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Dependence of Sum Frequency Generation (SFG) Spectral Features on Mesoscale Arrangement of SFG-Active Crystalline Domains Interspersed in SFG-Inactive Matrix: a Case Study With Cellulose in Uniaxially-Aligned Control Samples and Alkali-Treated Secondary Cell Walls of Plants Mohamadamin Makarem, Daisuke Sawada, Hugh M. O'Neill, Christopher M Lee, Kabindra Kafle, Yong Bum Park, Ashutosh Mittal, and Seong H. Kim J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 21 Apr 2017 Downloaded from http://pubs.acs.org on April 25, 2017

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The Journal of Physical Chemistry

Dependence of Sum Frequency Generation (SFG) Spectral Features on Mesoscale Arrangement of SFG-Active Crystalline Domains Interspersed in SFG-inactive Matrix: A Case Study with Cellulose in Uniaxially-Aligned Control Samples and Alkali-treated Secondary Cell Walls of Plants

Mohamadamin Makarem,1 Daisuke Sawada,2 Hugh M. O’Neill,2 Christopher M. Lee,1 Kabindra Kafle,1 Yong Bum Park,3 Ashutosh Mittal,4 and Seong H. Kim1* 1. Department of Chemical Engineering, Materials Research Insitute, Pennsylvania State University, University Park, PA, 16802, USA. 2. Biology & Soft Matter Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA. 3. Department of Biology, Pennsylvania State University, University Park, 16802, USA. 4. National Renewable Energy Laboratory, Biosciences Center, Golden, CO 80401, USA. * Corresponding author: [email protected]

Abstract: Vibrational sum frequency generation (SFG) spectroscopy can selectively detect not only molecules at two-dimensional (2D) interfaces but also noncentrosymmetric domains interspersed in amorphous three-dimensional (3D) matrices. However, the SFG analysis of 3D systems is more complicated than 2D systems because more variables are involved. One of such variables is the distance between SFG-active domains in SFG-inactive matrices. In this study, we fabricated control samples in which SFG-active cellulose crystals were uniaxially aligned in an amorphous matrix. Assuming uniform separation distances between cellulose crystals, the relative intensities of alkyl (CH) and hydroxyl (OH) SFG peaks of cellulose could be related to 1 ACS Paragon Plus Environment


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the inter-crystallite distance. The experimentally measured CH/OH intensity ratio as a function of the inter-crystallite distance could be explained reasonably well with a model constructed using the theoretically calculated hyperpolarizabilities of cellulose and the symmetry cancellation principle of dipoles antiparallel to each other. This comparison revealed physical insights into the inter-crystallite distance dependence of the CH/OH SFG intensity ratio of cellulose, which can be used to interpret the SFG spectral features of plant cell walls in terms of mesoscale packing of cellulose microfibrils.

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Introduction Hierarchical structures of cellulose and matrix polymers in plant cell walls play important roles in biological and mechanical functions of plant cells in various growth stages and their responses to external stimuli,1-4 and understanding the recalcitrance of lignocellulose biomass to chemical and biological degration processes.5-7 The hierarchy in plant cell wall structure spans multiple length scales – ranging from hydrogen bonding interactions between cellulose chains at the molecular level, to assembly of cellulose chains into microfibrils in the nanoscale, packing of celluloe microfibrils along with matrix polymers in the mesoscale (between 10’s nm to 100’s nm), cellular heterogeniety in the microscale (or at the tissue level), and all the way to phenotypes in the macroscale. It is critical to obtain accurate information at all length scales to test and validate numerous structural models proposed for plant cell walls and devise efficent schemes for conversion of lignocellulose biomass to useful chemicals.8-13 Among these length scales, the mesoscale structural characterization is the most challenging. The spectroscopic techniques such as infrared (IR), Raman, and nuclear magnetic resonance (NMR) analyze chemical structures at the molecular level.14-16 X-ray diffraction (XRD) can determine the crystalline order within the unit cell whose dimensions are on the order of nanometers.17-18 X-ray and neutron scattering can reveal recurring spatial orders up to ~100 nm.19-20 Optical imaging can provide structural information usually in the microscale, although some recent advancements in high resolution imaging pushes the limit to submicrons.21 Although scanning electron microscopy (SEM) and transmission electron microscopy (TEM) can be used to probe the mesoscale structure, they often require destructive sample preparation.14,

22-23

Recently, atomic force microscopy (AFM) has been demonstrated to probe physical structures of



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cell walls in the native hydrated state;24-25 but, AFM can image only the exposed surface and cannot provide information about the internal structure of cell walls. An important breakthrough in structural characterization of plant cell walls came from the demonstration of using sum frequency generation (SFG) vibrational spectroscopy for selective and nondestructive detection of “crystalline” cellulose microfibrils.26-28 As a nonlinear optical process, SFG requires noncentrosymmetry.29-31 Plants, bacteria, and tunicates synthesize cellulose into crystalline forms which are intrinsically noncentrosymmetric;32 thus, native crystalline celluloses are SFG-active.33 Other matrix components in plant cell walls are amorphous; thus, they are SFG-inactive. This allows SFG to detect crystalline cellulose in plant cell walls nearly free from interference from the matrix polymers. In addition, SFG can distinguish the polymorphic structures of cellulose.32, 34 Another important characteristic of SFG spectroscopy is that it requires the coherence of three waves with different wavelengths;30,

35

this means that the spatial arrangement of SFG-

acitve domains within the so-called coherence length is important. The coherence length in detection of cellulose in amorphous polymer matrices with SFG at a scattering or reflection angle is in the order of 100’s nm.32 This principle was used to explain spectral differences for primary cell walls and secondary cell walls of land plants and algae as well as bacterial cellulose and tunicates.32 Especially, SFG data compiled from various plant samples suggested that the relative intensities of CH peaks versus OH peaks vary with the degree of packing between cellulose microfibrils in cell walls. These examples include cotton fibers originated from different species,36 reaction woods,37 and pretreated lignocellulose biomass samples.11, 38 Although such empirical relationships between the CH/OH SFG intensity ratio and the mesoscale packing of


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cellulose microfibrils in plant cell walls were frequently documented in previous studies, the origin for such relationships is not clearly understood yet. In this paper, we address this missing link between the CH/OH SFG intensity ratio and the packing of crystalline cellulose microfibrils. The experimental design is schematically illustrated in Figure 1. Here, the control samples were prepared by mixing cellulose Iβ crystals (isolated from the tunicate) with a polymer matrix followed by stretching the mixture with a high draw ratio. Fibrin was used as the matrix since it is SFG-inactive and highly stretchable. Then, cellulose crystallites in the fibrin fiber were highly aligned along the draw direction (i.e., uniaxially aligned). Since cellulose crystallites were distributed randomly in the initial mixture, the overall polarity of crystallites in the drawn fiber must be random; in other words, the number of cellulose crystallites with reducing ends pointing to one direction must be the same as those pointing the opposite direction. This is equivalent to “antiparallel packing of crystallites” in average across the cross-section of the cellulose-fibrin composite fiber. Then, an average intercrystallite distance (d in Figure 1) could be estimated from the volumetric concentration of cellulose in the composite sample assuming the uniform distribution of cellulose crystals with uniform dimensions. Here, the experimentally-observed trend in the CH/OH SFG intensity ratio as a function of the estimated inter-crystallite distance (d) was compared with a simple model constructed taking into account the symmetry cancellation propensity of the CH and OH stretch modes found from quantum mechanical calculations for the uniaxial and antiparallel packing of cellulose crystallites.39 The CH/OH vs. d correlation found from the control sample study validated the previous interpretation of the SFG spectra of native plant cell walls32,

36, 38

and explained the

reason that the CH SFG intensity can be related to the fraction of crystalline cellulose in plant 5 ACS Paragon Plus Environment


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cell walls.27, 33,

36

Finally, the CH/OH vs. d correlation was used to estimate the polymorphic

conversion of cellulose microfibrils in plant cell walls upon treatment with high concentration NaOH solutions which is used for mercerization or biomass pretreatments.40-41

Figure 1. Schematic representation of SFG measurements using a fiber containing uniaxiallyaligned cellulose Iβ crystals interspersed in a polymer matrix. The arrows indicate the chain polarity of each crystal (represented with square rod). The fiber diameter is marked as c and the inter-crystallite distance is marked as d, which can be estimated from the volume fraction of cellulose. The fiber is aligned along the laser incidence plane.

Experimental Details To prepare uniaxially-aligned cellulose control samples, never-dried cellulose samples from tunicin were suspended in water and centrifuged into a pellet. The pellet was re-suspended in 4M HCl and the suspension was stirred for 4 hours. The resulting cellulose nanocrystals were washed several times with distilled water and then sonicated to disperse crystals. 100 mg of fibrinogen, purchased from Sigma Aldrich, was added to the suspension of cellulose Iβ nanocrystals which has 25, 12 and 7 mg of cellulose by dry weight and stirred for 20 minutes. 6 ACS Paragon Plus Environment

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Sodium chlorite was added to a concentration of 1.5%. A concentrated solution of thrombin, purchased from Sigma Aldrich, was then added and the mixture was immediately poured into a petri dish and left for several hours to coagulate at room temperature. Sections of approximately 2 cm2 in area and a ~3 mm thick were cut from the gel and stretched in one direction to about 3 times of their original length. When left to dry, the strips shrunk to ~1 mm thickness. 2D X-ray diffraction (XRD) patterns of cellulose-fibrin composite fibers were collected by the transmitting beam from a rotating anode X-ray generator, MicroMAX-007HF (RIGAKU) operated at 30mA and 40kV, using CuKα radiation (λ = 1.5418 Å). The samples were fixed by a block of clay on a goniometer perpendicular to the X-ray beam with a sample-to-detector distance of 100 mm. Azimuthal orientation angle was determined by fitting azimuthal intensity profiles with a Gaussian function using lmfit.42 The full width half maximum of the Gaussian function was used to evaluate the orientation angle. Avicel PH-101® was obtained from Sigma-Aldrich and was used as received. A cellulose II reference sample was produced by treating Avicel with ~4M NaOH as described previously.34 As a model biomass, a land plant stem and corn stover were analyzed. After 4 days of cold treatment at 4°C, Arabidopsis thaliana wild-type [Colombia (Col-0)] plants were grown on 1× MS medium16 containing 1% sucrose for 1 week, and transferred onto soil and grown under 70 µmol m-2s-1 light intensity (day/night: 16/8 h, temperature: 22/16°C) as described.

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Inflorescence stems of 8-week old Arabidopsis were ground in liquid nitrogen, incubated in 100% ethanol with exchanging twice for 24 h to remove chlorophylls, and rinsed three times with ddH2O. The ground Arabidopsis inflorescence walls were incubated in 4M NaOH containing 20 mM NaBH4 for 3 h at room temperature to mercerize the cellulose. The



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mercerized walls were neutralized, washed with excess of deionized water, and freeze-dried for SFG measurements. Corn stover internodes were harvested and extracted using the previously described protocol.43 Briefly, a majority of the xylan and lignin were removed using an organosolv pretreatment (0.57wt% H2SO4, 130°C for 1 h)44 followed by further delignification with 0.67wt% NaClO2 at 60°C for 2 hr.45 Then, the sample was washed with deionized water and dried in air. This extracted corn stover (5g) was mercerized by stirring in 100 g 6 M NaOH solution at 25 °C under nitrogen for 50 minutes. This solution was then further heated at 70 °C under a nitrogen cover at successive NaOH concentrations of 15, 12, 10, 8, 4, and 1 wt.% NaOH, each for 50 min. The sample was then extensively washed with deionized water at 70°C until the pH decreased to ~7 followed by freeze drying. XRD analyses of pristine and NaOH-treated samples were performed using a Rigaku (Tokyo, Japan) Ultima IV diffractometer with CuKα radiation having a wavelength λ(Kα1) = 0.15406 nm generated at 40 kV and 44 mA in reflection geometry. The diffraction intensities of freeze dried samples placed on a quartz substrate were measured in the range of 8 to 42° 2θ using a step size of 0.02° at a rate of 2°/min. A scanning SFG system equipped with a 27 ps Nd-YAG laser (10 Hz pulse repetition rate) and a broadband system equipped with a 85 fs Ti-Sapphire amplifier system (2kHz pulse repetition rate) were used in this study.26, 46 The fiber samples with uniaxially-aligned cellulose crystallites were analyzed with the fs-broadband system at two different detection angles – 0o and 90o from the 800 nm beam propagation direction (Figure 1).46 The fiber sample was mounted horizontally within the laser incidence plane. Since the 800 nm and broadband IR pulses were aligned to propagate nearly parallel 45o from the vertical direction, the 0o detection can be called 8 ACS Paragon Plus Environment

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as a transmission mode; then, the 90o detection is a reflection mode. Two polarization combinations (ppp and ssp) were used. Here, the three-letter combination of p and s represents the polarizations of SFG signal, input 800 nm, and input IR in series. The measured SFG signal was normalized with the incident 800 nm and IR pulse powers. The Arabidopsis inflorescence stem and corn stover samples were analyzed with the psscanning system.26 All spectra were acquired in the ssp polarization combination. SFG spectra were taken at 4 cm-1 per step in the CH stretching region (2700−3050 cm-1) and 8 cm-1 per step in the OH stretching region (3050−3800 cm-1), using 100 laser shots per step. The measured SFG signal was normalized with the incident 532 nm and IR pulse powers.

Results and Discusion 1. Confirmation of Uniaxial Alignment of Cellulose in Control Sample with 2D-XRD 2D-XRD patterns of three control samples with different cellulose contents (4.9%, 9.4%, and 17.2%) are shown in Figure 2. The fibers were placed in the vertical direction in the 2D image shown in Figure 2. The 110, 110, and (200) diffraction peaks of cellulose are clearly seen in the equatorial direction. This confirms the uniaxial alignment of cellulose crystals along the fiber direction. Three sharp rings at high degree are due to sodium chlorite in the sample. The broad scattering ring close to the center is fibrin.



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Figure 2. 2D-XRD patterns of cellulose-fibrin composite fibers. The cellulose content in the fiber is (a) 4.9%, (b) 9.4%, and (c) 17.2%. The rocking curves along the azimuthal angle are shown in the Supporting Information.

2. SFG Analysis of Uniaxially-Aligned Cellulose in Fibrin Matrix In order to discuss the SFG spectral features in terms of inter-crystallite distance, we estimated the average distance between cellulose crystals from the cellulose contents. First, the cellulose weight fraction (4.9%, 9.4%, and 17.2%) was converted to the volume fraction using the density of fibrin (~1.4 g/cm3) and cellulose (~1.5 g/cm3). The cellulose Iβ crystals isolated from the tunicate are known to be approximately 15 nm in diameter (i.e., c = 15 nm in Figure 1) and ~1 µm long.47 Since cellulose crystals are well dispersed in the fibrin matrix, it could be assumed that the lateral distances between adjacent crystallites are uniform (as illustrated schematically in Figure 1). As a first order approximation, the head-to-tail separation of crystals along the length direction could be assumed to be similar to the lateral distance (d) between crystals. With these assumptions, the average inter-crystallite distance is estimated to be ~55 nm for 4.9 wt.% cellulose, ~36 nm for 9.4 wt.%, and ~23 nm for 17.2 wt.% (see Supporting Information). Hereafter, the inter-crystallite distance will be used in describing data, instead of the cellulose concentration in the control sample. 10 ACS Paragon Plus Environment

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Figure 3 compares the ppp and ssp SFG spectra of three control samples with different inter-crystallite distances detected in the reflection detection geometry (Figure 1). The general features and trends of the SFG spectra collected in the transmission direction are similar (see Figure S2 in Supporting Information). The peak at 2944 cm-1 is tentatively assigned to the CH2 asymmetric stretch mode of the exocyclic CH2OH side group coupled with the CH stretch modes in the axial positions of the 6-membered ring.39 The small peak at ~2850 cm-1 is presumed to have the contribution from the CH2 symmetric stretch mode.48 The peak at 3320~3330 cm-1 is the OH stretch modes that are delocalized throughout the entire crystalline domain.49 Since all vibration modes detected are highly coupled,39, 48-49 they will be referred to as the CH and OH bands, hereafter, without distinguishing specific symmetry or position in the crystal. The sharp small peaks at ~3630 cm-1 and 3700 cm-1 are isolated OH groups; they are believed to originate from the OH groups at the cellulose crystal surface without hydrogen bonding interactions with adjacent molecules or adsorbed water. The origin of these sharp peaks is beyond the scope of this paper and will be discussed separately in a subsequent paper.



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Figure 3. SFG spectra of cellulose-fibrin composite fibers collected with (a) ppp and (b) ssp polarizations at the 45o scattering direction. The average inter-crystallite distance marked in the figure is estimated from the cellulose concentration.

In Figure 3 (and Figure S2), it is noted that the intensities of the CH peaks vary substantially with the inter-crystallite distance (d), whereas those of the OH peaks show only minor changes. The areas of the CH and OH peaks were calculated by integrating the SFG intensity under each peak and plotted in the inset to Figure 4. In SFG analyses of plant cell walls, the absolute intensity can vary with external factors such as physical dimensions and surface roughness of samples. Thus, the relative intensity of the CH and OH peaks is a more meaningful measure since such artifacts can be cancelled out when one peak intensity is normalized with the other from the same sample. The CH/OH ratio of the SFG peak areas is plotted in Figure 4. These results clearly show that the CH/OH SFG ratio decreases as the inter-crystallite distance (d) increases.


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Figure 4. Plot of SFG intensity ratio (CH/OH) measured with (a) ppp and (b) ssp polarizations with uniaxially-aligned cellulose crystals assumed to be evenly separated by three different distances in the SFG-inactive fibrin matrix. The insets show the measured intensity at each polarization.

3. Origin of the Inter-Crystallite Distance Dependence of the CH/OH SFG Ratio In general, the SFG process in resonance with vibraitonal modes can be described as:30-31, 35

∆ ∝ sinc 2

(1)

' #$%& ∝ !" ( ) (' * +Γ'

(2)

' #$%& ∝

-.$% -0& -/' -/'

(3)

where ISF, Ivis, and IIR are the intensities of SFG, visible (532 nm or 800 nm), and IR beams,

respectively; is the effective second-order nonlinear susceptibility; and sinc 1

∆ 2

3 is the

synchronization factor which is a function of the phase mismatch between SFG, visible, and IR 56 ) 56 ) 56 where 56 is the momentum of each light) and the sample size (. beams (∆ 4 The SFG coherence length (& ) can be estimated from the synchronization factor: & 7 2⁄∆.30, 13 ACS Paragon Plus Environment


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35

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The term depends on the number density of the SFG-active molecules (N), the resonance

of the IR frequency (( ) with the frequency of a q-th vibration normal mode ((' ) and its ' damping factor (Γ' ), and the hyperpolarizability of that specific normal mode (#$%& ) that can be 9:;

) and dipole tensors (

9?@

9=>

). 30, 35, 39

One of the main difficulties in analyzing biological samples (such as cellulose in plant cell walls) with SFG is the lack of a single-crystalline material to use for quantitative

determination of the terms at various polarization combinations and experimental geometries. Another difficulty comes from the complex structure of large biological molecules. Cellulose is not a simple isolated molecule; it is a nanocrystalline solid material with varying degree of crystallinity. These challenges could be circumvented if the hyperpolarizabilities of all vibrational modes can be calculated from a first-principles theory. We recently carried out quantum mechanical calculations of all hyperpolarizabilities of a simple truncated structural model of cellulose using time-dependent density functional theory (TD-DFT).39

So, the

computation results can be used to predict how the SFG intensities of the CH and OH vibration modes of cellulose would vary with the distance between cellulose crystalline domains interspersed in the SFG-inactive matrix.

TD-DFT calculations found that the SFG spectra of cellulose are governed by the

terms with large components along the chain axis when cellulose crystallites are packed in parallel; in contrast, those containing components perpendicular to the (200) plane of cellulose Iβ crystal are dominant in the antiparallel packing of crystallites.39 The OH modes have their transition dipoles within the (200) plane and have strong directionality with respect to the chain axis.39 Thus, when cellulose crystallites are arranged in the antiparallel fashion within the SFG 14 ACS Paragon Plus Environment

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coherence length, their overall dipoles would be small due to symmetry cancellation. Unlike the OH modes, the stretch modes of axial CH groups and the asymmetric stretch of the exocyclic CH2 group have the transition dipole perpendicular to the (200) plane.39 Depending on whether

cellulose crystallites are arranged parallel versus antiparallel, different terms of the CH modes become more significant; but, the difference in their magnitude is relatively small.39 For this reason, the overall intensity of the CH mode is not very sensitive to the packing polarity of cellulose crystallites.39 When cellulose crystallites are aligned uniaxially with no net polarity (i.e., the number of cellulose crystallites with dipoles pointing to one direction are the same as the number of those pointing the opposite direction), then the system is effectively antiparallel when averaged over the entire sample space (Figure 1). In this situation, the extent of symmetry cancellation between the OH modes of adjacent crystallites antiparallel to each other will depend on how closely they are located. This effect can be estimated considering the inter-crystallite distance (d in Figure 1) with respect to the coherence length (& ). When d

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