Location: Cotton Structure and Quality Research
Title: Electron density studies of methyl cellobioside Authors
Submitted to: National Cotton Council Beltwide Cotton Conference
Publication Type: Proceedings
Publication Acceptance Date: February 15, 2011
Publication Date: April 25, 2011
Citation: Stevens, E.D., Dowd, M.K., Johnson, G.P., French, A.D. 2011. Electron density studies of methyl cellobioside. National Cotton Council Beltwide Cotton Conference. 1416-1418. Interpretive Summary: To understand how cotton responds to moisture, it is important to better understand the forces that hold the cotton fiber together. This paper investigates the forces that hold the major molecule in cotton, cellulose, to other cellulose molecules. In the past it was widely stated that weak bonding forces called hydrogen bonds were responsible for holding the fiber together, but other, even weaker forces may be more important because there are many of them than there are of the normal hydrogen bonds. This work independently analyzes the electrons around the individual atoms that were determined by experiment and also by quantum mechanics calculations for a small fragment of the cellulose molecule. Both the ordinary hydrogen bonds and the other, weaker bonds could be observed. Information from the small molecule will be combined with information from other small carbohydrate molecules and applied to the specific situation for cellulose.
Technical Abstract: Experimental X-ray diffraction crystallography determines the variations in electron density that result from the periodic array of atoms in a crystal. Normally, the positions and type of atom are determined from the electron density based on an approximation that the atoms are spherical. However, the atoms can also be refined based on a multipole approximation that permits the electron density to show a more resolved picture. For example, the lone pairs of electrons become visible, and the characteristics of the electron density between atoms can indicate when bonding is occurring. Even rather weak interactions, such as hydrogen bonds, can be resolved with this technique. Comparable information can be obtained from theoretical electronic structure theory calculations, and the two methods can work together to gain added insight. Methyl cellobioside has structural similarities to cellulose III which results from ammonia mercerization of cotton, but unlike the cellulose polymorph, provides sufficiently high quality crystals for these determinations.