The helical structure of cellulose I

The unit cell dimension along the crystallographic b ‐axis of cellulose is widely accepted to be 10.3 Å, as against the distance of 10.3912 Å between the terminal oxygen atoms of a cellobiose molecule, estimated from the now well‐established crystal structure of cellobiose. Since cellulose is only a polymer of the cellobiose residue, it has been possible to derive the crystal structure of cellulose I from that of cellobiose. The strict application of stereochemical principles to the sucessive residues in the cellulose chain and a consideration of the formal geometric characteristics of a helix suggest a helical form to the cellulose molecule, with seven cellobiose residues per turn, radius r = 1.5830 Å, and angle of helix 7°51′ which is close to the x‐ray orientation angle of 8°21′ observed in ramie, the best‐oriented native cellulose. An analysis of intensity data both for the equatorial and for the meridional reflections leads to a unit cell with central reversed and corner chains and a relative shift between them of one fourth of the repeat length along the b ‐axis.