Electronic structure tilt angle

The modeling of electron diffraction by the pattern decomposition method, for which no structural information is required, can be successfully applied for extraction of the diffraction information from the pattern. Several parameters can be refined during the procedure of decomposition, including the tilt angle of the specimen the unit cell parameters peak-shape parameters intensities. The procedure consists of fitting, usually with a least-squares refinement, a calculated model to the whole observed diffraction pattern. 

On the basis of novel electron diffraction methods, Unwin and Henderson have shown that each protein molecule consists of seven ot-hexical rods, about 40 A long and 10 A apart, all perpendicular to the plane of the membrane (39,40). The lattice structure results in clustering of the BR molecules as trimers around one of the threefold axes. Analysis of CD spectra in terms of exciton interactions between the chromophores (41,42), as well as linear dichroism measurements (43-45), have yielded values of 20-24° for the tilt angle (out of the membrane plane) of the retinal transition moment, which closely coincides with the long axis of the molecule. The absence of any rotational mobility of bacteriorhodopsin in the purple membrane (on a time scale of 60 min) was also confirmed by linear dichroism measurements (45). 

Since the three-dimensional structure ofrhodopsin was determined in 1975, it has become one of the most-investigated and best understood membrane proteins. By using a high-resolution electron microscope at a low specimen temperature (4 K), and a uniformly thin specimen of the two-dimensional bacteriorhodospin crystals to allow data collection at steep tilt angles, Kimura et recently obtained a 3 A map that... 

The short N—C(Ph) bond (138 pm) found for (188) indicated partial doublebond character owing to electron donation by the amino-group. The conformations of triphenylphosphine in the free and solid states and the nitrile tilt angle for (189) have been calculated. The five-membered ring of (190) has been found to be significantly non-planar. Crystal and molecular structures of the ylides (191),(192),and (181) have been determined. The... 

The proposed structure of the SmCca phase shown in Fig. 23 has recently been supported by the results of a study using electron spin resonance spectroscopy [159]. Specifically, it is clear that within the SmCca phase there are two distinct directors with tilt directions differing by 180°. The tilt angle within a layer is reported to have a maximum value of 18° which is consistent with the bent geometry of an odd membered dimer in which the mesogenic groups make an angle of 15° with the spacer. 

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