Solid-state NMR reveals a structural variation from the Iβ polymorphic form in bacterial cellulose

Solid-state 13C nuclear magnetic resonance (NMR) spectroscopy is a powerful tool for identifying the various allomorphs of cellulose, for example quantifying the proportions of cellulose Iα and Iβ allomorphs in the crystalline domains of various native celluloses. While marine invertebrate animals known as tunicates produce nearly pure cellulose Iβ, solid-state NMR reveals that bacterial cellulose is dominated by the Iα allomorph, but also has additional signals arising from a secondary crystalline form that are normally attributed to cellulose Iβ. However, in this paper we show that the 13C chemical shifts and correlation patterns in 2D NMR spectra for this secondary crystalline form in bacterial cellulose are not the same as those found in cellulose Iβ spectra reported for tunicate cellulose. In order to reach this conclusion, it was necessary to ensure all spectra were correctly set to the same chemical shift scale. In doing so, it became apparent there were spectral features for bacterial cellulose that were entirely consistent with previously reported spectra of bacterial cellulose but did not match those of the Iβ allomorph found in tunicate cellulose. Through a careful and detailed analysis of the 1D and 2D NMR spectra of three 13C-enriched bacterial cellulose samples, two sets of correlated 13C chemical shifts for this secondary crystalline form were identified. The fact that these chemical shifts and correlation patterns are different than those of tunicate cellulose suggests that there exists some sort of structural variation from the Iβ allomorph in bacterial cellulose since the 13C NMR spectrum closely resembles, but is not identical to, the 13C spectrum of the Iβ allomorph found in tunicate cellulose.