Hydrogen-bond structure in carbohydrate crystals

Abstract

So that possible relationships between hydrogen-bond lengths and hydrogen-bond types could be investigated, it was decided to study by neutron diffraction the crystal structures of several molecules having nearly the same size and shape. Feasibility of crystallization of a homogeneous species was a prime criterion. The three methyl pyranosides selected for the studies had been previously investigated by X-ray diffraction. These were the crystals of methyl ..cap alpha..-glucopyranoside, methyl ..cap alpha..-mannopyranoside, and methyl ..cap alpha..-altopyranoside. Hydrogen bonding in these three structures is especially significant because they contain examples of five of the ten possible types of O-H-O interactions shown in Table I. The results of these neutron diffraction studies provided a systematic relationship between hydrogen-bond length and hydrogen-bond type in the three structures. Not all crystalline carbohydrates fall into their classifications as ''neatly'' as did those from the first three pyranosides. Complicating factors include the competitive effects of hydrogen bonds, polar and dipolar interactions, and van der Waals forces. Most noticeable feature of Table I is the predominance of the bonds of the donor-acceptor type which correspond to the shorter, stronger bonds and are part of the infinite chains or are the inner bonds of finite chains, a consequence ofmore » the ''cooperative effect'' which is also discussed. The anomeric effect of hydrogen bonding is also evaluated and the results are tabulated. Table II illustrates comparative X-ray data and neutron diffraction data on H--O bond lengths. Authors believe that the systematics of the hydrogen-bond data presented will form a useful framework with which to compare future experimental measurements and theoretical calculations.« less