Eng. Costel Agache

Conformational study of the lignin model compounds using ab initio methods

SCIENTIFIC COORDINATOR: Prof. dr. eng. Valentin I. Popa

PhD Thesis asserted at "Gh. Asachi" Technical University of Jassy - Faculty of Industrial Chemistry, May 2005

Lignin, a biopolymer with technological importance, due to its extremely complicated structure and to the multiple linkage possibilities of the phenylpropane units through ether or C-C bonds still represents a challenge for elucidating its molecular structure due to the limits of experimental methods. As an add-on to the experimental techniques, computational methods represent a promising  technique for investigating lignin structural properties.

The main objective of the present thesis was the conformational study of the lignin structural units based on the methods of the ab initio molecular orbital theory. Among the most important contributions to the field brought by this thesis, the following can be cited:

• complete study of the conformation of the main linkages: β-O-4 (arylglycerol β-aryl ether), β-5 (phenylcoumaran), 5-5 (biphenyl), 4-O-5 (diphenyl ether), β-β (pinoresinol), β-1 (arylglycerol β-aryl), of the conformation of the guaiacyl ring and of the propane side chain (coniferyl alcohol, α-carbonyl units);
• calculation of the potential energy surfaces, of the transition states through which the interconversion of conformers takes place, of the contribution of each equilibrium structure, of the role of the hydrogen bonds in molecular stabilisation;
• elucidation of the spatial conformation of hydroxyl and methoxy groups from the guaiacyl nucleus, evaluation of the energy of the O-H…O intramolecular hydrogen bond typical for the guaiacyl  rings, of the dipole moment (SM5.42R, PCM and SCRF);
• identification, classification and characterisation of the 200 structures for the β-O-4 compounds,characterisation of all conformers of the biphenyl and diphenyl ether structures, conformational analysis of 24 β-β structures and 90 β-1 structures.

The present study is aligned to the modern tendencies of investigating lignin molecular structure, which, taking into account its complexity, imposes to adopt a step by step approach for molecular modelling. Thus, the first step necessary for the validation of the modelling protocol before attempting to study larger molecular structures is the study of the predominant structural units for which large amounts of experimental data are available.