| Abstract: | A new series of x-ray fiber diffraction patterns is reported from agarose films dried at temperatures close to 100°C from water or dimethylacetamide solutions, cooled to room temperature, stretched above 40°C, and finally x-rayed at room temperature. This procedure gives x-ray patterns showing highly crystalline forms of agarose with projected axial advances (h) per repeating disaccharide in the range of 0.888–0.973 nm, and favoring extended and single agarose chains. Stretching the films below 40°C yields an oriented but fairly diffuse x-ray pattern, similar to that reported previously for agarose, and that has been interpreted in terms of a model with two semicontracted agarose chains, with a value of h = 0.634 nm, draping around each other to form a double helix. The new data requires that double-helix to single-chain transitions be considered in condensed films as well as in solution. Alternatively, two populations of double-helix and single-helix conformations may coexist in the same film, or the original case for the double-helix model may be flawed, perhaps through misinterpretation of the diffuse x-ray pattern. The new results have prompted a critical discussion of evidence for the double-helix model and to indicate that there is potential to improve the data experimentally with a view to gaining a clearer insight into the molecular mechanisms involved in gelation of agarose. |
