The Hydrophobic Topographies of Amylose and its Blue Iodine Complex

Abstract

Two polymorphs of amylose, the native double-helical A-form and the single-stranded VH counterpart, as well as the amylose polyiodide complex were subjected to a molecular modeling study. Based on their X-ray diffraction-derived structures, the "solvent-accessible" contact surfaces were generated, onto which the computed molecular lipophilicity profiles (MLPs) were projected in color-coded form, easily allowing to locate hydrophobic and hydrophilic surface regions. Their detailed analysis revealed the double-stranded A-form to be a rather compact structure with an irregular distribution of hydrophilic and hydrophobic regions over the entire outer surface, with the interior of the double helix being inaccessible even for small molecules. By contrast, VH-amylose, in accord with its water solubility, has a pronouncedly hydrophilic outside surface with an as distinctly hydrophobic channel. — In the darkblue amylose-polyiodide complex, the hydrophobic channel of the single-helical VH-form serves as a well-ordered matrix for incorporation and alignment of the iodine-iodide species to elaborate a linear polyiodide chain in a nearly perfect steric fit and in full complementarity of hydrophobic regions of guest and host. Thus, the MLPs provide substantive credence to the view, that not only the amylose-iodine complex formation is mediated to an essential degree by hydrophobic attractions at the guest-host interface, but that the same factors determine the stability of this unique supramolecular assembly.