Determination of the Through-Bond Carbon−Carbon and Carbon−Proton Connectivities of the Native Celluloses in the Solid State

Using the two-dimensional (2D) refocused CP-INADEQUATE spectra of natural abundance Cladophora and tunicate celluloses, we determined the 13C homonuclear through-bond correlations of cellulose Iα and Iβ, respectively. Two sets of the 13C−13C connectivities from C1 through C6 were observed in the 2D INADEQUATE spectrum of the respective cellulose where two directly bonded carbons share the common frequency in the double quantum dimension, which indicated that both cellulose Iα and Iβ contain two magnetically nonequivalent anhydroglucose residues in the unit cells. After the 13C assignment of each carbon of the cellulose Iα and Iβ, assignments of the 1H chemical shifts of protons attached to each carbon of the both allomorphs were performed by use of the 2D MAS-J-HMQC spectra of the cellulose samples for the first time. These spectra gave the through-bond 13C−H correlations, which allowed the assignment of the 1H chemical shifts of protons that bind to C1, C3, C4, and C6 of the cellulose Iα and Iβ. From the differences in the 13C and 1H shifts of cellulose Iα and Iβ, it was revealed that the primary difference between two forms of cellulose I was in the conformations of anhydroglucose residues contained in the cellulose chains. In addition, the conformational difference in the torsion angle around the β-1,4 linkage between cellulose Iα and Iβ was suggested by the notable differences in their 1H chemical shifts of protons attached to C1.