Structural studies of bacterial cellulose through the solid-phase nitration and acetylation by CP/MAS 13C NMR spectroscopy
The solid-phase nitration and acetylation processes of bacterial cellulose have been investigated mainly by CP/MAS 13C NMR spectroscopy to clarify the features of these reactions in relation to the characterization of the disordered component included in the microfibrils. CP/MAS 13C NMR spectra of bacterial and Valonia cellulose samples are markedly changed as the nitration progresses, in a similar way to the case of cotton linters previously reported; and the relative reactivity of the OH groups in the glucose residues is found to decrease in the order of O(6)H>O(2)H>O(3)H. Moreover, the nitration rate and mode greatly depend on the concentration of nitric acid in the reaction media. At dilute and medium concentrations, the O(6)H groups in the crystalline and disordered components are subjected to nitration at nearly the same rate, indicating that these two components are distributed almost at random in the entire region of each microfibril. The preferential penetration of nitric acid into each microfibril also occurs prior to nitration at the medium concentration, resulting in an increase in the mole fraction of the disordered component. In contrast, all OH groups undergo nitration very rapidly at the higher concentration, although nitration levels off to a certain extent for O(3)H groups. In solid-phase acetylation, no regio-selective reactivity is observed among the three kinds of OH groups, which may be due to the characteristic reaction that proceeds in a very thin layer between the acetylated and nonacetylated regions in each microfibril. The almost random distribution of the disordered component in the entire region of the microfibrils is also confirmed in this solid-phase acetylation. On the basis of these results, the mechanism of the solid-phase reactions and the microfibril structure are discussed.
KeywordsBacterial cellulose CP/MAS 13C NMR Disordered structure Microfibril structure Solid-phase acetylation Solid-phase nitration
The authors thank Professor Masaki Tsuji, Institute for Chemical Research, Kyoto University, for kindly allowing us to use a 200 kV JEOL JEM-200CS transmission electron microscope.
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