Structural characteristics of different softwood lignins according to 1D and 2D NMR spectroscopy

Along with cellulose, lignin is one of the most common polymers of plant origin. Numerous studies show that the complex and irregular structure of lignin is still unknown. First of all, the existing problems are connected with the peculiarities of lignins as a natural chemical compound. In this paper the general regularities of the chemical structure of dioxane lignins isolated from the softwood - larch (Larix sibirica), pine (Pinus cembra) and juniper (Juniperus communis). The studies were carried out using a combination of C-13, P-31 and 2D NMR spectroscopy. According to HSQC spectra it was established that the main structural fragments are the structures of beta-aryl ether, followed by phenylcoumarans, with lower amounts of other substructures. Signals belonging to a methyl substituted phenylcoumaron were first identified on the NMR spectra of softwood dioxane lignins. Signals related to the structure of 3 '-O-methylated dihydroquercetin were found in the composition of larch dioxane lignin.