Infrared and Raman spectra of lignin substructures: Dibenzodioxocin.

Peter Bock,Notburga Gierlinger,Paula Nousiainen,Thomas Elder,Hassan Amer,Antje Potthast,Markus Blaukopf,Ronald Zirbs

doi:10.1002/jrs.5808

Abstract

Vibrational spectroscopy is a very suitable tool for investigating the plant cell wall in situ with almost no sample preparation. The structural information of all different constituents is contained in a single spectrum. Interpretation therefore heavily relies on reference spectra and understanding of the vibrational behavior of the components under study. For the first time, we show infrared (IR) and Raman spectra of dibenzodioxocin (DBDO), an important lignin substructure. A detailed vibrational assignment of the molecule, based on quantum chemical computations, is given in the Supporting Information; the main results are found in the paper. Furthermore, we show IR and Raman spectra of synthetic guaiacyl lignin (dehydrogenation polymer—G‐DHP). Raman spectra of DBDO and G‐DHP both differ with respect to the excitation wavelength and therefore reveal different features of the substructure/polymer. This study confirms the idea previously put forward that Raman at 532 nm selectively probes end groups of lignin, whereas Raman at 785 nm and IR seem to represent the majority of lignin substructures.

Highlights

Vibrational spectroscopy is a very suitable method for plant cell wall research, because chemical information can be related to spatial information and measurements can be performed on samples containing several cells
These are only selected features of the lignin polymer; for example, cinnamaldehyde end groups only account for about 4% of lignin substructures.[29,30,31,32]
Here, we report spectra of a Gbased dehydrogenation polymer (G-DHP), which should serve as a lignin model and reevaluate its previous literature assignments of the IR bands

Summary

| INTRODUCTION

Vibrational spectroscopy is a very suitable method for plant cell wall research, because chemical information can be related to spatial information and measurements can be performed on samples containing several cells. Lignin is the second most abundant plant polymer, and contents of 20–40% are usually found in wood.[8] In the plant cell wall, it can be distinguished from other constituents by IR and Raman microscopy.[9,10,11,12] Published work has mainly focused on the estimation of ethylenic residues (cinnamyl alcohols and aldehydes)[13,14,15,16,17,18,19] and determination of S/G ratios[20,21,22,23,24,25,26,27,28] in lignin These are only selected features of the lignin polymer; for example, cinnamaldehyde end groups only account for about 4% of lignin substructures.[29,30,31,32] To the best of our knowledge, in none of the studies using Raman or IR spectroscopy, abundant lignin linkages like ß-O-4 or ß-ß were studied. This, together with our previous work, updates the lignin band assignments to the current research level

| METHODS

| RESULTS AND DISCUSSION

C O stretching of p-substituted aryl ketone

C C stretching of coniferyl alcohol Ring stretch Φ8b of G-rings and Φ8a of

| CONCLUSION