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George B. Benedek Group
Alfred H. Caspary Professor of Physics and Biological Physics in the Department of Physics
Professor of Physics and Health Sciences and Technology in the
Harvard-MIT Division of Health Sciences and Technology

Helical Ribbons as Metastable Intermediates in Cholesterol Crystallization

Personnel: Boris Khaykovich, Chintan Hossain, Jennifer J. McManus, Aleksey Lomakin and George B. Benedek

Helical ribbons with characteristic pitch angles (11 or 54░) form spontaneously in a variety of quaternary surfactant-phospholipid-sterol-water solutions. Such helical ribbons were first found in human bile, where it was shown that they are metastable intermediate structures on the pathway of cholesterol crystallization. These helical ribbons form in a variety of axial lengths, widths and radii. Remarkably, however, they all have pitch angles of either 11 or 54░. We believe that the remarkable stability of each of the two pitch angles is due to a crystalline structure of the ribbons and that the two different pitch angles may be due to either different crystal structures or different growth directions of the ribbons. In order to probe the molecular structure of the ribbons, we are conducting X-ray diffraction and micro-diffraction measurements of individual helical ribbons. 
The microscopic size of the ribbons (on the order of 100 x 10 x 1 Ám3), in addition to their curvature present formidable challenges for the X-ray diffraction measurements and subsequent data analysis. We have developed special sample preparation techniques and an appropriate data analysis method. We believe that our techniques can be useful in future research on other biomolecular structures, such as membranes, or small or flexible crystals. We have conducted a number of X-ray diffraction experiments on single ribbons of various sizes at the Advanced Photon Source at Argonne National Laboratory (Argonne, IL).
We have succeeded in measuring X-ray diffraction peaks from various individual ribbons and deduce their unit cell size. The most interesting observation comes from comparing the unit cell of the ribbons with that of cholesterol monohydrate (ChM). Similar to ChM, the ribbons are quasi-two-dimensional crystals. Unexpectedly, the out-of-plane unit cell size of the ribbons is about three times that of ChM (c = 34.36 ┼), while the in-plane lattice constants are slightly smaller than ChM (a ? b ? 12.4 ┼). The differences are probably due to the difference in water content between ChM and the ribbons. Future measurements should give us a better understanding of the reason for these differences.
Our findings suggest a discovery of a new crystalline phase of cholesterol. We are working on confirming this observation by additional measurements and to understand the reasons for the differences in the unit cell size between the ribbons and cholesterol monohydrate crystals. In addition, smaller helical ribbons should be measured to confirm that their crystal structure is the same as that of large ribbons.