Surface structure of native cellulose microcrystals by AFM

Atomic force microscopy (AFM) has been used to study the surface of native cellulose I microcrystals from Valonia ventricosa. High-resolution images show clear structural details of the surface, namely the 0.52 nm repeat along the cellulose chains resulting from the glucose sub-unit and the inter-molecular spacing of similar to 0.6 nm. Cellulase from Valonia exists naturally in both a triclinic (I-alpha) and a monoclinic (I-beta) crystal form within the same microfibril; the main difference between them being a displacement of adjacent chains by a quarter of the c-axis period to give either a diagonally shifted or staggered arrangement of the cellobiose units. The most significant finding in this work is that it has been possible to image the cellobiose repeat along the chain because of topographic differences associated with the asymmetric glucose unit, and thus identify triclinic structure on the microcrystal surface. Computer modelling has been used to construct pseudo-AFM topography images from Connolly surfaces of the facets of the two different crystal forms, and the triclinic models are in excellent agreement with the real images obtained by AFM.