2D NMR Characterization of Cellobiose Sulfate Hydrolyzed from Cellulose Nanocrystals
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There has been growing interest in the study of Cellulose nanocrystals (CNC) and the development of CNC-based new materials due to its renewable nature, unique aspect ratio and high elastic moduli. Characterization of CNC physiochemical properties, especially its surface chemistry, is essential for the CNC-based materials development. Two types of CNC have been produced and characterized in this study, they are sulfuric acid hydrolyzed CNC (CNC-S) and hydrochloric acid hydrolyzed CNC (CNC-H). The approach of enzymatic hydrolysis was used to characterize the physiochemical properties difference between CNC-S and CNC-H. We determined the Langmuir adsorption isotherm of cellulase on CNC-S, CNC-H and Avicel, and observed higher adsorption capacity (Γ_max=240 mg/g) and higher affinity (R= 0.159 L/g) of enzyme on CNC-S than those (Γ_max=50.17 mg/g, R= 0.124 L/g) on CNC-H. Correspondingly, the hydrolysis yield was higher (55 %) in CNC-H than that (20 %) in CNC-S. Interestingly, we also observed a small peak presented in the HPLC analysis of CNC-S hydrolysis samples. The peak compound was analyzed and confirmed as cellobiose sulfate by LC-MS and NMR. The structural elucidation of cellobiose sulfate was analyzed by 2D NMR (COSY, HSQC, TOSCY and HMBC). The NMR analysis indicated the sulfation occurred on the C6 (primary alcohol) of non-reducing end. In addition, we used XRD to determine the CNC crystallinity and observed no considerable difference between CNC-S and CNC-H. FT-IR has been used to characterize the functional groups on CNC-H and CNC-S. Zeta potential has been compared between CNC-S and CNC-H, which has a potential effect on electrostatic interaction between enzyme and CNC. The significance of this work provides a better understand of sulfate groups on CNC and their potential effects on enzymatic hydrolysis of cellulose and surface modification of CNC.