SURFACE-STRUCTURE OF NATIVE CELLULOSE MICROCRYSTALS BY AFM

Atomic force microscopy (AFM) has been used to study the surface of na tive cellulose I microcrystals from Valonia ventricosa. High-resolutio n images show clear structural details of the surface, namely the 0.52 nm repeat along the cellulose chains resulting from the glucose sub-u nit and the inter-molecular spacing of similar to 0.6 nm. Cellulase fr om Valonia exists naturally in both a triclinic (I-alpha) and a monocl inic (I-beta) crystal form within the same microfibril; the main diffe rence between them being a displacement of adjacent chains by a quarte r of the c-axis period to give either a diagonally shifted or staggere d 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 th e asymmetric glucose unit, and thus identify triclinic structure on th e microcrystal surface. Computer modelling has been used to construct pseudo-AFM topography images from Connolly surfaces of the facets of t he two different crystal forms, and the triclinic models are in excell ent agreement with the real images obtained by AFM.