Helix opening

DnaA dnaA Initiator protein Binds oriC promotes double-helix opening DnaB loading... 

Fig. 34. Model for the ioata double helix. Open circles represent oxygen atoms and larger (dotted) circles S03-groups (after Anderson et...

Fig. 22.5. Relative peak intensity of and C=0 amide and ester carbons in the R-helix, o)L-helix and -sheet forms as deconvoluted computer fitting (closed circle) Cp, aR-helix, (open square) C=0, aR-helix, (open triangle) C=0, WL-helix (closed triangle) C, /3-sheet and (open circle) C=0, 8-sheet.

Figure 14 An experimental setting for single-crystal X-ray diffraction study at 30 K, showing a Bruker three-circle diffractometer with a SMART 1000 CCD area detector, a laser device for crystal irradiation and a HeliX open flow helium cryostat in action...

Fig. 1.4 A comparison of the I helix region in ferric and oxy-P450cam (PDB 2A1M). When Oj binds, the I helix opens up and the H-bond between Thr252 and Glu248 is...

Even though we stated at the beginning of this section that it is not our purpose to discuss the biological function of the macromolecules described, it is hard to resist saying a few words about some of their most important interactions. First, it is easy to visualize how the structure of DNA leads to its replication and hence the transfer of genetic information which is coded in the sequence of bases along the chain. Specifically, when the DNA double helix opens up it exposes the sequence of bases in each strand of the molecule. In a solution where bases, sugars and phosphates are available or can be synthesized, it is possible to form the complementary strand of each of the two strands of the parent molecule. This results in... 

The guest host effect realized in polymers on addition of a dichroic dye and untwisting of the cholesteric helix open up certain prospects for the construction of solid optical elements based on LC polymers such as dichroic polaroids and selective filters. The important possibility of obtaining ferroelectric polymeric LC materials was recently demonstrated in [67]. 

Fig. 14. Choleic acid inclusion chemistry (a) crystal stmcture of DCA inclusion compound with phenanthrene (b) view along a DCA inclusion helix accommodating DMSO and water guest molecules (oxygen and sulfur atoms and methyl groups are represented by open circles and large and small black...

Figure 4.2 A p-a-p motif is a right-handed structure. Two such motifs can be joined into a four-stranded parallel p sheet in two different ways. They can be aligned with the a helices either on the same side of the p sheet (a) or on opposite sides (b). In case (a) the last p strand of motif I (red) is adjacent to the first p strand of motif 2 (blue), giving the strand order 1 2 3 4. The motifs are aligned in this way in barrel structures (see Figure 4.1a) and in the horseshoe fold (see Figure 4.11). In case (b) the first p strands of both motifs are adjacent, giving the strand order 4 3 12. Open twisted sheets (see Figure 4.1b) contain at least one motif alignment of this kind. In both cases the motifs ate joined by an ct helix (green).

In the next class of a/p structures there are a helices on both sides of the p sheet. This has at least three important consequences. First, a closed barrel cannot be formed unless the p strands completely enclose the a helices on one side of the p sheet. Such structures have never been found and are very unlikely to occur, since a large number of p strands would be required to enclose even a single a helix. Instead, the p strands are arranged into an open twisted p sheet such as that shown in Figure 4.1b. 

The C-terminal transmembrane helix, the inner helix, faces the central pore while the N-terminal helix, the outer helix, faces the lipid membrane. The four inner helices of the molecule are tilted and kinked so that the subunits open like petals of a flower towards the outside of the cell (Figure 12.10). The open petals house the region of the polypeptide chain between the two transmembrane helices. This segment of about 30 residues contains an additional helix, the pore helix, and loop regions which form the outer part of the ion channel. One of these loop regions with its counterparts from the three other subunits forms the narrow selectivity filter that is responsible for ion selectivity. The central and inner parts of the ion channel are lined by residues from the four inner helices. 

It is well known that native collagen containes tripeptide sequences, which alone are not capable of building up a triple helix (e.g. Gly-Pro-Leu, Gly-Pro-Ser) when they exist as homopolypeptides. The synthesis of threefold covalently bridged peptide chains opens up the possibility of investigating the folding properties of such weak helix formers, because the bridging reduces the entropy loss during triple-helix formation and thereby increases the thermodynamic stability of the tertiary structure. Therefore, we have... 

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