Molecular dynamics simulation study of xyloglucan adsorption on cellulose surfaces: effects of surface hydrophobicity and side-chain variation

The effect of surface hydrophobicity and side-chain variation on xyloglucan adsorption onto cellulose microfibrils (CMF) is investigated via molecular dynamics simulations. A molecular model of CMF with (100), (010), (1-10), (110) and (200) crystal faces was built. We considered xylogluco-oligosaccharides (XGO) with three repeating units, namely (XXXG)(3,) (XXLG)(3), and (XXFG)(3) (where each (1,4)-beta-d-glucosyl residue in the backbone is given a one-letter code according to its substituents: G = beta-d-Glc; X = alpha-d-Xyl-(1,6)-beta-d-Glc; L = beta-d-Gal-(1,2)-alpha-d-Xyl-(1,6)-beta-d-Glc; F = alpha-l-Fuc-(1,2)-beta-d-Gal-(1,2)-alpha-d-Xyl-(1,6)-beta-d-Glc). Our work shows that (XXXG)(3) binds more favorably to the CMF (100) and (200) hydrophobic surfaces than to the (110), (010) and (1-10) hydrophilic surfaces. The origin of this behavior is attributed to the topography of hydrophobic CMF surface, which stabilizes (XXXG)(3) in flat conformation. In contrast, on the rough hydrophilic CMF surface (XXXG)(3) adopts a less favorable random-coil conformation to facilitate more hydrogen bonds with the surface. Extending the xyloglucan side chains from (XXXG)(3) to (XXLG)(3) hinders their stacking on the CMF hydrophobic surface. For (XXFG)(3), the interaction with the hydrophobic surface is as strong as (XXXG)(3). All three XGOs have similar binding to the hydrophilic surface. Steered molecular dynamics simulation was performed on an adhesive model where (XXXG)(3) was sandwiched between two CMF hydrophobic surfaces. Our analysis suggests that this sandwich structure might help provide mechanical strength for plant cell walls. Our study relates to a recently revised model of primary cell walls in which extensibility is largely determined by xyloglucan located in limited regions of tight contact between CMFs.