A study of lignin formation at the molecular level by scanning tunneling microscopy

Abstract

A scanning tunneling microscope (STM) was used to observe the temporal formation and organization of dehydrogenative polymer (DHP) synthesized from coniferyl alcohol. The images obtained elucidate this structure for the first time. The structure of DHP, as seen from STM images, shows long-range order. It appears that DHP consists of building units or modules assembled into larger assemblies called supermodules. Supermodules are interconnected into the overall lattice-like polymer structure with or without spherical regions. One module consists of about 20 monomers, while the supermodule contains about 500 monomers. Calculated molecular weights for modules and supermodules agree with DHP molecular weight distribution peaks. Samples prepared at two different pH values, 6.4 and 7.6, have the same characteristics. The results presented demonstrate that the process of lignification, even in in vitro conditions, is highly ordered, and as such contribute to our understanding of the structure of lignin, a significant constitutive and functional element of cell walls.

A Comparative Study of Enzymatically and Photochemically Polymerized Artificial Lignin Supramolecular Structures Using Environmental Scanning Electron Microscopy

“…This has been recently determined by gel permeation chromatography. This observation is also consistent with a lower degree of structural order in PCP than in DHP, as observed in films formed on both polymers by STM in the nanoscale range (13,14).…”

Visualization of artificial lignin supramolecular structures

“…There is a slight discrepancy between previously published STM results (Radotic et al 1994) and herein presented ESEM images of fragments of a globule surfaces. While STM images reveal the modular structure of the globule, ESEM images show a smooth surface of a globule.…”

“…This value is calculated for an assumed polymer density of 1.3 g/cm 3 . However, the experimentally determined molecular mass of the DHP lignin model compound is on the order of magnitude of 10 5 (Radotic et al 1994, Wayman et al 1974, which is six orders of magnitudes less than previously indicated in the calculated value based on the average size of the observed globules. We strongly believe that this discrepancy can be explained in terms of the modular structure of the globule.…”

“…Since naturally occurring lignin cannot be isolated in unaltered form, enzymatically polymerized dehydrogenate polymer (DHP) of coniferyl alcohol, obtained in vitro, is used as the best model compound for the purpose of structural studies of lignin (Lewis et al 1988). Using scanning tunneling microscopy (STM) images of DHP, combined with gel permeation chromatography (GPC) measurements (Radotic et al 1994), it was shown that lignin has a well-ordered structural organization with fractal dimensionality, even if it is synthesized in vitro. It was found that the lignin polymer is built of modules containing about 20 monomers, which are further polymerized and cross-linked into the supermodules containing about 500 monomers.…”

Supramolecular Self-Assembled Chaos: Polyphenolic Lignin’s Barrier to Cost-Effective Lignocellulosic Biofuels

“…However, a globular object is not a fractal [13]. To reconcile the fractal properties of lignin, it is assumed that lignin adopts a coiled or helical conformation possessing fractal dimensions [131,141,145,146].…”