Lignin films from spruce, eucalyptus and wheat straw studied with electroacoustic and optical sensors: Effect of composition and electrostatic screening on enzyme binding

32 páginas.-- 6 figuras.-- 4 tablas.-- 53 referencias.-- SUPPORTING INFORMATION The following data are available online as complementary information: Assignments of lignin 13C/1H correlation signals in the 2D HSQC spectra of the MWL isolated from eucalyptus, wheat straw and spruce; size and zeta potential of the MWL measured in aqueous dispersions at different background electrolyte concentrations; water contact angles before and after CTec2 and CBH-I adsorption; AFM images of wheat straw films corresponding to SPR sensograms after rinsing with 50 mM background electrolyte; schematics of the different scenarios for enzymes adsorption on the surfaces for the calculation of surface coverage.

Lignins were isolated from spruce, wheat straw and eucalyptus by using the milled wood lignin (MWL) method. Functional groups and compositional analyses were assessed via 2D NMR and 31P to realize their effect on enzyme binding. Films of the lignins were fabricated and ellipsometry, atomic force microscopy and water contact angle measurements were used for their characterization and to reveal the changes upon enzyme adsorption. Moreover, lignin thin films were deposited on quartz crystal microgravimetry (QCM) and surface plasmon (SPR) resonance sensors and used to gain further insights into the lignin-cellulase interactions. For this purpose, a commercial multicomponent enzyme system and a monocomponent Trichoderma reesei exoglucanase (CBH-I) were considered. Strong enzyme adsorption was observed on the various lignins but compared to the multicomponent cellulases, CBH-I displayed lower surface affinity and higher binding reversibility. This resolved prevalent questions related to the affinity of this enzyme with lignin. Remarkably, a strong correlation between enzyme binding and the S/G ratio was found for the lignins, which presented a similar hydroxyl group content (31P NMR): higher protein affinity was determined on isolated spruce lignin (99% G units) while the lowest adsorption occurred on isolated eucalyptus lignin (70% S units). The effect of electrostatic interactions in enzyme adsorption was investigated by SPR, which clearly indicated that the screening of charges allowed more extensive protein adsorption. Overall, this work furthers our understanding of lignin-cellulase interactions relevant to biomass that has been subjected to no or little pretreatment and highlights the widely contrasting effects of the nature of lignin, which gives guidance to improve lignocellulosic saccharification and related processes

This study was partially funded by the INDOX EU-project (KBBE-2013-7-613549); the LIGNOCELL, LIGNIN, NOESIS, and BIORENZYMERY Spanish MICINN (co-financed by FEDER funds) projects (AGL2011-25379, CTQ2014-60764-JIN, BIO2014-56388 R and AGL2014-53730-R) and the CSIC Project 201440E097. A.P. is grateful to the Spanish MINECO for a FPI fellowship. O.J.R. would like to thank the Academy of Finland for funding through its Centers of Excellence Program (2014–2019), under Project 132723612 HYBER.

Peer reviewed