The multi-scale architecture of cellulose in plant cell wall systems investigated by small angle scattering techniques

Transcription

1 The multi-scale architecture of cellulose in plant cell wall systems investigated by small angle scattering techniques Marta Martinez-Sanz

2 The Plant Cell Wall The plant cell wall (PCW) is the structural component covering plant cells, providing a number of functions: Strength to support the plant Rigidity to fix cell shape Flexibility to enable tissue growth Porosity, protection against environmental stress, signaling and sensing Shape, growth rate and resistance of plants

3 Cellulose in the plant cell wall Cellulose crystallites (dimensions, allomorph I α /I β ) Cellulose microfibrils (cross-section, crystalline, amorphous and paracrystalline cellulose localization) Microfibril aggregates (bundles/ribbons) (cellulose interactions with PCW components) Hierarchical assembly of cellulose in PCWs Need to combine different characterisation techniques to cover the whole size range

4 Cellulose in the plant cell wall Microscopy techniques involve specimen preparation (often drying) possible structural changes Scattering techniques powerful tool to characterise native PCW (partially hydrated)

5 Characterization of PCWs by SAS techniques Based on interactions between incident radiation (light, X-ray, neutrons) and particles By analysing the scattered radiation we can obtain information about size, shape, orientation and particle correlations. Incident-Scattered radiation Scattering vector: Modification of scattering angle (θ) q range size range Real-space dimension Wide angle scattering (WAS): High q d ~ nm Small angle scattering (SAS): Low q d=1-100s nm Ultra-small angle scattering (USAS): Low q d=100s nm-10 µm

6 Characterization of PCWs by SAS techniques Isotropic behaviour radial average

7 Characterization of PCWs by SAS techniques SANS SAXS Size range Å-1 < q < 0.7 Å-1 (d nm) 0.02 Å-1 < q < 0.3 Å-1 (d 2-30 nm) ρ crystalline cellulose (10 10 cm -2 ) ρ paracrystalline cellulose (10 10 cm -2 ) physical density ρ amorphous cellulose (10 10 cm -2 ) ρ D 2 O exchanged cellulose (10 10 cm -2 ) ρ Arabinoxylan (10 10 cm -2 ) ρ Xyloglucan (10 10 cm -2 ) ρ crystalline d-cellulose (10 10 cm -2 ) ρ H 2 O (10 10 cm -2 ) ρ D 2 O (10 10 cm -2 ) SANS enables modification of cellulose ρ by H/D exchange - Soaking samples in D 2 O or H 2 O/ D 2 O mixtures (labile OH groups replaced by OD) - Deuterium labelling (C 6 H 10 O 5 C 6 D 10 O 5 )

8 Approaches to study the structure of PCW PCW structure deconstruction: Progressive removal of PCW components and cellulose isolation. Application: Lignocellulosic biomass for the production of biofuels Bottom-up approach: Use of PCW analogues to mimic the biosynthesis process Bacterial cellulose as a model system. Incorporation of PCW components into the culture media Application: Investigation of the biosynthesis process and roles of different PCW components

9 Characterization of PCWs by SAS techniques MODEL SYSTEMS: Highly hydrated (~98-99% H 2 O) layer of cellulose synthesised by bacteria (G. xylinus) CELLULOSE HYDROGELS Investigation of the interaction mechanism of cellulose with PCW matrix components: - BC hydrogels with AX and XG - BC hydrogels with pectins (solutions and Ca 2+ gels) - dbc hydrogels with AX, XG and MLG PCW SYSTEMS: Mature cotton fibres Food-extracted CWs

10 BC hydrogels with AX and XG BC BC-AX BC-XG AX Martinez-Sanz et al. Cellulose (2015) 22, SAXS XRD XG affects the arrangement of cellulose microfibrils within the ribbon XG promotes the crystallization of I β allomorph (typically found in plants)

11 BC hydrogels with AX and XG SANS Contrast variation experiments CORE-SHELL model Ribbons SHELL - > 90% bound solvent + Fully exchanged paracrystalline cellulose - Surface AX/XG domains (non-specific adsorption interaction mechanism) Ribbons CORE % bound solvent (40-30% non-exchanged H 2 O) - Partially exchanged cellulose (crystalline + paracrystalline) - XG domains strongly interacting with cellulose microfibrils Martinez-Sanz et al. Soft Matter, (2016) 12,

12 BC hydrogels with Pectins (a) Hydrogels prepared in the presence of high DM pectin solutions with different viscosities Lopez-Sanchez et al. Carbohyd.Polym., (2016) 153, Non-interacting pectin (60-80%) Removed after washing. Leads to phase separation upon drying. Mainly located filling in the pores between the ribbons Denser hydrogels Bound pectin (20-40%) Remains after washing. Reduces the X C slightly but does not affect the crystalline configuration. Interacts directly with cellulose µfibrils (without affecting crystallisation process) forming domains of nm

13 Deuterated BC hydrogels Aim: Increase neutron SLD contrast by replacing H atoms in cellulose with D Ф=32 ± 12 nm Ф=27 ± 11 nm H-CH D-CH Molecular structure : C 6 D 5 H 5 O 5 Martinez-Sanz et al. Carbohyd.Polym., (2016) 147, Similar X C (97-98%) and crystallite cross-sections - Predominant cellulose I α allomorph

14 Deuterated BC hydrogels SANS SAXS Ribbon Microfibril

15 Deuterated BC hydrogels PROPOSED BIOSYNTHESIS MECHANISM vs. INVESTIGATED STRUCTURAL FEATURES Brown R.M., J.Macromol.Sci. (1996) 33, Cellulose µfibrils are synthesised by TC sub-units Proximity of TC sub-units Association of adjacent µfibrils Interaction of µfibrils with strongly bound H 2 O through H bonding network Ribbon

16 Martinez-Sanz et al. Polymer, (2016) 105, D-BC composite hydrogels Extending the q range by using SANS (4 config.) + USANS C 6 D 5 H 5 O 5 ~ 5 cm Larger samples required Small nodules D-CH D-CH-AX 13% AX D-CH-XG D-CH-MLG D-CH crystallinity (84%) decreases with 39% XG 32% MLG the presence of MLG (68%) and XG (59%). Only XG promotes I β crystallisation Ribbon cross-links

17 D-BC composite hydrogels New structural features - Core-shell + Beaucage model to fit the entire q range CORE-SHELL RIBBON STRUCTURE - D-CH slower synthesis rate less dense ribbon - AX and MLG modify shell properties - XG affects core and shell properties LONGITUDINAL STRUCTURE - Individual µfibril Crystalline cellulose domains 1.6 nm nm - Ribbon Periodical twisting µm

18 D-BC composite hydrogels Air-dried samples Native hydrogels D-CH D-CH-AX D-CH-XG D-CH-MLG

19 To summarize Approach to investigate the structure of native cellulose hydrogels: Combination of small angle scattering techniques with diffraction, microscopy and spectroscopy Hierarchical architecture of cellulose modelled by multi-scale core-shell formalism SAXS microfibril structure, SANS ribbon structure (H/D exchange process) Partial deuteration of cellulose enhances the appearance of SANS structural features Extending the q range with USANS Scattering features likely related to cellulose longitudinal structure Elucidation of the distinct interaction mechanism of PCW matrix polysaccharides with cellulose

20 AX XG - Interferes with cellulose crystallisation - Promotes I β allomorph - Interfibrillar domains crosslinking and interspacing µfibrils - Surface domains cross-linking Surface interactions via nonspecific adsorption mechanism ribbons Pectin MLG - Interferes with cellulose crystallisation but does not create a network of cross-linked µfibrils - Surface domains cross-linking ribbons (i) non-interacting fraction filling in ribbon pores (ii) Interacting fraction coating cellulose µfibrils

21 Ongoing work Extraction of different lignocellulosic fractions from different algae and seaweed species Investigation of the cellulose architecture and structural roles of matrix polysaccharides by means of scattering techniques (SAXS/WAXS synchrotron experiments Application of extracted carbohydrates as encapsulation matrices, bioactive materials and reinforcing agents for packaging structures

22 Acknowledgements ANSTO Elliot Gilbert Christine Rehm Liliana de Campo UQ Mike Gidley Patricia Lopez-Sanchez Deirdre Mikkelsen Bernadine Flanagan Dongjie Liu Si-Qian Chen Dongjie Wang