What Factors Control Dimerization of Coniferyl Alcohol?

Abstract

Data suggest that the dimerization of coniferyl alcohol is not under thermodynamic control. In this study, molecular dynamics calculations were used to estimate the effect of the solvent environment. In water, the coniferyl alcohol radicals were forced to associate by the formation of a solvent cage. In glycerol, the solvent cage effect appeared to be absent. These results suggest that in water, the product distribution of the dimers will be modified by interactions with the solvent. The computed results are consistent with experimental observations of coniferyl alcohol reactions in various solvents.

Biomimetic Synthesis of Natural and “Unnatural” Lignans by Oxidative Coupling of Caffeic Esters

“…The formation of one main product by 5-8Ј coupling can be explained on the basis of other arguments. 6294 Then, taking into account the fact that the activation energies for the radical coupling reactions are nearly zero, [27] we compared the stability of all the possible diastereomeric products arising from the 5-5Ј, 5-8Ј and 8-8Ј coupling reactions of the methyl caffeate 4-OH radical to establish the expected percentage distribution. A preliminary Monte Carlo [28] -simulated annealing [29] was used to sample geometries from a Boltzmannn-weighted distribution.…”

Optical properties in Biopolymers: lignin fragmen

“…Despite this roles, the study of lignin has been limited by its own nature, the fact that the macromolecular pattern can vary from species to species has make almost impossible to generate a detailed model of it. [1,2] The chemical and technological aspects of lignin modification products are huge, beginning with the great amounts of raw material. All these products are available from the paper production industries that are usually disposed or burn as a secondary energy source; they have great energy content because of the presence of aromatic rings, and besides the carbon skeleton presents a wide range of substitution and addition reaction sites.…”

“…In softwoods the most abundant precursor is the Coniferyl Alcohol (90-75%) [6] and the preferred polymerization mechanisms are a free radical ones, called β-O-4 (50%), β-5 (9-12%), and 5-5 (10-11%) [7]. These structures are named according to the reactions sites that the formation of a specific structure involves, these patterns and reactions define and regulate the entire structure at the macromolecular level.…”

Molecular dynamics study of lignin constituents in water

“…In theory, there may be 25 combinations of intermonomeric linkages, the most frequent ones in both softwood and hardwood being b-O4, b-5, b-b, b-1, 5-5 and 5-O4 (Monties 1989a, and references therein). The relative distribution of the linkages is not clearly established (Monties 1989b(Monties , 1998, since many factors exist that affect the linkage ratios, including the radical production rate, the presence of carbohydrates, the type of solvent, ionic strength, and pH (Chioccara et al 1993;Terashima and Atalla 1995;Houtman 1999). It is still a matter of dispute as to whether the linkage distribution is under thermodynamic or kinetic control.…”

“…The energy barrier and reaction energy of the initial radical-radical coupling that produces the b-O4 QM intermediate, and the energetics of the alternative mechanism of coupling coniferyl alcohol to a coniferyl alcohol radical have also been explored (Durbeej and Eriksson 2003c). Houtman estimated the effect of the environment on the coupling frequency and also explored the character of the motion of lignin model compounds near a cellulose surface (Houtman and Atalla 1995;Houtman 1999). Roussel and Lim (1995a,b) were able to reproduce both the distribution of the intermonomeric couplings and the number of bonds per monomer using Monte Carlo simulations.…”