Symmetry Synchrotron

Some experiments are aiming at the study of structure evolution. In general, the studied material is isotropic or exhibits simple anisotropy (e.g., fiber symmetry). Most frequently the material is irradiated in normal-transmission geometry. A synchrotron beamline is necessary, because in situ recording during the materials processing is requested with a cycle time of seconds between successive snapshots (time-resolved measurements). 

It took the short time of one year or so to solve the structure of rhinovirus which causes the common cold. This relied on two major advances in methods. The first was the use of synchrotron radiation in data collection. Nearly a million reflections were collected on the protein crystallography facility at the Cornell Synchrotron source in a matter of days. This conveyed a speed advantage over data collection on a conventional source and also ameliorated an otherwise impossible problem of radiation damage when long exposure times were used. The far greater rate of radiation damage in the X-ray beam in relation to plant viruses is symptomatic of an inherently less stable protein capsid and the absence of quasi-symmetry. The capsid consists of 60 copies each of four proteins and the virus with about 30 % RNA has a total molecular weight of about 8.5 million. 

CL-DNA complexes form spontaneously when solutions of cationic liposomes (typically containing both a cationic lipid and a neutral helper lipid) are combined. We have discovered several distinct nanoscale structures of CL-DNA complexes by synchrotron X-ray diffraction, three of which are schematically shown in Fig. 1. These are the prevalent lamellar phase with DNA sandwiched between cationic membranes (Lo,c) [22], the inverted hexagonal phase with DNA encapsulated within inverse lipid tubes (Hnc) [23], and the more recently discovered Hj0 phase with hexagonally arranged rod-like micelles surrounded by DNA chains forming a continuous substructure with honeycomb symmetry [24]. Both the neutral lipid and the cationic lipid can drive the formation of specific structures of CL-DNA complexes. The inverse cone shape of DOPE favors formation of the... 

The structure of solid C6o has been determined by synchrotron-X-ray diffraction [44] and neutron diffraction on powder samples [45]. At room temperature C60 solid forms a face-centered cubic (fee) lattice with lattice constant a = 1.417 nm (space group Fm3m) [44], Room temperature 13C NMR measurements indicate that C60 molecules are rotating rapidly [46-48]. The shell radius is 0.352 nm and the van der Waals radius of the molecule is 0.501 nm [44]. The solid undergoes a first-order transition at ca 250 K [49], and below the transition temperature the molecules are orientationally ordered, forming a simple-cubic (sc) structure with a four-molecule basis (space group Pa3) [44,45,50,51]. Below the transition the motion becomes slower and the molecules execute jumps between symmetry-equivalent orientations [46-48], A typical powder XRD pattern of C6o and its simulation are given in Fig. 11(a) [52]. 

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