CONFORMATIONAL-ANALYSIS AND 2D NMR ASSIGNMENT STRATEGIES FOR LIGNIN MODEL COMPOUNDS - THE STRUCTURE OF ACETOGUAIACYL-DEHYDRO-DIISOEUGENOL METHYL-ETHER

The resolved H-1 and C-13 chemical shifts for the lignin model trimer acetoguaiacyl-dehydro-diisoeugenol methyl ether have been completely a ssigned using one- and two-dimensional NMR techniques. Identification of spin families and assignments within the spin families were made us ing routine one-dimensional experiments for chemical shifts and multip licities, and COSY and heteronuclear correlation experiments to determ ine connectivities. NOESY, HOESY, and long-range heteronuclear correla tion experiments were then used to determine the juxtaposition of the assigned groups and to provide a confirmation of assignments. Steady s tate NOE difference and time-dependent NOESY and HOESY experiments wer e then used to provide experimental measures of the trimer conformatio n. Semiempirical molecular orbital methods were used to calculate mini mum energy structures and energy barriers for bond rotations within th e molecule. We have found that for the trimer-size molecule the NOE ex periments provide a less detailed picture of the molecular conformatio n than the molecular orbital results. A comparison of NOE and MO resul ts with crystal structures from the literature indicates the extent to which sidechain and methoxyl torsion angles may be estimated using ea ch method. The correlation between NOE and MO predictions for interato mic distances apparently is related to the narrow minima for beta-O-4 torsion angles for which the different results confirm each other. Bro ad minima or low-energy barriers for torsion angle rotations may yield minimized structures by computational methods that are not experiment ally justified.