Conformational analysis and 2D NMR assignment strategies for lignin model compounds. The structure of acetoguaiacyl-dehydro-diisoeugenol methyl ether

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Abstract
The resolved 1H and 13C chemical shifts for the lignin model trimer acetoguaiacyl-dehydro-diisoeugenol methyl ether have been completely assigned using one- and two-dimensional NMR techniques. Identification of spin families and assignments within the spin families were made using routine one-dimensional experiments for chemical shifts and multiplicities, and COSY and heteronuclear correlation experiments to determine connectivities. NOESY, HOESY, and long-range heteronuclear correlation experiments were then used to determine the juxtaposition of the assigned groups and to provide a confirmation of assignments. Steady state NOE difference and time-dependent NOESY and HOESY experiments were then used to provide experimental measures of the trimer conformation. Semiempirical molecular orbital methods were used to calculate minimum energy structures and energy barriers for bond rotations within the molecule. We have found that for the trimer-size molecule the NOE experiments provide a less detailed picture of the molecular conformation than the molecular orbital results. A comparison of NOE and MO results with crystal structures from the literature indicates the extent to which side-chain and methoxyl torsion angles may be estimated using each method. The correlation between NOE and MO predictions for interatomic distances apparently is related to the narrow minima for β-O-4 torsion angles for which the different results confirm each other. Broad minima or low-energy barriers for torsion angle rotations may yield minimized structures by computational methods that are not experimentally justified. Key words: semiempirical molecular orbital calculation, 2D NMR, conformation, lignin model, nuclear Overhauser effect.