Reverse-tilt state

In the O-like state the extracellular ends of helices A, B, C, and D are tilted outward, but their cytoplasmic ends are not displaced. Helix E is tilted also, but around a pivot point near its middle, so its extracellular and cytoplasmic ends are displaced outward and inward, respectively. If this structure is indeed like that of the O state, the implication is that the protein undergoes a scissoring motion in the second half of the photocycle. It begins with a splaying of the cytoplasmic side of the seven helical bundle in M, which continues in N but reverses in O and opens the extracellular cavity instead. These suggested large-scale global motions are in sharp contrast with the relatively small (1-2 A) and more local atomic displacements in the first half of the photocycle. The rationale must be that the structure of the protein in the unilluminated state predisposes it to the early reactions in the cycle, but the later reactions require drastically different conformations. 

Alkene insertions into M—CR3 bonds proceed via a similar transition state, with the CR3 moiety tilting towards the terminal alkene-C, with retention of configuration of the CR3 group. Note that in either case in the M(R)(alkene) complex two coordination sites are involved, whereas in the product only one is occupied. For this reason the stability of alkyls depends on blocking this second coordination site to prevent the reverse reaction. 

Knowledge of the electrooptic behavior of the FLCPs is of the utmost importance for display device applications. One relevant parameter in this respect is the response time. As for the spontaneous polarization, the determination of the response time requires a uniformly aligned sample. The test cell is placed between crossed polarizers so that one tilt direction is parallel to the direction of one polarizer. The electrooptic effect is achieved by applying an external electric field across the cell, which switches the side chains from one tilt direction to the other as the field is reversed. A photodiode measures the attenuation of a laser beam when the cell is switched between the two states. Generally, the electrooptical response time is defined as the time corresponding to a change in the light intensity from 10 to 90% when the polarity of the applied field is reversed ( 10-9o)-... 

Ferroelectricity in SmC liquid crystals is directly observed in the surface-stabilized state in which macroscopic domains of opposite tilt direction are connected to opposite directions of the spontaneous polarization cf. Sect. 1.2). An electric field-induced reversal of the direction of spontaneous polarization also reverses the direction of tilt and thus leads to a polar bistable electro-optic response. In thermotropic SmC liquid crystals this unique combination of a fluid ferroelectric material was and is extensively studied. 

Figure 7.20 Images showing the photoelastic response of disks that are subject to simple shear, after Ren et al. [63]. Top to bottom shows states that are, respectively, fragile, near shear jamming, and well above shear jamming. In these experiments, a layer of particles is sheared, using a special apparatus that allows for uniform shear strain across the whole layer. The initial state is one that is stress free and that has a parallelogram shape that is "Tilted down"" at the lower left corner. The final state is one where the tilt is reversed. Note that throughout the shearing process, the density of the system remains constant.

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