Pitch of helix

Chira.lNema.tlc, If the molecules of a Hquid crystal are opticaHy active (chiral), then the nematic phase is not formed. Instead of the director being locaHy constant as is the case for nematics, the director rotates in heHcal fashion throughout the sample. This chiral nematic phase is shown in Figure 7, where it can be seen that within any plane perpendicular to the heHcal axis the order is nematic-like. In other words, as in a nematic there is only orientational order in chiral nematic Hquid crystals, and no positional order. Keep in mind, however, that there are no planes of any sort in a chiral nematic Hquid crystal, since the director rotates continuously about the heHcal axis. The pitch of the helix formed by the director, ie, the distance it takes for the... 

This jacket is considered a special case of a helical coil if certain factors are incorporated into equations for calculating outside-film coefficients. In the equations at left and below, the equivalent heat transfer diameter D. for a rectangular cross-section IS equal to four times the width of the annular space, w and IS the mean or centerline diameter of the coil helix. Velocities are calculated from the actual cross-section of the flow area. pw. where p IS the pitch of the spiral baffle, and from the effective mass flowrate. W. through the passage. The leakage around spiral baffles is considerable, amounting to 35-50% of the total mass flowrate. The effective mass flowrate is about 60% of the total mass flowrate to the jacket W =... 

And diis meant that die chain could turn corners only at die alpha carbons.. .. I creased die paper in parallel creases dirough the alpha carbon atoms,. so that I could bend it and make die bonds to the alpha carbons, along die chain, have tetrahedral value. And then I looked to see if I could form hydrogen bonds from one part of the chain to the next. He. saw diat if he folded die. strip like a chain of paper dolls into a helix, and if he got die pitch of the screw right, hydrogen bonds could be shown to form,... 

FIGURE 17.14 The helical arrangement of actin monomers in F-actin. The F-actin helix has a pitch of 72 nm and a repeat distance of... 

The screw compressor can be visualized as a development of the gear pump. For gas pumping, the rotor shapes are modified to give maximum swept volume and no clearance volume where the rotors mesh together, and the pitch of the helix is such that the inlet and outlet ports can be arranged at the ends instead of at the side. The solid portions of the screws slide over the gas ports to separate one stroke from the next, so that no extra inlet or outlet valves are needed. 

Extrapolation of the molecular structure of an a-maltohexaose duplex com-plexed with triiodide in single crystals leads to a left-handed, 8-fold, antiparallel double-helix for amylose.90 The pitch of this idealized helix is 18.6 A, so h is only 2.33 A. Although this model is no contender to the fiber data, in terms of biosynthesis, it is doubtful that the native amylose helix favors antiparallel chains. 

Here, ry is the separation between the molecules resolved along the helix axis and is the angle between an appropriate molecular axis in the two chiral molecules. For this system the C axis closest to the symmetry axes of the constituent Gay-Berne molecules is used. In the chiral nematic phase G2(r ) is periodic with a periodicity equal to half the pitch of the helix. For this system, like that with a point chiral centre, the pitch of the helix is approximately twice the dimensions of the simulation box. This clearly shows the influence of the periodic boundary conditions on the structure of the phase formed [74]. As we would expect simulations using the atropisomer with the opposite helicity simply reverses the sense of the helix. 

A peptoid pentamer of five poro-substituted (S)-N-(l-phenylethyl)glycine monomers, which exhibits the characteristic a-helix-like CD spectrum described above, was further analyzed by 2D-NMR [42]. Although this pentamer has a dynamic structure and adopts a family of conformations in methanol solution, 50-60% of the population exists as a right-handed helical conformer, containing all cis-amide bonds (in agreement with modeling studies [3]), with about three residues per turn and a pitch of 6 A. Minor families of conformational isomers arise from cis/trans-amide bond isomerization. Since many peptoid sequences with chiral aromatic side chains share similar CD characteristics with this helical pentamer, the type of CD spectrum described above can be considered to be indicative of the formation of this class of peptoid helix in general. 

Fig. 11 Phase diagram of a twisted ribbon solution that forms fibrils. The axes represent relative helix pitch of isolated ribbons and relative stacking attraction of ribbons. Reproduced from Aggeli et al. [20]. Copyright 2001 National Academy of Sciences, USA...

To quantitatively characterize the PM populations using chiroptical characteristics, it may be useful to use the gabs values of 16 at each temperature with reference to the regression curve of these gabs values in 17, which is assumed to adopt a purely P 73-helical structure, instead of the Ae value. The analysis is based on the assumption that the weak temperature dependence of the ymax for 17 is due to a minute modification in the screw pitch of the P helix, rather than any formation of the M-helical motif. 

---