converting some heavy elements into more mobile or more lethal derivatives that can enter the [[food chain]]. The [[biomethylation]] of [[arsenic]] compounds starts with the formation of [[methanearsonate]]s.  Thus, trivalent inorganic arsenic compounds are methylated to give methanearsonate. [[S-adenosylmethionine]] is the methyl donor.  The methanearsonates are the precursors to dimethylarsonates, again by the cycle of [[Redox|reduction]] (to methylarsonous acid) followed by a second methylation.<ref name=Cullen>{{cite journal|title=Comparative toxicity of trivalent and pentavalent inorganic and methylated arsenicals in rat and human cells|author1=Styblo, M. |author2=Del Razo, L. M. |author3=Vega, L. |author4=Germolec, D. R. |author5=LeCluyse, E. L. |author6=Hamilton, G. A. |author7=Reed, W. |author8=Wang, C. |author9=Cullen, W. R. |author10=Thomas, D. J. |journal=Archives of Toxicology|year=2000|volume=74|issue=6|pages=289–299|doi=10.1007/s002040000134|pmid=11005674|bibcode=2000ArTox..74..289S |s2cid=1025140}}</ref> Related pathways are found in the [[Mercury methylation#Microbial|microbial methylation]] of mercury to [[methylmercury]].

=== Epigenetic methylation ===
<!-- NOTE: "Epigenetic methylation" redirects here; if this section heading is removed or changed, add {{Anchor|Epigenetic methylation}} to another appropriate section. -->

====DNA methylation ====
[[DNA methylation]] is the conversion of the cytosine to [[5-methylcytosine]]. The formation of Me-CpG is [[Catalysis|catalyzed]] by the enzyme [[DNA methyltransferase]]. In vertebrates, DNA methylation typically occurs at [[CpG site]]s (cytosine-phosphate-guanine sites—that is, sites where a [[cytosine]] is directly followed by a [[guanine]] in the DNA sequence). In mammals, DNA methylation is common in body cells,<ref name="pmid19842073">{{cite journal| author=Tost J| title=DNA methylation: an introduction to the biology and the disease-associated changes of a promising biomarker. | journal=Mol Biotechnol | year= 2010 | volume= 44 | issue= 1 | pages= 71–81 | pmid=19842073 | doi=10.1007/s12033-009-9216-2 | s2cid=20307488 }}</ref> and methylation of CpG sites seems to be the default.<ref name="ReferenceC">{{cite journal | vauthors = Lister R, Pelizzola M, Dowen RH, Hawkins RD, Hon G, Tonti-Filippini J, Nery JR, Lee L, Ye Z, Ngo QM, Edsall L, Antosiewicz-Bourget J, Stewart R, Ruotti V, Millar AH, Thomson JA, Ren B, Ecker JR | title = Human DNA methylomes at base resolution show widespread epigenomic differences | journal = Nature | volume = 462 | issue = 7271 | pages = 315–22 | date = November 2009 | pmid = 19829295 | pmc = 2857523 | doi = 10.1038/nature08514 | bibcode = 2009Natur.462..315L }}</ref><ref>{{cite journal | vauthors = Stadler MB, Murr R, Burger L, Ivanek R, Lienert F, Schöler A, van Nimwegen E, Wirbelauer C, Oakeley EJ, Gaidatzis D, Tiwari VK, Schübeler D | title = DNA-binding factors shape the mouse methylome at distal regulatory regions | journal = Nature | volume = 480 | issue = 7378 | pages = 490–5 | date = December 2011 | pmid = 22170606 | doi = 10.1038/nature11086 | doi-access = free }}</ref> Human DNA has about 80–90% of CpG sites methylated, but there are certain areas, known as [[CpG site#CpG islands|CpG islands]], that are CG-rich (high cytosine and guanine content, made up of about 65% CG [[Residue (chemistry)|residues]]), wherein none is methylated. These are associated with the [[Promoter (genetics)|promoters]] of 56% of mammalian genes, including all [[Housekeeping gene|ubiquitously expressed genes]]. One to two percent of the human genome are CpG clusters, and there is an inverse relationship between CpG methylation and transcriptional activity. Methylation contributing to epigenetic inheritance can occur through either DNA methylation or protein methylation. Improper methylations of human genes can lead to disease development,<ref name = "Rotondo_2013">{{cite journal | vauthors = Rotondo JC, Selvatici R, Di Domenico M, Marci R, Vesce F, Tognon M, Martini F | title = Methylation loss at H19 imprinted gene correlates with methylenetetrahydrofolate reductase gene promoter hypermethylation in semen samples from infertile males | journal = Epigenetics | volume = 8 | issue = 9 | pages = 990–7 | date = September 2013 | pmid = 23975186 | pmc = 3883776 | doi = 10.4161/epi.25798 }}</ref><ref name = "Rotondo_2012">{{cite journal | vauthors = Rotondo JC, Bosi S, Bazzan E, Di Domenico M, De Mattei M, Selvatici R, Patella A, Marci R, Tognon M, Martini F | title = Methylenetetrahydrofolate reductase gene promoter hypermethylation in semen samples of infertile couples correlates with recurrent spontaneous abortion | journal = Human Reproduction | volume = 27 | issue = 12 | pages = 3632–8 | date = December 2012 | pmid = 23010533 | doi = 10.1093/humrep/des319 |url= https://academic.oup.com/humrep/article/27/12/3632/651064| doi-access = free | hdl = 11392/1689715 | hdl-access = free }}</ref> including cancer.<ref name="pmid27223861">{{cite journal |vauthors=Rotondo JC, Borghi A, Selvatici R, Magri E, Bianchini E, Montinari E, Corazza M, Virgili A, Tognon M, Martini F | title = Hypermethylation-Induced Inactivation of the IRF6 Gene as a Possible Early Event in Progression of Vulvar Squamous Cell Carcinoma Associated With Lichen Sclerosus | journal = JAMA Dermatology | volume = 152| issue = 8 | pages = 928–33 |date=2016 | pmid = 27223861 | doi = 10.1001/jamadermatol.2016.1336 }}</ref><ref name = "Rotondo_2018">{{cite journal | vauthors = Rotondo JC, Borghi A, Selvatici R, Mazzoni E, Bononi I, Corazza M, Kussini J, Montinari E, Gafà R, Tognon M, Martini F | title = Association of Retinoic Acid Receptor β Gene With Onset and Progression of Lichen Sclerosus-Associated Vulvar Squamous Cell Carcinoma | journal = JAMA Dermatology | volume = 154| issue = 7| pages = 819–823 | date = 2018 | pmid = 29898214| doi = 10.1001/jamadermatol.2018.1373| pmc = 6128494 }}</ref>

In [[Honey bee|honey bees]], DNA methylation is associated with alternative splicing and gene regulation based on functional genomic research published in 2013.<ref>{{cite journal |last1=Li-Byarlay |first1=Hongmei |last2=Li |first2=Yang |last3=Stroud |first3=Hume |last4=Feng |first4=Suhua |last5=Newman |first5=Thomas C. |last6=Kaneda |first6=Megan |last7=Hou |first7=Kirk K. |last8=Worley |first8=Kim C. |last9=Elsik |first9=Christine G. |last10=Wickline |first10=Samuel A. |last11=Jacobsen |first11=Steven E. |last12=Ma |first12=Jian |last13=Robinson |first13=Gene E. |title=RNA interference knockdown of DNA methyl-transferase 3 affects gene alternative splicing in the honey bee |journal=Proceedings of the National Academy of Sciences |date=30 July 2013 |volume=110 |issue=31 |pages=12750–12755 |doi=10.1073/pnas.1310735110 |doi-access=free |pmid=23852726 |pmc=3732956 |bibcode=2013PNAS..11012750L }}</ref> In addition, DNA methylation is associated with expression changes in immune genes when honey bees were under lethal viral infection.<ref>{{cite journal |last1=Li-Byarlay |first1=Hongmei |last2=Boncristiani |first2=Humberto |last3=Howell |first3=Gary |last4=Herman |first4=Jake |last5=Clark |first5=Lindsay |last6=Strand |first6=Micheline K. |last7=Tarpy |first7=David |last8=Rueppell |first8=Olav |title=Transcriptomic and Epigenomic Dynamics of Honey Bees in Response to Lethal Viral Infection |journal=Frontiers in Genetics |date=24 September 2020 |volume=11 |doi=10.3389/fgene.2020.566320 |doi-access=free |pmid=33101388 |pmc=7546774 }}</ref> Several review papers have been published on the topics of DNA methylation in social insects.<ref>{{cite journal |last1=Li-Byarlay |first1=Hongmei |title=The Function of DNA Methylation Marks in Social Insects |journal=Frontiers in Ecology and Evolution |date=19 May 2016 |volume=4 |doi=10.3389/fevo.2016.00057 |doi-access=free }}</ref><ref>{{cite book |doi=10.1016/bs.aiip.2015.06.002 |title=Physiological and Molecular Mechanisms of Nutrition in Honey Bees |series=Advances in Insect Physiology |date=2015 |last1=Wang |first1=Ying |last2=Li-Byarlay |first2=Hongmei |volume=49 |pages=25–58 |isbn=978-0-12-802586-4 }}</ref>

====RNA methylation====
'''RNA methylation''' occurs in different RNA species viz. [[tRNA]], [[rRNA]], [[mRNA]], [[tmRNA]], [[snRNA]], [[snoRNA]], [[miRNA]], and viral RNA. Different catalytic strategies are employed for RNA methylation by a variety of RNA-methyltransferases. RNA methylation is thought to have existed before DNA methylation in the early forms of life evolving on earth.<ref>{{cite journal |last1=Rana |first1=Ajay K. |last2=Ankri |first2=Serge |title=Reviving the RNA World: An Insight into the Appearance of RNA Methyltransferases |journal=Frontiers in Genetics |date=6 June 2016 |volume=7 |page=99 |doi=10.3389/fgene.2016.00099 |pmid=27375676 |pmc=4893491 |doi-access=free }}</ref>

[[N6-Methyladenosine|N6-methyladenosine (m6A)]] is the most common and abundant methylation modification in RNA molecules (mRNA) present in eukaryotes. 5-methylcytosine (5-mC) also commonly occurs in various RNA molecules. Recent data strongly suggest that m6A and 5-mC RNA methylation affects the regulation of various biological processes such as RNA stability and mRNA translation,<ref>{{cite journal |last1=Choi |first1=Junhong |last2=Ieong |first2=Ka-Weng |last3=Demirci |first3=Hasan |last4=Chen |first4=Jin |last5=Petrov |first5=Alexey |last6=Prabhakar |first6=Arjun |last7=O'Leary |first7=Seán E |last8=Dominissini |first8=Dan |last9=Rechavi |first9=Gideon |last10=Soltis |first10=S Michael |last11=Ehrenberg |first11=Måns |last12=Puglisi |first12=Joseph D |title=N6-methyladenosine in mRNA disrupts tRNA selection and translation-elongation dynamics |journal=Nature Structural & Molecular Biology |date=February 2016 |volume=23 |issue=2 |pages=110–115 |doi=10.1038/nsmb.3148 |pmid=26751643 |pmc=4826618 }}</ref> and that abnormal RNA methylation contributes to etiology of human diseases.<ref>{{cite web|last=Stewart|first=Kendal|title=Methylation (MTHFR) Testing & Folate Deficiency|date=15 September 2017|url=https://www.genomixnutrition.com/methylation-testing-s/2.htm|access-date=11 October 2017|archive-url=https://web.archive.org/web/20171012202147/https://www.genomixnutrition.com/methylation-testing-s/2.htm|archive-date=12 October 2017}}</ref>

In social insects such as honey bees, RNA methylation is studied as a possible epigenetic mechanism underlying aggression via reciprocal crosses.<ref>{{cite journal |last1=Bresnahan |first1=Sean T. |last2=Lee |first2=Ellen |last3=Clark |first3=Lindsay |last4=Ma |first4=Rong |last5=Rangel |first5=Juliana |last6=Grozinger |first6=Christina M. |last7=Li-Byarlay |first7=Hongmei |title=Examining parent-of-origin effects on transcription and RNA methylation in mediating aggressive behavior in honey bees (Apis mellifera) |journal=BMC Genomics |date=12 June 2023 |volume=24 |issue=1 |page=315 |doi=10.1186/s12864-023-09411-4 |doi-access=free |pmid=37308882 |pmc=10258952 }}</ref>

====Protein methylation====
[[Protein methylation]] typically takes place on [[arginine]] or [[lysine]] [[amino acid]] residues in the protein sequence.<ref>{{cite book |last1=Walsh |first1=Christopher |chapter=Protein Methylation |pages=121–149 |chapter-url={{GBurl|JGBfQXIzdwgC|p=121}} |title=Posttranslational Modification of Proteins: Expanding Nature's Inventory |date=2006 |publisher=Roberts and Company Publishers |isbn=978-0-9747077-3-0 }}</ref> {{anchor|Arginine methylation}}Arginine can be methylated once (monomethylated arginine) or twice, with either both methyl groups on one terminal nitrogen ([[asymmetric dimethylarginine]]) or one on both nitrogens (symmetric dimethylarginine), by [[protein arginine methyltransferases]] (PRMTs). Lysine can be methylated once, twice, or three times by [[lysine methyltransferases]]. Protein methylation has been most studied in the [[histone]]s. The transfer of [[methyl]] groups from [[S-adenosyl methionine]] to histones is catalyzed by enzymes known as [[histone methyltransferase]]s. Histones that are methylated on certain residues can act [[Epigenetics|epigenetically]] to repress or activate gene expression.<ref name="Grewal2004">{{cite journal |last1=Grewal |first1=Shiv IS |last2=Rice |first2=Judd C |title=Regulation of heterochromatin by histone methylation and small RNAs |journal=Current Opinion in Cell Biology |date=June 2004 |volume=16 |issue=3 |pages=230–238 |doi=10.1016/j.ceb.2004.04.002 |pmid=15145346 |url=https://zenodo.org/record/1258832 }}</ref><ref name="Nakayama2001">{{cite journal |last1=Nakayama |first1=Jun-ichi |last2=Rice |first2=Judd C. |last3=Strahl |first3=Brian D. |last4=Allis |first4=C. David |last5=Grewal |first5=Shiv I. S. |title=Role of Histone H3 Lysine 9 Methylation in Epigenetic Control of Heterochromatin Assembly |journal=Science |date=6 April 2001 |volume=292 |issue=5514 |pages=110–113 |doi=10.1126/science.1060118 |pmid=11283354 |bibcode=2001Sci...292..110N }}</ref> Protein methylation is one type of [[post-translational modification]].

===Evolution===
Methyl metabolism is very ancient and can be found in all organisms on earth, from bacteria to humans, indicating the importance of methyl metabolism for physiology.<ref name=Kozbial>{{cite journal |last1=Kozbial |first1=Piotr Z |last2=Mushegian |first2=Arcady R |title=Natural history of S-adenosylmethionine-binding proteins |journal=BMC Structural Biology |date=December 2005 |volume=5 |issue=1 |page=19 |doi=10.1186/1472-6807-5-19 |pmid=16225687 |pmc=1282579 |doi-access=free }}</ref> Indeed, pharmacological inhibition of global methylation in species ranging from human, mouse, fish, fly, roundworm, plant, algae, and cyanobacteria causes the same effects on their biological rhythms, demonstrating conserved physiological roles of methylation during evolution.<ref name=Fustin>{{cite journal |last1=Fustin |first1=Jean-Michel |last2=Ye |first2=Shiqi |last3=Rakers |first3=Christin |last4=Kaneko |first4=Kensuke |last5=Fukumoto |first5=Kazuki |last6=Yamano |first6=Mayu |last7=Versteven |first7=Marijke |last8=Grünewald |first8=Ellen |last9=Cargill |first9=Samantha J. |last10=Tamai |first10=T. Katherine |last11=Xu |first11=Yao |last12=Jabbur |first12=Maria Luísa |last13=Kojima |first13=Rika |last14=Lamberti |first14=Melisa L. |last15=Yoshioka-Kobayashi |first15=Kumiko |last16=Whitmore |first16=David |last17=Tammam |first17=Stephanie |last18=Howell |first18=P. Lynne |last19=Kageyama |first19=Ryoichiro |last20=Matsuo |first20=Takuya |last21=Stanewsky |first21=Ralf |last22=Golombek |first22=Diego A. |last23=Johnson |first23=Carl Hirschie |last24=Kakeya |first24=Hideaki |last25=van Ooijen |first25=Gerben |last26=Okamura |first26=Hitoshi |title=Methylation deficiency disrupts biological rhythms from bacteria to humans |journal=Communications Biology |date=6 May 2020 |volume=3 |issue=1 |page=211 |doi=10.1038/s42003-020-0942-0 |pmid=32376902 |pmc=7203018 |doi-access=free }}</ref>

==In chemistry==
The term methylation in [[organic chemistry]] refers to the [[alkylation]] process used to describe the delivery of a {{chem2|CH3}} group.<ref>{{cite book |doi=10.1002/9780470084960.ch13 |chapter=Aromatic Substitution, Nucleophilic and Organometallic |title=March's Advanced Organic Chemistry |date=2006 |pages=853–933 |isbn=978-0-471-72091-1 }}</ref>

===Electrophilic methylation===
Methylations are commonly performed using [[electrophile|''electrophilic'']] methyl sources such as [[iodomethane]],<ref>{{cite journal|last1=Vyas|first1=G. N.|last2=Shah|first2=N. M.|title=Quninacetophenone monomethyl ether |journal=[[Organic Syntheses]] |date=1951|volume=31|page=90|doi=10.15227/orgsyn.031.0090}}</ref> [[dimethyl sulfate]],<ref>{{cite journal|last1=Hiers|first1=G. S.|title=Anisole|journal=Organic Syntheses|date=1929|volume=9|page=12|doi=10.15227/orgsyn.009.0012}}</ref><ref>{{cite journal|last1=Icke|first1=Roland N.|last2=Redemann|first2=Ernst|last3=Wisegarver|first3=Burnett B.|last4=Alles|first4=Gordon A.|title=m-Methoxybenzaldehyde|journal=Organic Syntheses|date=1949|volume=29|page=63|doi=10.15227/orgsyn.029.0063}}</ref> [[dimethyl carbonate]],<ref>{{cite journal|last1=Tundo|first1=Pietro|last2=Selva|first2=Maurizio|last3=Bomben|first3=Andrea|title=Mono-C-methylathion of arylacetonitriles and methyl arylacetates by dimethyl carbonate: a general method for the synthesis of pure 2-arylpropionic acids. 2-Phenylpropionic acid|journal=Organic Syntheses|date=1999|volume=76|page=169|doi=10.15227/orgsyn.076.0169}}</ref> or [[tetramethylammonium chloride]].<ref>{{cite journal|last1=Nenad|first1=Maraš|last2=Polanc|first2=Slovenko|last3=Kočevar|first3=Marijan|title=Microwave-assisted methylation of phenols with tetramethylammonium chloride in the presence of K<sub>2</sub>CO<sub>3</sub> or Cs<sub>2</sub>CO<sub>3</sub>|journal=Tetrahedron|date=2008|volume=64|issue=51|pages=11618–11624|doi=10.1016/j.tet.2008.10.024}}</ref> Less common but more powerful (and more dangerous) methylating reagents include [[methyl triflate]],<ref>{{cite journal|last1=Poon|first1=Kevin W. C.|last2=Albiniak|first2=Philip A.|last3=Dudley|first3=Gregory B.|title=Protection of alcohols using 2-benzyloxy-1-methylpyridinium trifluoromethanesulfanonate: Methyl (R)-(-)-3-benzyloxy-2-methyl propanoate|journal=Organic Syntheses|date=2007|volume=84|page=295|doi=10.15227/orgsyn.084.0295}}</ref> [[diazomethane]],<ref>{{cite journal|last1=Neeman|first1=M.|last2=Johnson|first2=William S.|title=Cholestanyl methyl ether|journal=Organic Syntheses|date=1961|volume=41|page=9|doi=10.15227/orgsyn.041.0009}}</ref> and methyl fluorosulfonate ([[magic methyl]]). These reagents all react via S<sub>N</sub>2 [[nucleophilic substitution]]s. For example, a [[carboxylate]] may be methylated on oxygen to give a methyl [[ester]]; an [[alkoxide]] salt {{chem2|RO-}} may be likewise methylated to give an [[ether]], {{chem2|ROCH3}}; or a ketone [[enolate]] may be methylated on carbon to produce a new [[ketone]].

:[[Image:Iodomethane rxn1.png|class=skin-invert-image|350px|Methylation of a [[carboxylic acid]] salt and a [[phenol]] using [[iodomethane]]]]

The [[Purdie methylation]] is a specific for the methylation at oxygen of [[carbohydrate]]s using [[iodomethane]] and [[silver oxide]].<ref name="Purdie1903">{{Cite journal | last1 = Purdie | first1 = T. | last2 = Irvine | first2 = J. C. | doi = 10.1039/CT9038301021 | title = C.?The alkylation of sugars | journal = Journal of the Chemical Society, Transactions | volume = 83 | pages = 1021–1037 | year = 1903 | url = https://zenodo.org/record/2039403 }}</ref>
:[[Image:Purdie methylation.png|class=skin-invert-image|500px|Purdie methylation]]

===Eschweiler–Clarke methylation===
The [[Eschweiler–Clarke reaction]] is a method for methylation of [[amine]]s.<ref>{{cite journal|last1=Icke|first1=Roland N.|last2=Wisegarver|first2=Burnett B.|last3=Alles|first3=Gordon A.|title=β-Phenylethyldimethylamine|journal=Organic Syntheses|date=1945|volume=25|page=89|doi=10.15227/orgsyn.025.0089}}</ref> This method avoids the risk of [[quaternization]], which occurs when amines are methylated with methyl halides.
[[Image:Eschweiler-Clarke Reaction.svg|class=skin-invert-image|center|300px|The [[Eschweiler–Clarke reaction]] is used to methylate amines.]]

===Diazomethane and trimethylsilyldiazomethane===
[[Diazomethane]] and the safer analogue [[trimethylsilyldiazomethane]] methylate carboxylic acids, phenols, and even alcohols:
:<chem>RCO2H  +  tmsCHN2  +  CH3OH  -> RCO2CH3  +  CH3Otms  +  N2</chem>
The method offers the advantage that the side products are easily removed from the product mixture.<ref>{{cite encyclopedia|chapter=Trimethylsilyldiazomethane|vauthors=Shioiri T, Aoyama T, Snowden T|title=Encyclopedia of Reagents for Organic Synthesis|encyclopedia=e-EROS Encyclopedia of Reagents for Organic Synthesis|year=2001|doi=10.1002/047084289X.rt298.pub2|isbn=978-0-471-93623-7}}</ref>

===Nucleophilic methylation===
Methylation sometimes involve use of [[nucleophile|''nucleophilic'']] methyl reagents.  Strongly nucleophilic methylating agents include [[methyllithium]] ({{chem2|CH3Li}})<ref>{{cite journal|last1=Lipsky|first1=Sharon D.|last2=Hall|first2=Stan S.|title=Aromatic Hydrocarbons from aromatic ketones and aldehydes: 1,1-Diphenylethane|journal=Organic Syntheses|date=1976|volume=55|page=7|doi=10.15227/orgsyn.055.0007}}</ref> or [[Grignard reagent]]s such as [[methylmagnesium bromide]] ({{chem2|CH3MgX}}).<ref>{{cite journal|last1=Grummitt|first1=Oliver|last2=Becker|first2=Ernest I.|title=trans-1-Phenyl-1,3-butadiene|journal=Organic Syntheses|date=1950|volume=30|page=75|doi=10.15227/orgsyn.030.0075}}</ref> For example, {{chem2|CH3Li}} will add methyl groups to the [[carbonyl]] (C=O) of ketones and aldehyde.:

:[[Image:MeLi on acetone.png|class=skin-invert-image|250px|Methylation of [[acetone]] by [[methyl lithium]]]]

Milder methylating agents include [[tetramethyltin]], [[dimethylzinc]], and [[trimethylaluminium]].<ref>{{cite journal|last1=Negishi|first1=Ei-ichi|last2=Matsushita|first2=Hajime|title=Palladium-Catalyzed Synthesis of 1,4-Dienes by Allylation of Alkenyalane: α-Farnesene|journal=Organic Syntheses|date=1984|volume=62|page=31|doi=10.15227/orgsyn.062.0031}}</ref>

== See also ==
{{Portal|Chemistry|Biology}}

===Biology topics===
*[[Bisulfite sequencing]] – the biochemical method used to determine the presence or absence of methyl groups on a DNA sequence
*[[MethDB]] DNA Methylation Database
*[[Microscale thermophoresis]] – a biophysical method to determine the methylisation state of DNA<ref name=Wienken2>{{cite journal |vauthors=Wienken CJ, Baaske P, Duhr S, Braun D | title=Thermophoretic melting curves quantify the conformation and stability of RNA and DNA | journal=Nucleic Acids Research | year=2011 |  doi = 10.1093/nar/gkr035 | volume=39 | issue=8 | pages=e52 | pmid=21297115 | pmc=3082908}}</ref>
*[[Remethylation]], the reversible removal of methyl group in [[methionine]] and [[5-Methylcytosine|5-methylcytosine]]

===Organic chemistry topics===
*[[Alkylation]]
*[[Methoxy]]
*[[Organozinc compound#Titanium–zinc methylenation|Titanium–zinc methylenation]]
*[[Petasis reagent]]
*[[Nysted reagent]]
*[[Wittig reaction]]
*[[Tebbe's reagent]]

== References ==
{{reflist}}

== External links ==
{{Commons category|Methylation}}
{{Wiktionary}}
*[https://web.archive.org/web/20160418034750/http://www.detectorvision.com/deltaMasses/ deltaMasses] Detection of Methylations after Mass Spectrometry

{{Protein posttranslational modification}}

{{Authority control}}

[[Category:Methylation| ]]
[[Category:Epigenetics]]
[[Category:Organic reactions]]
[[Category:Post-translational modification]]