X-ray structure of ammonia-cellulose I: New insights into the conversion of cellulose i to cellulose III _i

A supercritical ammonia treatment has been used to trap an ammonia-cellulose complex during the conversion of cellulose I to cellulose III _I. The crystal and molecular structure of this complex, designated ammonia-cellulose I, has been determined by using X-ray fiber diffraction data (space group P2 ₁; a = 4.47 Å, b = 8.81 Å, c = 10.34 Å, γ = 92.7°). Although the existence of ammonia-cellulose I has been known for some time, this is the first report of its crystal structure. A one-chain monoclinic unit cell has an asymmetric unit that contains only one glucosyl residue and one ammonia molecule. The ammonia molecule acts as a bridge between hydrogen-bonded sheets, forming extended chains of cooperative hydrogen bonds. The sheets are similar to those found in cellulose III _I, with O2⋯O6 intrasheet hydrogen bonds and the gt conformation of the hydroxymethyl group providing potential bifurcated O3⋯O5 and O3⋯O6 intrachain hydrogen bonds. This new structure provides a number of insights into the structural transition pathway followed during the conversion of cellulose I to cellulose III _I.