Molecular Interactionsatthe Cellulose-Lignin Interface Exploredvia Molecular Simulation
Josh Vermaas, Gregg Beckham, and Michael Crowley
Fall 2019ACSNational Meeting San Diego, California
August28th,2019 NREL/PR-2700-74305
LigninIntroduction
• Ligninisanaromatic heteropolymerthatmakesup between15-40%ofthedry weightofterrestrialplants
• Largestsourceofrenewable aromatics
• Manypotentialindustrialuses • Implicatedasasignificant
driverofcellwallrecalcitrance
2
LignocelluloseSimulation
Vermaasetal.2015 3
LigninBindstoCellulose…
• Lignincoversroughlya quarterofthecellulose surface
• Doesitcoatthesurface evenly?
• Doesligninchemistry matter?
Vermaasetal.2015 4
…ParticularlyHydrophobicCelluloseFaces
• Prefershydrophobicfaces • Blockscellulasebinding • Canwequantifythethermodynamic
preferencebetweenfaces?
Vermaasetal.2015 5
TheManyFacesofCellulose
• Onlythe200faceis hydrophobic
• Otherprimaryfacesare hydrophilic
• Normalcellulosetwists • Infinitecellulosedoesnot
6
Fernandesetal.2015
200 110
1-10 010
TheNextExperiment
• Construct“infinite”sheetsof cellulosewithdifferentfaces exposedtosolution
• Placeligninderivedcompounds insolution
• Simulate for 200 ns • Determine binding free energies
ofcompoundstoindividualfaces
7
WhatCompounds? OH OH OH OH
O OH
R' R Gua
iaco
l (G) Ph
enol (H)
HO
β1
* *
HO
O O
ββ
* *
* *
OH
OH
O
OH
OH O O O
OH OH
O
Syring
ol (S)
OH
O O α-ester γ-ester Phenolics OH OH
O *
*
OH
OH OH Ca
tech
ol (C)
OH HO 4O5 HO
OH β5 OH
OH
* O HO
O HO O
OHHO
*
OH O
R R' R R' OH
* *
Chiralcenter
OH O OH
R
OH OH R' OH OH
Monolignols 5-5 αO4 & βO4 βO4 8
PotentialBindingMetricsforFreeEnergyDetermination
• Free energy isrelatedto probability
• Trajectoriescanbeusedto determinerelative probabilities
• Howdowedefineabound vs unbound state?
9
DistanceandContactMetrics, Compared
• Distance • LigninCenter of
masstocellulose centerofmass
• Veryeasytoexplain • Contactnumber
• Pairwisecontacts betweencellulose andlignin
• Isnotbiasedby ligninshape
Unb
ound
Bo
und
Unb
ound
10
CelluloseBindingforMonomers
• Hydrophobic200face predominantforbinding
• Ferulatepreferssolution • Fitswellwithits
physiologicalrole • 110facebetterfor
bindingthantheother OH OH OH OH OH OH OH OH OH OH OH MeO OMe OMe OH MeO MeO OMe OH MeO OH MeO MeO hydrophilicfaces
OH
O OH OH O O OH O OH
11
MultimericBinding
• 200>>110>010>1-10 • Planarligninmoleculeslike
estersdemonstrateparticular affinitytothe200face
• Comparisonsbetween stereoisomerssuggestlonger simulationmaybebeneficial
12
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1
2
3
4
5
6
CelluloseFace-DependentBindingRelationships
100
G
bindi
ng
(kT
) 0
-
-
-
-
-
-r2 = 0.77 r2 = 0.56 r2 = 0.44
P henols Alcohols Carboxylates ↵O4& O4 O4
55 4O5 Esters 1 5
-2 0 -2 0 -2 0 110 Gbinding (kT) 1-10 Gbinding (kT) 010 Gbinding (kT) 13
14
HowDoesBindingScalewithPolymerSize?
• Monomersanddimersarenot perfectmodelsforreallignin
• Drawfromestablishedlignin polymerlibraries
• Determine ligninbinding affinity
UnbiasedLigninContactstoCellulose
15
TechnicalChallenges
• Bindingisspontaneousand irreversibleontypicalMD timescalesforlargerlignin
• Usereplicaexchangeumbrella samplingonacontact-number reactioncoordinatetoestimate binding free energy
16
SurfaceAreaComparison
• Contactnumberasdefinedisa strongproxyforcontactsurface areabetweenligninandcellulose
16.0 kDa lignin polymer 1200 200
110
1-10 1000 010
800
600
400
200
0
0 500 1000 1500 A2) 17 Lignin-Cellulose Contact Area (˚
Lig
nin
-Cel
lulo
se C
onta
cts
BindingFreeEnergyProfiles
18
SizeDependenceofLignin-CelluloseInteractions
• Bindingfreeenergygrows withthelogarithmofthe ligninmass
• Trendisweak, highly inconsistent
19
• Bindingfreeenergygrows directlywiththenumberof contactspresentatthefree energy minimum
• Slopesareconsistentacross faces
CelluloseFace 200 0.0111 110 0.0083 1-10 0.0107 010 0.0107
Summary
• Bindingstrengthquantificationcaninformligninmodification • Ligninrelatedcompoundsprefer tobindtohydrophobic cellulose
facesbyaboutafactorof2 • Mayberelatedtothe“flatness”ofthe200face, whichfits
wellwithplanararomaticrings, creatingmorenonspecific interactionsites
• Lignincontactnumber/contactareaismoreimportantthan polymersizeindeterminingbindingstrength • Ligninagglomerationisnotconsidered, butwouldlikely
additivelyincreasebindingstrength 20
GreggBeckham MichaelCrowley
Questions?
www.nrel.gov
This work was authored by the National Renewable Energy Laboratory, operated by Alliance for Sustainable Energy, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding provided by the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy Bioenergy Technologies Office. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes.
