Effect of water models on structure and dynamics of lignin in solution

Md Masrul Huda,Neeraj Rai,Nusrat Jahan

doi:10.1063/5.0047974

Abstract

Lignin, a major biomass component, can be an excellent source for different monomers in the polymer industry. However, the complex and heterogeneous structure of lignin poses a significant challenge for designing energy-efficient processes for depolymerization. As many proposed depolymerization processes are solvothermal, it is essential to understand the structure and dynamics of lignin in solution, in particular aqueous solution. Here, we utilize molecular dynamics simulations to understand the effect of water models on the structure and dynamics of different model lignin oligomers (softwood and hardwood) as a function of temperature. We have examined three different water models: TIP3P, TIP4P/Ew, and flexible SPC/Fw. We find that the diffusion constant of lignin oligomers in an aqueous solution differs significantly depending on the water model used. The diffusion constant of lignin in the TIP3P water model is almost twice as large as that in SPC/Fw and TIP4P/Ew. The softwood and hardwood oligomers adopt an extended structure in TIP3P water compared to SPC/Fw and TIP4P/Ew. Given the different levels of sensitivity of transport and structural properties of aqueous lignin on water models, it is important to take these into account when discussing results from a specific water model.

Highlights

After generating structures using LigninBuilder,37 the end state was converted to a lignin topology through TopoGromacs 1.8.40 We examined the effect of the water model on lignin solvation and dynamics at five different temperatures: 300, 340, 380, 420, and 460 K; the pressure was set to 1 atm
From the mean squared displacement (MSD) plots, it is apparent that the slope of the MSD curve is larger for the TIP3P water model than for SPC/Fw or TIP4P/Ew, signifying higher self-diffusivity
We examined the effect of water models on the structure and dynamics of hardwood and softwood lignin

Summary

INTRODUCTION

Lignocellulosic biomass, with cellulose, hemicellulose, and lignin as the primary components, is a sustainable alternative to fossil fuels to produce fuels and chemicals. Among the primary components, lignin, a heterogeneous, cross-linked polymer, presents a significant challenge for its utilization as a feedstock. Lignin represents between 15% and 25% of the dry weight of biomass and is composed of phenylpropanoid subunits: p-coumaryl, coniferyl, and sinapyl alcohol cross-linked through many C–C (β–β, β–1, 5–5) and C–O (β–O–4, 4–O–5, α–O–4) bonds, which makes it the only renewable source with a phenolic monomer for the production of high-value aromatic compounds as well as non-ethanol biofuel. The major monolignol composition of lignin varies with the source of the biomass, and the randomness in oxidative coupling leads to a highly polydispersed and branched lignin structure. For example, lignin derived from softwood mainly contains guaiacyl units with a small amount of p-hydroxyphenyl. Lignin, a heterogeneous, cross-linked polymer, presents a significant challenge for its utilization as a feedstock.. The structural and thermodynamical properties of lignin depend on the types of monomers and linkages present.. The heterogeneity of lignin presents a significant challenge for experimental characterization of solvation structure and dynamics in the condensed phase. As the structural and transport properties of biopolymers in the aqueous phase depend on the water model’s accuracy, it is important to investigate the sensitivity of aqueous phase lignin solvation and transport properties to different water models. SPC/Fw and TIP4P/Ew predict correct densities and energetic properties and provide a good estimate of the diffusion constant and the dielectric constant.. SPC/Fw and TIP4P/Ew predict correct densities and energetic properties and provide a good estimate of the diffusion constant and the dielectric constant.20 These water models perform better than TIP3P. The native lignin is heterogeneous, a large body of previous computational work has used homogeneous lignin. In our work, we have used both homogeneous and heterogeneous models of lignin

LIGNIN MODEL AND SIMULATION DETAILS

Diffusion constant of lignin oligomers

Lignin structure in solution

CONCLUSIONS