Two-Dimensional 1H-Nuclear Magnetic Resonance Relaxometry for Understanding Biomass Recalcitrance

Low-field nuclear magnetic resonance (LFNMR) relaxometry examines the influence of the local environments within porous materials on the responses of the water-associated 1 H to magnetic fields, yielding information on the chemical and physical surroundings of the water. 1D NMR relaxometry has been used to examine the relationship between water constraint within lignocellulosic biomass microstructure and its enzymatic digestibility; however, the effect of physical confinement and magnetic dephasing by the local chemistry could not be decoupled. This limitation is overcome by two-dimensional T 1 T 2 1 H NMR relaxometry, where simultaneously probing the spin–lattice and spin–spin relaxation times of water 1 H resolves physical and chemical contributions to relaxation times of unique water environments within the sample. 2D T 1 T 2 relaxation revealed four water environments in Norway spruce assigned to lumen and cell wall water based on water mobility in the pools. Sulfur dioxide (SO 2 ) pretreatment of the spruce eliminated the cell wall water environments, while increasing the mobility of water in the lumens. Subsequent dewatering of pretreated spruce to high dry matter content in the samples significantly decreased water mobility in the lumens without changing the local chemical composition. For the first time, the use of 2D T 1 T 2 relaxation revealed that osmotic pressure exerted by solute (glucose or bovine serum albumin (BSA)) uptake into the microstructure of lignocellulosic biomass expands the volume of the confined spaces such as the cell lumens. The uptake of BSA was associated with increased water retention and enzymatic digestibility of SO 2 pretreated spruce. Overall, 2D T 1 T 2 relaxation results suggest a relationship where increasing water mobility in the biomass microstructure reduces its recalcitrance.