Ice, vitreous

When water condenses from the vapour on to a surface at a temperature below about — i6o°C a clear deposit of ice-like material is formed which apparently has an amorphous or glassy structure. This is, indeed, a phenomenon which occurs also with germanium deposited on to a cold substrate, though room temperature is cold in this case. The two structures are probably related, as the relation between the parent crystal structures might lead one to suspect. 

When the vitreous deposit is warmed above about —160 C it transforms irreversibly to Ice Ig. Transformation is fairly rapid above —135 °C so that at warming rates of a few degrees per minute the transformation appears to take place at about —130 °C (Dowell Rinfret, i960). The heat evolved in the transition seems to be 7-24 cal g- (Pryde Jones, 1952 Ghormley, 1956, 1968 Beaumont et al. 1961) with the most probable value lying near the top of this range. 

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Ice I is one of at least nine polymorphic forms of ice. Ices II to VII are crystalline modifications of various types, formed at high pressures ice VIII is a low-temperature modification of ice VII. Many of these polymorphs exist metastably at liquid nitrogen temperature and atmospheric pressure, and hence it has been possible to study their structures without undue difficulty. In addition to these crystalline polymorphs, so-called vitreous ice has been found within the low-temperature field of ice I. It is not a polymorph, however, since it is a glass, i.e. a highly supercooled liquid. It is formed when water vapour condenses on surfaces cooled to below — 160°C. 

Cross-/] structure has been demonstrated for Sup35pNM filaments. Serio et al. (2000) observed a 0.47-nm reflection by X-ray diffraction, and subsequently this reflection was shown to be meridional both by X-ray fiber diffraction (Kishimoto et al., 2004) and electron diffraction (King and Diaz-Avalos, 2004). In the Ure2p system, cross-/ structure has been established by electron diffraction from prion domain filaments preserved in vitreous ice (Fig. 7 Baxa et al, 2005). In addition, a 0.47-nm reflection was detected by both X-ray diffraction and electron diffraction from filament preparations of full-length Ure2p and the Ure2p1 65-GFP fusion, indicating that they contain the same structure (Fig. 7 Baxa et al, 2005). 

Gantz, D.L., Wang, D.Q., Carey, M.C., and Small, D.M. (1999). Cryoelectron microscopy of a nucleating model bile in vitreous ice formation of primordial vesicles. Biophys. J. 76, 1436-1451. 

Record a number of micrographs of the particles embedded in vitreous ice. 

Figure 3.7 Transmission electron micrographs of eicosane sulfate micelles in vitreous ice (two different magnifications...

Practical realisation of vitreous ice embedded samples of individual biological macromolecules is usually achieved with a holey carbon grid (Figure 6.21), which is a thin carbon-based support him (50-200 nm in dimensions) perforated with holes and mounted on a standard electron microscope sample grid. An aqueous sample of a biological macro molecule... 

Figure 6.21 Vitreous Ice Freezing Device A thin (60-200 nm) carbon support film perforated with holes is placed on a standard EM carbon grid and then a sample of biological macromolecule is applied and rapidly frozen in liquid ethane (<138 K). After this, the vitrified sample is transferred into a cryoholder (under liquid nitrogen conditions) and then transferred to the microscope for visualisation. The holes in the thin film allow for the formation of monolayers of biological macromolecules in a range of orientations embedded within a thin (20-60 pm) layer of vitreous ice. The visualisation of such monolayer regions gives the best possible cryo-EM images of the embedded biological macromolecules.

Fig. 28-3. Structure of an alphavirus. Shown is the three-dimensional reconstruction of Sindbis virus at 28 A resolution from computer-processed images taken by electron cryomicroscopy, (a) The original electron micrograph shows virus particles in vitreous ice. (b) The surface view of the virus shows details of the 80 trimeric spikes, which are arranged in a T=4 icosahedron. Each spike protrudes 50 A from the virion surface and is believed to be composed of three E1-E2 glycoprotein heterodimers, (c) The cross-sectional view shows the outer surface spikes (yellow) and the internal nucleocapsid (blue), composed of the capsid and viral RNA. The space between the spikes and the nucleocapsid would be occupied by the lipid envelope. The green arrows mark visible points of interaction between the nucleocapsid and trans-membranal tails of the glycoprotein spikes, (d) The reconstructed capsid also exhibits a T=4 icosahedral symmetry. Computer models Courtesy of Angel M. Paredes, Cell Research Institute and Department of Microbiology, The University of Texas at Austin, Austin, Tex. Similar but not identical versions of these computer models were published in Paredes AM, Brown DT, Rothnagel R, et al. Three-dimensional structure of a membrane-containing virus. Proc Natl Acad Sci USA. 1993 90 9095-9099.

First, ice has a large number of solid modifications hexagonal or ordinary ice, 1 cubic ice, 1 ices II-IX vitreous ice. Most of them exist at elevated pressure or need special formation conditions. Under ordinary conditions, only hexagonal ice is formed, which has therefore been investigated more frequently than other modifications. In this chapter we shall deal only with ordinary ice. 

Figure 2 (A) A noisy transmission electron micrograph of vitrified earthworm hemoglobin and (B) the 3D single particle reconstruction produced with the IMAGIC-5 image processing software. Note in (A) the faint images of the hemoglobin molecules at different orientations within the vitreous ice, due to varying molecular rotation with respect to the axis of the electron beam. (Micrographs kindly provided by Dr. Marin van Heel, Fritz-Haber Institute, Berlin.)...

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