Scissors, conformational

Scaling function 211,213,233,235 Scissors, conformational 167 Shear rate/flow 226 Sodium dodecyl sulfate 123 Solvatation 66, 72 Spin trap 153... 

FIGURE 5.19 Role of cholesterol in the exocytosis of synaptic vesicles. Cholesterol constrains the conformation of v-SNARE proteins in such a way that their transmembrane domains adopt a parallel ("closed scissors") orientation. This particular conformation is required for the fusion process because it induces a fusion-compatible curvatiue of the s)fnaptic vesicle. In this case, the interaction between v-SNARE and t-SNARE proteins triggers the fusion mechanism (lower inset, 2). Without cholesterol, v-SNARE remains in an "open scissors" conformation (upper inset, 1), and the curvature of the vesicle is not appropriate for fusion. ... 

With reference to hosts and a guest, molecular assemblies have to conform to certain circumstances, generally called complementary relationships. They involve both steric and electronic terms. The objects may be achieved by the use of properly chosen sensor groups and by a suitably tailored basic skeleton as exemplified by the present scissor- or roof-shaped host molecules. From the point of view of the introductory thoughts of this chapter (cf. Sect. 3.1), it is a matter of consideration to see how consistent the scissor or the roof simile is in the light of crystal structures. 

Considerable experimental evidence (1-6,11) suggests that the methylene chains inside of a spherical micelle are almost as disordered as in the bulk liquid state (i.e. they contain a significant proportion of gauche conformers). The FTTR spectra of micellar SDS support this assertion, exhibiting CH2 stretching and scissoring band frequencies which are comparable to those found in the spectra of liquid hydrocarbons (1-6,11). A recent quantitative analysis of the CH2 defect modes of SDS has shown that the disorder of me methylene tails is similar to that found in liquid tridecane (11). 

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. 

Mendelsohn and Mantsch, 1986). The less intense band at 1470 cm which corresponds to the scissoring mode of CH2 groups, is in some cases useful. Mendelsohn used the rocking mode wavenumbers of selectively deuterated methylene groups to monitor the conformational disorder in bilayers (Mendelsohn, 1991). 

It follows from the existence of conformational scissors (Fig. 4) that in the polymerization of symmetric or quasisymmetric dienes in hydrocarbon media on an active center with a slightly polar carbon-metal bond the primary acts of monomer attachment lead to the cis-conformation of the end unit. This conclusion is in good agreement with the modern concepts of the formation mechanism of the cis-structure of anionic polydienes [70]. According to these concepts this is followed by either the attachment of the next monomer molecule or by the cis-trans isomerization of the end unit. The microstructure is fixed at the moment of the attachment of a new monomer unit to the active center, the configuration (cis- or trans-) of the end unit being retained in the polymer chain ... 

The bands due to methylene and methyl groups occur in the 1500-1350 cm" region. At around 1470 cm, there are bauds due to CH2 bending, with the number and frequency of these bands being dependent on acyl chain packing and conformation. While the asymmetric deformation modes of the CH3 group are obscured by the scissoring bands, the symmetric deformation mode appears at 1378 cm-i. 

---