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ONTHEPERFECTHEXAGONALPACKINGOFTUBESE.L.Starostin
Max-Planck-InstitutfurPhysikkomplexerSysteme,Dresden,GermanyE-mail:[email protected]
Abstract.Inmostcasesthehexagonalpackingoffibrousstructuresextremizestheenergyofinteractionbetweenstrands.Ifthestrandsarenotstraight,thenitisstillpossibletoformaperfecthexaticbundle.Conditionsunderwhichtheperfecthexag-onalpackingofcurvedtubularstructuresmayexistareformulated.Ofparticularin-terestareclosedbundleslikeDNAtoroidsorspools.Theclosureorreturncon-
straintsofthebundleresultinanallow-ablegroupofautomorphismsofthecross-sectionalhexagonallattice.Thestructureofthisgroupisexplored.Examplesofanopenhelical-likeandclosedtoroidal-likebundlesarepresented.ApossibleimplicationonacondensationofDNAintoroidsanditspackinginsideaviralcap-sidisbrieflydiscussed[1].
IntroductionItisknownthatthedensestpackingofin-finitestraightcylindersishexagonalwhenalltheiraxesareparallel[2].Itevi-dentlycorrespondstohexagonalpackingofdisksinaplane.Thehexagonalpack-ingoftubularobjectsoccursinnumerousinstancesatnanotomacroscale.Amongexamplestherearenanotubes[3],highdensitycolumnarhexaticliquidcrystallineDNAmesophases[4]andothers.Inmostcases,thispackingextremizestheinter-actionenergybetweenfilaments.Geo-metrically,itmeansthatallpairsofneigh-bouringaxesarelocatedatconstantdis-tancetoeachother.
Insomeinstances,thefilamentsarenotstraight.Then,thenaturalquestionarisesofwhetheritisstillpossibletoreachthesamemaximaldensityofpacking.Ifyes,thenthesecondquestioncanbeformu-latedas:whatisthesetofconfigurationsofinfinite(orclosed)tubesthathavethemaximaldensity?Byatube(oratubu-larneighbourhood)hereweunderstandthesetofallpointsinspacewhosedis-tancefromthesmoothaxialcurvedoesnotexceedtheconstantthicknessradius.Wecansetthescalebyfixingthisra-diusto1.Moreoverwewillassumethattheglobalcurvatureoftheaxesislessorequalto1.Thus,thetubescannotover-lap,buttheyareperfectlyflexible.Itwill
beshowninthefollowingthatthedensestpackingclassincludescurvilinearaxes,whichshouldberelativelyparallel.Thisim-pliesthatanarbitrarysmalltwistofoneaxisaroundanotherimmediatelydestroysthehexagonalpacking.
Acomplicatedstructureariseswhenthetubesareincontactwiththemselves.AnimportantexampleisacondensationofDNAintoroids[5,6,7]oraDNAarrange-mentinsideofviralcapsids[8,9,10].
ab
Thesepicturesaretakenfrom[7].TheyshowcryoelectronmicrographsofDNAtoroidswiththeplaneofthetoroid(col-umna)approximatelycoplanarwiththemicroscopeimageplane(top-view)and(columnb)orthogonaltothemicroscopeimageplane(edge-view).HexagonalpackingofDNAisclearlyseenontherightfigure(A).
UnconstrainedtubepackingWestartwithconsiderationofaperfecttubeofsomelengthwithaxisr0(s),sbe-ingthearclengthparametrization.Letthetubebeinacontinuouscontactwiththemaximalallowednumberofothertubesofthesamethickness.Thisnumberequals6[11],thusitmaybesaidthatthetubesarehexagonallypacked.Denotetheaxesoftheneighbouringtubesbyrj(s),j=1,...,6.Wecanchoosethesamepara-metrizationforallthetubessuchthatforeverysthepointsrj(s),j=1,...,6aretheclosesttothecentralaxisr0(s)andtheylieintheverticesofaregulartrian-gularlattice.
Thevectorfieldmj0(s)≡rj(s)−r0(s)isrelativelyparallel[12].Itimpliesthatthereisnotwistofvectorsmj0(s)about
thecentralaxis.Wecanaddmorelay-ersofthetubesinthesamemannerasfirstsixtubes.Proceedingthiswaywillal-lowustobuildabundleofparalleltubesthatfillupsomedomaininspace.Thehexagonalpackingprovidesthemaximaldensityinthisdomain.
Letusnowobtainanequationthatgov-ernsthepositionoftheneighbouringtubeforagivencentralaxis.Weomittheindexjforclarity.Since‖m‖=const(=2),wecanwritedm
ds=ω×m,(1)
andthevectorωmayberepresentedasω=ω1m+ω2T×m[11],wherewede-notebyT=dr
dsthetangenttothecentralaxis.
Bydefinition,thevectormconnectstheclosestpointsontwocurves,whichim-pliesm·T=0.Differentiatingthisequa-tionandfurthersubstituteeq.(1)fordm
ds,wecometoω2m2=m·dT
ds,or,withthehelpoftheSerret-Frenetequations,ω2m2=κm·N.whereNistheprincipalnormaltor(s)andκthecurvatureofthis
curve.Finally,theequationfortheorien-tationvectormcanbegiventheform
dm
ds=−κ(m·N)T.(2)
Thisisthemainequationthatdescribesthearrangementoftubesinthehexago-nallypackedbundle.
Remark.Inthecontinuumlimit,theelasticFrank-Oseenenergydensity
e=1
2K1[∇·T]2+
1
2K2[T·(∇×T)]2+
1
2K3[T×(∇×T)]2
reducestotheterm12K3κ2,withK3beingthebendingrigidity.
–2 –1 0 1 2
–20
2
0
2
4
6
8
10
12
EXAMPLE:aregularhelicalcurve:r(s)=(cosas,sinas,
√1−a2s),0≤a≤1.Equa-
tion(2)transformstothesystemdξ
ds=aη,
dη
ds=a(a2−1)ξ,
dmz
ds=a2√
1−a2ξ,(3)andthefirsttwocomponentsofthevectorm=(mx,my,mz)areexpressedasmx=ξcosas−ηsinas,my=ξsinas+ηcosas.Theexplicitsolutionofeq.(3)iseasytofind:
ξ=c1cosτs+c2sinτs,
η=√
1−a2(c2cosτs−c1sinτs),
mz=a(c1sinτs−c2cosτs),whereτ=a
√1−a2isthetorsionand
m2=c21+c2
2.Thefigureshowsabun-dleofsixtubesarrangedatconstantdis-tancefromthecentraltube(showninblue)andfromtheneighbours.Ateverysectionwhichisorthogonaltotheiraxesthecrossingpointsformthehexagonallattice.Thetubesareshownthinnertoeaserepresentation.
CycledbundlesInthiswork,particularattentionisgiventoclosedbundlesofhexagonallypackedtubes.Theclosednessconditionimposesastrictconstraintonthewholestructure.Indeed,takeanorthogonalcross-sectionofthebundle.Then,westudythemap-pingofthe2Dhexagonallatticeinthecross-sectionontoitself.Theautomor-phismsthatpreserveboththedistancesandtheconnectivityformadiscreteinfi-nitegroup.
abc
def
Theautomorphismgroupofthelatticeisfinitelygeneratedbythefollowingsetoftransformations:1.anidentitymap,
2.translationsalongthelatticevectorse1ande2(fig.d),
3.arotationthrough13πaroundtheorigin
(figs.a,b,c),
4.arotationthrough23πaround
13(e1+e2)(fig.e),
5.arotationthroughπaround12e1(fig.f).
Thisallowsustocharacterizeallpossibleclosedhexagonallypackedbundles:thewrithingnumber[13]ofeachaxisthatreal-izesthemappingshouldequaln/6,wherenisinteger.Herearetheexamples:
Theperfectlypackedbundlemadeupoftwoclosedtubes(cf.case(a)).Thecore(dark)makesoneturnandthesecondtubewindssixtimes.Thecolourvaria-tioncodesthearclength.Thetubesareshownthinnertoeaserepresentation.
Theperfectlypackedbundlethatcorre-spondstocase(f).
Undercertainconditions,theDNAtoroidsmaydeformtakingonawarpedshape[14].Generally,thisdeformationaffectstheinterstranddistancesandtheinteractionenergybetweenstrands.Thiseffectmayinfluencethetwist-bendinsta-bilityoftheDNAcondensates[15].AperfectlypackedstructuremayhaveitsoverallshapethatcloselyresemblesthewarpedDNAtoroids.
Left.Theperfectlypackedbundlemadeupoffourclosedtubes(cf.case(e)).Thetubesareshownthinnertoeaserepre-sentation.Right.TheDNAbundlelooksliketwistedskeinofyarn(anelectronmi-crographfrom[14]).
ConcludingremarksItisshownthatcurvilineartubularfila-mentsmaybepackedinthemostcom-pactwayinsomespatialdomain.Suchapackingextremizestheinteractionenergybetweenneighbouringfilamentsanditisonlypossibleifthebundleistwist-free.
Anequationisderivedthatgovernstherelativepositionsofthetubesinthehexagonalbundle.
Particularattentionisgiventocycledarrangementsofthetubes.Thecorre-spondingautomorphismsofthehexago-
nallatticeinthecross-sectionareclassi-fied.
Thecycledstructuresareonlypossibleifthewritheofthecentralaxisisn/6,nanin-teger.Theautomorphismgroupstructureforbidsacycledhexagonallypackedbun-dlemadeupwithasinglefilament:frus-trationisinevitable(cf.[16]).
Theresultsareofauniversalnatureandareapplicabletovariousfibrousstruc-tures,inparticular,toacondensationofDNAintoroidsaswellastoitspackinginsidetheviralcapsids.
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