The triple helical structure of lentinan, a linear β-(1 → 3)-D-glucan

Probable models for the crystalline structure of lentinan, the β-(l → 3)-D-glucan of the fungus Lentinus elodes, were evaluated by X-ray fiber diffraction and theoretical conformational analysis of single and multiple helices. The X-ray diagram is consistent with a hexagonal unit cell with dimensions a = b = 15.8 Å and c (fiber repeat) = 6 Å. Conformational analysis predicts five probable models for this fiber repeat, of which one is a single helix, two are double helices, and two are triple helices. On the basis of packing of these helices into the unit cell and a comparison of their calculated Fourier transforms with the observed X-ray diffraction intensity distribution, all but the two triple helical models are eliminated from consideration. The two triple helices differ only in chirality and both left- and right-handed models are stabilized by interstrand [Formula: see text] hydrogen bonds, such as are found in the triple helical structure of β-(1 → 3)-D-xylan. The right-handed triple helix may be the more probable structure of the two, by analogy with the known structure of the β-(1 → 3)-D-xylan. It is likely that the β-(1 → 3)-D-glucan of curdlan and those of Armillaria mellea and Lilium longiflorum also possess the same structure.