Fig. 2

Structure of crystalline (a) and amorphous (b) cellulose as implemented in the CA model. a Cellulose nanocrystals were modeled in analogy to the natural elementary fibril of cellulose allomorph I and were composed of up to 144 cellulose chains (16 nm crystals). Their length was constant at 100 nm equaling 100 cellobiose units per chain, and the width/height varied between 8 and 16 nm. Yellow bars show the cellulose chains with their reducing end indicated in orange. Shown in brown are the so-called hydrophobic faces of the cellulose nanocrystal which is where attack by CBH I takes place. Two orientations of the nanocrystal within amorphous material are considered whereby the crystal’s hydrophobic faces are aligned horizontally (left) or vertically (right). b A random positioning algorithm (Additional file 1) was used to create amorphous cellulose layers with a final mean fragment length of 18 cellobiose units. The figure shows an exemplary amorphous plane. It also shows how amorphous chains with varying orientation are combined in one plane. Amorphous chains can be oriented in x or y direction in one z-plane. Tokens ‘±1’ and ‘±2’ are used within the amorphous chain orientation matrix to distinguish x and y direction, respectively. The internal direction of an amorphous chain is indicated by an algebraic sign (‘+’ or ‘−’) and relative to the origin of the z-plane (0, 0). A chain oriented from the reducing to the non-reducing end with respect to the origin has a positive sign (‘+’) and a vice versa oriented chain is indicated by a negative sign (‘−’). Amorphous cellulose material was obtained by stacking multiple layers of amorphous cellulose on top of each other. c The distribution pattern for the cellulose chain lengths in 30 independent planes making up the amorphous material is shown for x- (left panel) and y-oriented (right panel) cellulose chains demonstrating the essentially random distribution of the chains in all orientations