T2-Filtered T2 - T2 Exchange NMR.

This work introduces an alternative way to perform the T2 - T2 Exchange NMR experiment. Rather than varying the number of π pulses in the first CPMG cycle of the T2 - T2 Exchange NMR pulse sequence, as used to obtain the 2D correlation maps, it is fixed and small enough to act as a short T2-filter. By varying the storage time, a set of 1D measurements of T2 distributions can be obtained to reveal the effects of the migration dynamics combined with relaxation effects. This significantly reduces the required time to perform the experiment, allowing a more in-depth study of exchange dynamics and relaxation processes with improved signal-to-noise ratio. These aspects stand as basis of this novel experiment, T2-Filtered T2 - T2 Exchange NMR or simply T2 F-TREx.

Fast reconstruction of highly undersampled MR images using one and two dimensional principal component analysis.

Recent compressed sensing techniques allow signal acquisition with less sampling than required by the Nyquist-Shannon theorem which reduces the data acquisition time in magnetic resonance imaging (MRI). However, prior knowledge becomes essential to reconstruct detailed features when the sampling rate is exceedingly low. In this work, one compressed sensing scheme developed in wireless sensing networks was adapted for the purpose of reconstructing magnetic resonance images by using one-dimensional principal component analysis (1D-PCA). Moreover, another related reconstruction method was proposed based on two-dimensional principal component analysis (2D-PCA). When comparing with one wavelet compressed sensing method, we demonstrate that these techniques are feasible and efficient at high undersampling rates.

Nuclear magnetic resonance investigation of water accessibility in cellulose of pretreated sugarcane bagasse.

BACKGROUND: Enzymatic hydrolysis is a crucial step of biomass conversion into biofuels and different pretreatments have been proposed to improve the process efficiency. Amongst the various factors affecting hydrolysis yields of biomass samples, porosity and water accessibility stand out due to their intimate relation with enzymes accessibility to the cellulose and hemicellulose fractions of the biomass. In this work, sugarcane bagasse was subjected to acid and alkali pretreatments. The changes in the total surface area, hydrophilicity, porosity and water accessibility of cellulose were investigated by scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR). RESULTS: Changes in chemical and physical properties of the samples, caused by the partial removal of hemicellulose and lignin, led to the increase in porosity of the cell walls and unwinding of the cellulose bundles, as observed by SEM. (1)H NMR relaxation data revealed the existence of water molecules occupying the cores of wide and narrow vessels as well as the cell wall internal structure. Upon drying, the water molecules associated with the structure of the cell wall did not undergo significant dynamical and partial moisture changes, while those located in the cores of wide and narrow vessels kept continuously evaporating until reaching approximately 20% of relative humidity. This indicates that water is first removed from the cores of lumens and, in the dry sample, the only remaining water molecules are those bound to the cell walls. The stronger interaction of water with pretreated bagasse is consistent with better enzymes accessibility to cellulose and higher efficiency of the enzymatic hydrolysis. CONCLUSIONS: We were able to identify that sugarcane bagasse modification under acid and basic pretreatments change the water accessibility to different sites of the sample, associated with both bagasse structure (lumens and cell walls) and hydrophilicity (lignin removal). Furthermore, we show that the substrates with increased water accessibility correspond to those with higher hydrolysis yields and that there is a correlation between experimentally NMR-measured transverse relaxation times and the efficiency of enzymatic hydrolysis. This might allow for semiquantitative estimates of the enzymatic hydrolysis efficiency of biomass samples using inexpensive and non-destructive low-field (1)H NMR relaxometry methods.