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The PAH World

Discotic polynuclear aromatic compounds as a mesophase scaffolding at theorigin of life

S. N. Platts

nickplatts{*at*}yahoo.com"... le hasard ne favorise que les esprits prpars " (chance only favors

prepared minds)!ouis Pasteur #$%%$'()The *PAH World* is a novel chemical structural model for the plausi+le formation

of oligomeric proto&informational templating materials on the early ,arth-presuma+ly progenitors of the idely e/pected RNA World in chemicalevolution theory. The model is +ased on the self&assem+ling discoticmesogenic +ehaviors of polynuclear aromatic compounds0 theirphotochemical edge&derivati1ations0 and the selectivity of such stac2edsupramolecular *aromatic core* scaffolds for the edge&on +inding and 3 4.56nm plane&parallel spacing of essentially random collections of smallpre+iotic heterocycles0 ta2en up and concentrated directly from thepresumed and surrounding *dilute primordial soup.* The constrained o+lig7)inter&+ase separation distance ould select for oligomeri1ing *lin2ers*enlan1antes) of fairly +astante) specific si1e0 such as small methanaloligomers0 hich ould also +e ta2en up from the pre+iotic chemicalenvironment0 condensing ith the small heterocycles and also ith eachother to form the fle/i+le structural +ac2+one of a first generation of proto&informational oligomeric material0 sta+ili1ed against +oth hydrolytic and

photolytic degradations +y its association ith the parent discoticmesophase. A transient local pH decrease e.g.0volcanic S8%aq.)) oulddisrupt the hydrogen +ond interactions anchoring the oligomer to thediscotic scaffold0 there+y releasing segments or portions of oligomericmaterial to e/plore intramolecular degrees of motional freedom out insolution0 perhaps folding plega+le) +ac2 on themselves to match upempare9ar) fortuitous +ase residue pairings via familiar Watson&:ric2&li2ecomplementarities. Segments rich in such chance complementarities ouldli2ely persist +y virtue of the com+ined cooperative strength deriving fromi.) the multiple intramolecular hydrogen +ond interactions +eteen paired+ases0 and ii.) the attractive pi&pi stac2ing van der Waals interactions

occurring +eteen neigh+oring and stac2ed +asepairs. This com+ination ofinteractions esta+lishing the domains of essentially hydropho+ic quasi&discotic mesophases in the secondary and tertiary structures of theseoligomeric proto&informational materials- +ase mismatchings malempare9adas) ithin such domains naturally leading to point replacementsselecting to minimi1e conformational potential energies. The ne *PAHWorld* model is pleasing to chemical intuition0 and provides the firstsatisfying structural anser to the pro+lem of a li2ely origin for thephenomenon of life.

The figures +elo provide the details for this model.

http://nobelprize.org/chemistry/articles/altman/index.htmlhttp://nobelprize.org/chemistry/articles/altman/index.html

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;igure#Diagram illustrating a stac2ed and self&assem+led mesophase&typearrangement of discotic polynuclear aromatic molecules0 in hich thearomatic cores of these molecules have +ecome associated and the systemhas self&organi1ed +y virtue of the ea2 inter&molecular van der Waalsforces of attraction operating +eteen the < molecular or+itals of nearest&neigh+or PAHs- these ea2 nett attractive forces often simply +eingtermed

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;igure % Barious e/emplar edge structures for partially derivati1ed PAH&typeandCor graphene&type polynuclear aromatic molecules. The edges areshon vertically don the righthand side el lado derecho) of each of the si/PAHCgraphene fragments illustrated0 and hydro/y functionalities have +eenadded into the edge structures at random0 simply to +e illustrative ofphotochemical andCor geohydrothermal derivati1ations at the edges ofpolynuclear aromatic species. 8ther types of organicfunctionali1ationCderivati1ation e.g.0 &:88H0 8) are certainly possi+le0especially given the 2non productive richness rique1a) of organic

photochemistry. The PAHCgraphene edges shon are partially hydro/ylatedand are thus also partially o/idi1ed relative to the parent aromatic

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hydrocar+ons. There are clearly many more possi+le polynuclear edgestructures than those +eing shon here0 and the variety of conceiva+leedge structures is further increased +y the various chemical functionalitiesthat are possi+le. All common edge functions e.g.0 hydro/yl0 2eto0car+o/ylic acid0 etc.) can engage captar0 engranar) in hydrogen +onding0

and the planar dispositions of such functions around the edges of individualPAHs arranged in stac2ed discotic arrays offers immediate potential for theedge&on vista lateral) +inding and ca. 4.56 nm plane&parallel spacing ofsmall sp%& hy+ridi1ed heterocyclic molecules0 moving firmly toards the firstphysicochemically plausi+le mechanism for the selection and concentrationof pre+iotic nucleo+ases and other similar small molecules out of the longpresumed dilute pre+iotic soup . The ne PAH World structural modeldirectly addresses the long recogni1ed dilution and selection pro+lemsinherent in all dilute pre+iotic soup models of the past0 these pro+lemshaving hitherto constituted to of the ma9or chemical trope1ones)stum+ling&+loc2s in the origins&of&life field during the no fifty&to years

since Stanley !. Eiller s +rea2through discoveries in pre+iotic chemistry.

;igure 5 This figure is purely diagrammatic and is simply meant to helpconvey the essential features of the ne PAH World model system. Threepre+iotic nucleo+ases are shon hydrogen +onded to hydro/y functions inthe edge structures of some derivati1ed and neigh+oring PAHtypemolecules0 hich are themselves disposed in a stac2ed and discotic array.The e/emplar polynuclear aromatic compound +eing envisaged previsto0imaginado) here is #05&dihydro/y he/a&peri&+en1ocoronene :6%H#$8% 0

#05&dihydro/y he/a+en1o@+c0ef0hi02l0no0qrcoronene0 H=:&diol ). Thehydro/y functionalities0 and their #05 dispositions0 are merely meant to +e

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illustrative of the general chemical principle of photochemical andCorhydrothermal derivati1ation at the edges of PAH&type and graphene&typemolecules. The three nucleo+ases are +eing shon in their 2eto tautomericforms0 so as to +e consistent ith the seemingly reasona+le assumptionthat pre+iotic Hadean i.e.0 F ca. 5.$( Ga) surface and rain aters ere

pro+a+ly acidulous. The 2eto tautomers +ear cargan) the pro&glycosidicpurine and pyrimidine secondary amine nitrogens i.e.0 the NH functions atN') and N#)0 respectively) disposed dispuestos) outards hacia afuera)0and thus chemically accessi+le to the surrounding solution. To unspecifiedpre+iotic oligomers of formaldehyde0 H:H8)n 0 are shon +eteenneigh+oring heterocyclics0 and it seems li2ely that the fairly +astante0equitativa) constrained inter+ase spacing espaciada) ould lead to thesystem selecting for oligomeri1ing lin2ers hich are themselves of similarsi1e. The derivati1ed PAHCgraphene edge structures can no effectively +eseen to represent something of an electronic phase +oundary lmite0frotera) +eteen the discotically arrayed aromatic cores of the PAHs and

the presumed surrounding aqueous media of Darin s arm little pondlaguna0 estanque) . The phenomenon of life might no +e seen as derivingfrom hat is effectively a phase separation +eteen the discotic aromaticcores of the PAHs versus the surrounding aqueous+ased phase. >t seemspro+a+le that the essentially regular %p1) inter&+asepair plane&parallelspacing distance of ca. 4.56 nm as inherited +y molecular +iology as achemical memory of this unique mode of origin0 and that this structuralmolecular fossil has +een conserved to us in the essentially quasi&discoticmesophase regions of heterocyclic nucleo+ase pairings in the secondaryand higher order structures of e/tant nucleic acids.

;igure 6 The pyrimidyl and imida1ole NH functions of the three nucleo+ases

are no shon condensed via N&glycosidic&type covalent lin2ages to thepresumed H:H8)n oligomers0 and the formaldehyde oligomers are

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themselves no shon having condensed together to produce an e/tendedoligomeric +ac2+one for the primitive proto&informational molecular material-al+eit that this earliest +ac2+one material is pro+a+ly li2ely to have +een+ranched in structure0 rather than straightforardly linear as depicted in thefigure. At this stage0 the nucleo+ase residues remain hydrogen +onded to

the stac2ed and scaffolding array of functionali1ed PAH&type molecules0and the co&operative effects of these multiple hydrogen +onds together iththe multiple

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might +e either totally freed of the PAHCgraphene stac2 altogether0 or mightperhaps remain tethered to the discotic stac2 at one or +oth ends via somesort of structural +ranchingCcross&lin2ing. 8nce the pH catastrophe hadpassed0 the levels of protonation of the heterocyclic +ase functions oulddecrease as the am+ient pH returned to normal0 and the sections of nely

released oligomer ould then +e free to fold +ac2 on themselves to +ringelectrically neutral heterocyclic +ase residues into hydrogen +ondingpro/imities0 perhaps to match up adventitious Watson&:ric2&li2ecomplementarities.

;igure J To representations of a single section of nely released proto&informational oligomeric material hich0 having +ecome dissociated from astac2ed and scaffolding array of derivati1ed polynuclear aromatic species0

has +een free to e/plore conformational degrees of freedom out in solution.The ca. 4.56 nm inter&heterocyclic +ase&residue plane&parallel separationconstraint0 structurally inherited from the discotic derivati1ed PAHCgraphenescaffold0 ould +e e/pected to readily favor the intra&molecular matching upi.e.0 hydrogen +onded +asepairing) of complementary small heterocyclic+ase residues0 as segments of protoinformational material came upon oneanother in the essentially +lind process of e/ploring their individual availa+leconformational parameter spaces. >n the ideali1ed and simplified e/ample+eing shon here0 involving a single molecule of an oligomer having anentirely linear i.e0 un+ranched +ac2+one) primary structure0 a single strandof nascent proto&informational oligomer is shon having folded +ac2 upon

itself0 hen it at once +ecomes intuitively conceiva+le that fortuitousWatson&:ric2&li2e +asepaired complementarities ould +e matched up.

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;urther0 that in the case of a single oligomeric molecule folding +ac2 onitself to form a stem&loop type secondary structural motif li2e that shonhere0 the +ac2+one chains of the presumed intramolecular proto&duple/&li2estructure ould thus +e set to run anti&parallel0 this +eing immediatelyreminiscent of a crucial and recurring secondary structural feature of e/tant

nucleic acids0 originating ith the famous anti&parallel chain +ac2+onesdescri+ed +y Watson K :ric2 in their classic #'(5 dou+le helical structuralsolution for the =&form of DNA0 and +eing very evident in the higher&orderstructures of ri+osomal LNA and transfer LNA molecules. Having formedfortuitous Watson&:ric2&li2e +asepairs0 it is entirely physicochemicallyconceiva+le that the co&operative reinforcing strengths of the multiple intra&molecular inter&heterocyclic hydrogen +onds and inter&+asepair n further regard to the etiology +orroing Al+ert ,schenmoser suse of this term) of the oligomeric +ac2+one0 it ould seem reasona+le that

the configurational loc2ing&in of stereochemical chiral informationalspecificities at asymmetric car+on centers in the earliest +ac2+onematerials could certainly have moved these early systems toardsminimi1ing the conformational potential energies of these first segments ofproto&informational oligomeric material- this chiral loc2 in configurationalparameter space presuma+ly representing the long&e/pected andcontingent fro1en accident of causation of +iological homochirality0 and thusan early chance chemical evolutionary step in the donstream direction ofsomething more nearly resem+ling a recogni1a+le progenitor of the idelye/pected LNA World milestone in origins&of&life science. Theconformational&potentialenergy& loering selection for the conservation of

complementarity&rich regions of protoinformational oligomeric materials mayell +e the single most important physicochemical and etiological factor

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toards a narroing don of the informational possi+ilities in terms of thesu+set of small pre+iotic nitrogenous heterocycles from hich aseventually to +e derived the +iological genetic code of nucleo+ase residuesfor all life on ,arth.

;igure M To&dimensional representation of some possi+le secondarystructures for portions of nascent proto&informational oligomeric materialnely released from attachment to discotic polynuclear aromaticfunctionali1ed PAHCgraphene scaffolding0 perhaps oing to a transient

decrease in the local pH resulting from a sudden volcanic release of anacidic o/ide e.g.0 S8%aq.)). The 1ig1ag lines represent oligomeric+ac2+one material e.g.0 H:H8)n )- the short straight lines representindividual pre+iotic heterocyclic residues oriented ith their 1&a/es in theplane of the paper i.e.0 individual +ase residues are oriented edge&on to thevieer)- and the longer straight lines represent hydrogen +onded pairs ofcomplementary +ase residues. Note that the edge&on perspective has +eenmaintained throughout for simplicity and ease of illustration. Whereas theprevious figure ;ig. J) shoed the ideali1ed case of a single molecule of alinear proto&informational oligomer0 here are shon pro+a+ly more li2elye/amples of +ranched +ac2+one primary oligomeric structures0 shoing

regions of +oth intra& and inter&molecular inter+ase hydrogen +ondingand

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+asepairs have matched&up oing to chance complementarities ofsequence. The essential planarity of the various +ase pairings0 com+inedith degrees of rotational freedom in the +ac2+one material itself0 has led tothe +asepairs +eing a+le to +ehave as quasi&discotic mesogens0 ith theconsequent formation of mesophase&li2e regions in hich sta+ili1ing nternational nion on Pure and Applied :hemistry >PA:) G.P Eoss0 >PA:nomenclature for fused&ring systems)0 are phenanthreneand anthracene0 hich+oth contain three fused aromatic rings. Smaller molecules0 such as +en1ene0are not PAHs.

PAHs may contain four&0 five&0 si/& or seven&mem+er rings0 +ut those ith five orsi/ are most common. PAHs composed only of si/&mem+ered rings are calledaltrnantPAHs. :ertain alternant PAHs are called "+en1enoid" PAHs. Thename comes from +en1ene0 an aromatic hydrocar+on ith a single0 si/&mem+ered ring. These can +e +en1ene rings interconnected ith each other +y

single car+on&car+on +onds and ith no rings remaining that do not contain acomplete +en1ene ring.

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The set of alternant PAHs is closely related to a set of mathematical entitiescalled polyhe/es0 hich are planar figures composed +y con9oining regularhe/agonsof identical si1e.

PAHs containing up to si/ fused aromatic rings are often 2non as "small"

PAHs and those containing more than si/ aromatic rings are called "large"PAHs. Due to the availa+ility of samples of the various small PAHs0 the +ul2 ofresearch on PAHs has +een of those of up to si/ rings. The +iological activityand occurrence of the large PAHs does appear to +e a continuation of the smallPAHs. They are found as com+ustion products0 +ut at loer levels than thesmall PAHs due to the 2inetic limitation of their production through addition ofsuccessive rings. Additionally0 ith many more isomers possi+le for largerPAHs0 the occurrence of specific structures is much smaller.

PAHs possess very characteristic B a+sor+ance spectra. These often possessmany a+sor+ance +ands and are unique for each ring structure. Thus0 for a set

of isomers0 each isomer has a different B a+sor+ance spectrum than theothers. This is particularly useful in the identification of PAHs. Eost PAHs arealso fluorescent0 emitting characteristic avelengths of light hen they aree/cited hen the molecules a+sor+ light). The e/tended pi&electron electronicstructures of PAHs lead to these spectra0 as ell as to certain large PAHs alsoe/hi+iting semi&conductingand other +ehaviors.

Li!"id cry#tal#!:s) are a state of matterthat has properties +eteen thoseof a conventional liquidand those of a solidcrystal.@#;or instance0 an !: mayflo li2e a liquid0 +ut its moleculesmay +e oriented in a crystal&li2e ay. Thereare many different types of !: phases0 hich can +e distinguished +y theirdifferent optical properties such as +irefringence). When vieed under amicroscope using a polari1ed light source0 different liquid crystal phases illappear to have distinct te/tures. The contrasting areas in the te/turescorrespond to domains here the !: molecules are oriented in differentdirections. Within a domain0 hoever0 the molecules are ell ordered. !:materials may not alays +e in an !: phase 9ust as atermay turn into ice orsteam).

!iquid crystals can +e divided into thermotropic0 lyotropic and metallotropicphases. Thermotropic and lyotropic !:s consist of organic molecules.

Thermotropic !:s e/hi+it a phase transitioninto the !: phase as temperatureischanged. !yotropic !:s e/hi+it phase transitions as a function of +othtemperature and concentration of the !: molecules in a solvent typicallyater). Eetallotropic !:s are composed of +oth organic and inorganicmolecules- their !: transition depends not only on temperature andconcentration0 +ut also on the inorganic&organic composition ratio.

,/amples of liquid crystals can +e found +oth in the natural orld and intechnological applications. Eost modern electronic displays are liquid crystal+ased. !yotropic liquid&crystalline phases are a+undant in living systems. ;ore/ample0 many proteins and cell mem+ranes are !:s. 8ther ell&2non !:

e/amples are solutions of soap and various related detergents0 as ell asto+acco mosaic virus

http://en.wikipedia.org/wiki/Polyhex_(mathematics)http://en.wikipedia.org/wiki/Hexagonhttp://en.wikipedia.org/wiki/Ultraviolet-visible_spectroscopyhttp://en.wikipedia.org/wiki/Isomerhttp://en.wikipedia.org/wiki/Fluorescenthttp://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/State_of_matterhttp://en.wikipedia.org/wiki/Liquidhttp://en.wikipedia.org/wiki/Solidhttp://en.wikipedia.org/wiki/Crystalhttp://en.wikipedia.org/wiki/Mesophase#cite_note-0%23cite_note-0http://en.wikipedia.org/wiki/Moleculeshttp://en.wikipedia.org/wiki/Opticshttp://en.wikipedia.org/wiki/Birefringencehttp://en.wikipedia.org/wiki/Microscopehttp://en.wikipedia.org/wiki/Polarization_(waves)http://en.wikipedia.org/wiki/Texture_(crystalline)http://en.wikipedia.org/wiki/Waterhttp://en.wikipedia.org/wiki/Thermotropichttp://en.wikipedia.org/wiki/Lyotropichttp://en.wikipedia.org/wiki/Organic_moleculeshttp://en.wikipedia.org/wiki/Phase_transitionhttp://en.wikipedia.org/wiki/Temperaturehttp://en.wikipedia.org/wiki/Temperaturehttp://en.wikipedia.org/wiki/Concentrationhttp://en.wikipedia.org/wiki/Solventhttp://en.wikipedia.org/wiki/Detergenthttp://en.wikipedia.org/wiki/Tobacco_mosaic_virushttp://en.wikipedia.org/wiki/Polyhex_(mathematics)http://en.wikipedia.org/wiki/Hexagonhttp://en.wikipedia.org/wiki/Ultraviolet-visible_spectroscopyhttp://en.wikipedia.org/wiki/Isomerhttp://en.wikipedia.org/wiki/Fluorescenthttp://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/State_of_matterhttp://en.wikipedia.org/wiki/Liquidhttp://en.wikipedia.org/wiki/Solidhttp://en.wikipedia.org/wiki/Crystalhttp://en.wikipedia.org/wiki/Mesophase#cite_note-0%23cite_note-0http://en.wikipedia.org/wiki/Moleculeshttp://en.wikipedia.org/wiki/Opticshttp://en.wikipedia.org/wiki/Birefringencehttp://en.wikipedia.org/wiki/Microscopehttp://en.wikipedia.org/wiki/Polarization_(waves)http://en.wikipedia.org/wiki/Texture_(crystalline)http://en.wikipedia.org/wiki/Waterhttp://en.wikipedia.org/wiki/Thermotropichttp://en.wikipedia.org/wiki/Lyotropichttp://en.wikipedia.org/wiki/Organic_moleculeshttp://en.wikipedia.org/wiki/Phase_transitionhttp://en.wikipedia.org/wiki/Temperaturehttp://en.wikipedia.org/wiki/Temperaturehttp://en.wikipedia.org/wiki/Concentrationhttp://en.wikipedia.org/wiki/Solventhttp://en.wikipedia.org/wiki/Detergenthttp://en.wikipedia.org/wiki/Tobacco_mosaic_virus

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Li!"id cry#tal $ha## The various !: phases called mesophases) can +echaracteri1ed +y the type of ordering. 8ne can distinguish positional orderhether molecules are arranged in any sort of ordered lattice) and orientationalorder hether molecules are mostly pointing in the same direction)0 andmoreover order can +e either short&range only +eteen molecules close to

each other) or long&range e/tending to larger0 sometimes macroscopic0dimensions). Eost thermotropic !:s ill have an isotropic phase at hightemperature. That is that heating ill eventually drive them into a conventionalliquid phase characteri1ed +y random and isotropic molecular ordering little tono long&range order)0 and fluid&li2e flo +ehavior. nder other conditions forinstance0 loer temperature)0 an !: might inha+it one or more phases ithsignificant anisotropicorientational structureand short&range orientational orderhile still having an a+ility to flo.@$@'

The ordering of liquid crystalline phases is e/tensive on the molecular scale.This order e/tends up to the entire domain si1e0 hich may +e on the order of

micrometers0 +ut usually does not e/tend to the macroscopic scale as oftenoccurs in classical crystalline solids. Hoever some techniques0 such as theuse of +oundaries or an applied electric field0 can +e used to enforce a singleordered domain in a macroscopic liquid crystal sample. The ordering in a liquidcrystal might e/tend along only one dimension0 ith the material +eingessentially disordered in the other to directions. @#4@##

M#o%nis the fundamental unit of a liquid crystalthat induces structural orderin the crystals.

Typically0 a liquid&crystalline molecule consists of a rigid moiety and one ormore fle/i+le parts. The rigid part aligns molecules in one direction0 hereas thefle/i+le parts induce fluidity in the liquid crystal. This rigid part is referred to asmesogen0 and it plays a crucial role in the molecule. The optimum +alance ofthese to parts is essential to form liquid&crystalline materials.

Di#cotic $ha## Dis2&shaped !: molecules can orient themselves in a layer&li2e fashion 2non as the discotic nematic phase. >f the dis2s pac2 intostac2s0 the phase is called a di#cotic col"mnar. The columnsthemselves may +e organi1ed into rectangular or he/agonal arrays.:hiral discotic phases0 similar to the chiral nematic phase0 are also

2non.

Naphthalene :#4H$ constituent of moth+alls)0 consisting of to coplanar si/&mem+ered rings sharing an edge0 is another aromatic hydrocar+on. =y formalconvention0 it is not a true PAH0 though is referred to as a +icyclic aromatichydrocar+on.

Aqueous solu+ility decreases appro/imately one order of magnitude for eachadditional ring.

Stac&in%in supramolecular chemistryrefers to a stac2ed arrangement of often

aromaticmolecules0 hich is adopted due to interatomic interactions. The mostcommon e/ample of a stac2ed system is found for consecutive +ase pairs in

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DNA. Stac2ing also frequently occurs in proteins here to relatively non&polarrings overlap. Which intermolecular forcescontri+ute to stac2ing is a matter ofde+ate.

Stac2ing is often referred to as '-' intraction0 though effects due to the

presence of a Q&or+ital are only one source of such interactions0 and in manycommon cases appear not to +e the dominant contri+utors.

>n supramolecular chemistry0 an aromatic intractionor '-' intraction) is anoncovalent interaction +eteen organic compounds containing aromaticmoieties. Q&Q interactions are caused +y intermolecular overlapping of p&or+italsin Q&con9ugated systems0 so they +ecome stronger as the num+er of Q&electrons increases. 8ther noncovalent interactions include hydrogen +onds0van der Waals forces0 charge&transfer interactions0 and dipole&dipoleinteractions.

Q&Q interactions act strongly on flat polycyclic aromatic hydrocar+onssuch asanthracene0 triphenylene0 and coronene +ecause of the many delocali1ed Q&electrons. This interaction0 hich is a +it stronger than other noncovalentinteractions0 plays an important role in various parts of supramolecularchemistry. ;or e/ample0 Q&Q interactions have a large influence on molecule&+ased crystal structures of aromatic compounds.

>n DNA0 $i #tac&in% occurs +eteen ad9acent nucleotides and adds to thesta+ility of the molecular structure. The nitrogenous +asesof the nucleotidesare made from either purine or pyrimidine rings0 consisting of aromatic rings.Within the DNA molecule0 the aromatic rings are positioned nearlyperpendicular to the length of the DNA strands. Thus0 the faces of the aromaticrings are arranged parallel to each other0 alloing the +ases to participate inaromatic interactions. Through aromatic interactions0 the pi +onds0 e/tendingfrom atoms participating in dou+le +onds0 overlap ith pi +onds of ad9acent+ases. This is a type of non&covalent chemical +ond. Though a non&covalent+ond is ea2er than a covalent +ond0 the sum of all pi stac2ing interactionsithin the dou+le&stranded DNA molecule creates a large net sta+ili1ing energy.

Gra$hn is a one&atom&thic2 gruesa) planar sheet lRmina) of sp%&+ondedcar+onatoms that are densely pac2ed in a honeycom+ crystal lattice enre9ado0

entramado). >t can +e visuali1ed as an atomic&scalechic2en ire made of car+on atoms and their +onds.The name comes from graphite &ene- graphiteitself consists of many graphene sheets stac2edtogether. The car+on&car+on +ond length ingraphene is a+out 4.#6% nm. Graphene is the +asicstructural element of some car+on allotropesincluding graphite0car+on nanotu+esandfullerenes.>t can also +e considered as an infinitely largearomaticmolecule0 the limiting case of the family offlat polycyclic aromatic hydrocar+onscalled graphenes.

http://en.wikipedia.org/wiki/DNAhttp://en.wikipedia.org/wiki/Intermolecular_forceshttp://en.wikipedia.org/wiki/Supramolecular_chemistryhttp://en.wikipedia.org/wiki/Molecular_orbitalhttp://en.wikipedia.org/wiki/Hydrogen_bondshttp://en.wikipedia.org/wiki/Van_der_Waals_forceshttp://en.wikipedia.org/wiki/Charge_transfer_complexhttp://en.wikipedia.org/wiki/Intermolecular_forcehttp://en.wikipedia.org/wiki/Intermolecular_forcehttp://en.wikipedia.org/wiki/Polycyclic_aromatic_hydrocarbonshttp://en.wikipedia.org/wiki/Anthracenehttp://en.wikipedia.org/wiki/Triphenylenehttp://en.wikipedia.org/wiki/Coronenehttp://en.wikipedia.org/wiki/Delocalized_electronhttp://en.wikipedia.org/wiki/Delocalized_electronhttp://en.wikipedia.org/wiki/Aromaticityhttp://en.wikipedia.org/wiki/DNAhttp://en.wikipedia.org/wiki/Nucleotideshttp://en.wikipedia.org/wiki/Nitrogenous_baseshttp://en.wikipedia.org/wiki/Purinehttp://en.wikipedia.org/wiki/Pyrimidinehttp://en.wikipedia.org/wiki/Aromatic_ringshttp://en.wikipedia.org/wiki/Pi_bondhttp://en.wikipedia.org/wiki/Chemical_bondhttp://en.wikipedia.org/wiki/Covalent_bondhttp://en.wikipedia.org/wiki/Sp2_bondhttp://en.wikipedia.org/wiki/Sp2_bondhttp://en.wikipedia.org/wiki/Sp2_bondhttp://en.wikipedia.org/wiki/Carbonhttp://en.wikipedia.org/wiki/Chicken_wire_(chemistry)http://en.wikipedia.org/wiki/Chicken_wire_(chemistry)http://en.wikipedia.org/wiki/Graphitehttp://en.wikipedia.org/wiki/-enehttp://en.wikipedia.org/wiki/Nanometerhttp://en.wikipedia.org/wiki/Allotropyhttp://en.wikipedia.org/wiki/Graphitehttp://en.wikipedia.org/wiki/Carbon_nanotubeshttp://en.wikipedia.org/wiki/Fullereneshttp://en.wikipedia.org/wiki/Aromaticityhttp://en.wikipedia.org/wiki/Polycyclic_aromatic_hydrocarbonshttp://en.wikipedia.org/wiki/DNAhttp://en.wikipedia.org/wiki/Intermolecular_forceshttp://en.wikipedia.org/wiki/Supramolecular_chemistryhttp://en.wikipedia.org/wiki/Molecular_orbitalhttp://en.wikipedia.org/wiki/Hydrogen_bondshttp://en.wikipedia.org/wiki/Van_der_Waals_forceshttp://en.wikipedia.org/wiki/Charge_transfer_complexhttp://en.wikipedia.org/wiki/Intermolecular_forcehttp://en.wikipedia.org/wiki/Intermolecular_forcehttp://en.wikipedia.org/wiki/Polycyclic_aromatic_hydrocarbonshttp://en.wikipedia.org/wiki/Anthracenehttp://en.wikipedia.org/wiki/Triphenylenehttp://en.wikipedia.org/wiki/Coronenehttp://en.wikipedia.org/wiki/Delocalized_electronhttp://en.wikipedia.org/wiki/Delocalized_electronhttp://en.wikipedia.org/wiki/Aromaticityhttp://en.wikipedia.org/wiki/DNAhttp://en.wikipedia.org/wiki/Nucleotideshttp://en.wikipedia.org/wiki/Nitrogenous_baseshttp://en.wikipedia.org/wiki/Purinehttp://en.wikipedia.org/wiki/Pyrimidinehttp://en.wikipedia.org/wiki/Aromatic_ringshttp://en.wikipedia.org/wiki/Pi_bondhttp://en.wikipedia.org/wiki/Chemical_bondhttp://en.wikipedia.org/wiki/Covalent_bondhttp://en.wikipedia.org/wiki/Sp2_bondhttp://en.wikipedia.org/wiki/Carbonhttp://en.wikipedia.org/wiki/Chicken_wire_(chemistry)http://en.wikipedia.org/wiki/Chicken_wire_(chemistry)http://en.wikipedia.org/wiki/Chicken_wire_(chemistry)http://en.wikipedia.org/wiki/Graphitehttp://en.wikipedia.org/wiki/-enehttp://en.wikipedia.org/wiki/Nanometerhttp://en.wikipedia.org/wiki/Allotropyhttp://en.wikipedia.org/wiki/Graphitehttp://en.wikipedia.org/wiki/Carbon_nanotubeshttp://en.wikipedia.org/wiki/Fullereneshttp://en.wikipedia.org/wiki/Aromaticityhttp://en.wikipedia.org/wiki/Polycyclic_aromatic_hydrocarbons

8/13/2019 The PAH World

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Allotro$yor allotro$i#mis a +ehavior e/hi+ited +y some chemical elements?these elements can e/ist in to or more different forms0 2non as allotropesofthat element. >n each allotrope0 the element*s atomsare +onded together in adifferent manner. Allotropes are different structural modifications of an element.@# Allotropes should not +e confused ith isomers0 hich are chemical

compoundsthat share the same molecular formula +ut have different structuralformulae. ;or e/ample0 car+on has 5 common allotropes? diamond0 here thecar+on atoms are +onded together in a tetrahedral lattice arrangement0graphite0 here the car+on atoms are +onded together in sheets of a he/agonallattice0 and fullerenes0 here the car+on atoms are +onded together inspherical0 tu+ular0 or ellipsoidal formations.

Ta"tomr# are isomers of organic compounds that readily interconvert +y achemical reaction called ta"tomri(ation. >t is common that this reactionresults in the formal migration of a hydrogenatomor proton0 accompanied +y asitch of a single +ond and ad9acent dou+le +ond. The concept of

tautomeri1ations is called ta"tomri#m. Tautomerism is a special case ofstructural isomerism and can play an important role in non&canonical +asepairingin DNAand especially LNAmolecules.

http://en.wikipedia.org/wiki/Chemical_elementhttp://en.wikipedia.org/wiki/Atomshttp://en.wikipedia.org/wiki/Chemical_bondhttp://en.wikipedia.org/wiki/Allotropy#cite_note-0%23cite_note-0http://en.wikipedia.org/wiki/Isomerhttp://en.wikipedia.org/wiki/Carbonhttp://en.wikipedia.org/wiki/Diamondhttp://en.wikipedia.org/wiki/Tetrahedralhttp://en.wikipedia.org/wiki/Graphitehttp://en.wikipedia.org/wiki/Fullereneshttp://en.wikipedia.org/wiki/Isomerhttp://en.wikipedia.org/wiki/Organic_compoundhttp://en.wikipedia.org/wiki/Chemical_reactionhttp://en.wikipedia.org/wiki/Hydrogenhttp://en.wikipedia.org/wiki/Atomhttp://en.wikipedia.org/wiki/Protonhttp://en.wikipedia.org/wiki/Single_bondhttp://en.wikipedia.org/wiki/Double_bondhttp://en.wikipedia.org/wiki/Structural_isomerismhttp://en.wikipedia.org/wiki/Base_pairhttp://en.wikipedia.org/wiki/Base_pairhttp://en.wikipedia.org/wiki/DNAhttp://en.wikipedia.org/wiki/RNAhttp://en.wikipedia.org/wiki/Chemical_elementhttp://en.wikipedia.org/wiki/Atomshttp://en.wikipedia.org/wiki/Chemical_bondhttp://en.wikipedia.org/wiki/Allotropy#cite_note-0%23cite_note-0http://en.wikipedia.org/wiki/Isomerhttp://en.wikipedia.org/wiki/Carbonhttp://en.wikipedia.org/wiki/Diamondhttp://en.wikipedia.org/wiki/Tetrahedralhttp://en.wikipedia.org/wiki/Graphitehttp://en.wikipedia.org/wiki/Fullereneshttp://en.wikipedia.org/wiki/Isomerhttp://en.wikipedia.org/wiki/Organic_compoundhttp://en.wikipedia.org/wiki/Chemical_reactionhttp://en.wikipedia.org/wiki/Hydrogenhttp://en.wikipedia.org/wiki/Atomhttp://en.wikipedia.org/wiki/Protonhttp://en.wikipedia.org/wiki/Single_bondhttp://en.wikipedia.org/wiki/Double_bondhttp://en.wikipedia.org/wiki/Structural_isomerismhttp://en.wikipedia.org/wiki/Base_pairhttp://en.wikipedia.org/wiki/Base_pairhttp://en.wikipedia.org/wiki/DNAhttp://en.wikipedia.org/wiki/RNA