Conformation helical

Fig. 5.17 Histogram of the normal modes calculated for a polyalanine polypeptide in an a-helical conformation. The height of each bar indicates the number of normal modes in each 50cm section.

Figure 1.10 Helical conformations in polymer molecules, (a) A vinyl polymer with R substituents has three repeat units per turn, (b) The a helix of the protein molecule is stabilized by hydrogen bonding. [From R. B. Corey and L. Pauling,/ end. Inst. Lombardo Sci. 89 10 (1955).]...

Compared to Ras, Gq subunits are extended at their N-termini by about 30 residues that in free Ga are disordered but, as we will see, obtain a helical conformation in association with Gpy. 

Figure 13.30 Ribbon diagram of the structure of Src tyrosine kinase. The structure is divided in three units starting from the N-terminus an SH3 domain (green), an SH2 domain (blue), and a tyrosine kinase (orange) that is divided into two domains and has the same fold as the cyclin dependent kinase described in Chapter 6 (see Figure 6.16a). The linker region (red) between SH2 and the kinase is bound to SH3 in a polyproline helical conformation. A tyrosine residue in the carboxy tail of the kinase is phosphorylated and bound to SH2 in its phosphotyrosine-binding site. A disordered part of the activation segment in the kinase is dashed. (Adapted from W. Xu et al.. Nature 385 595-602, 1997.)...

The conformation adopted by a molecule in the crystalline structure will also affect the density. Whereas polyethylene adopts a planar zigzag conformation, because of steric factors a polypropylene molecule adopts a helical conformation in the crystalline zone. This requires somewhat more space and isotactic polypropylene has a lower density than polyethylene. 

The ct-form which forms on rapid crystallisation from the melt and which has a helical conformation. 

FIGURE 7.22 Suspensions of amylose in water adopt a helical conformation. Iodine (b) can insert into the middle of the amylose helix to give a bine color that is characteristic and diagnostic for starch. 

X-ray diffraction studies indicate the existence of a novel double-stranded DNA helical conformation in which AZ (the rise per base pair) = 0.32 nm and P (the pitch) = 3.36 nm. What are the other parameters of this novel helix (a) the number of base pairs per turn, (b) Abase pair), and (c) c (the true repeat) ... 

Derivatized amylose is the basis for the Chiralpak AD CSP. This CSP has been utilized for the resolution of ibuprofen and flurbiprofen, as well as other members of the family of nonsteroidal inflammatory drugs (NSAIDs) [39, 61]. Ibuprofen was not resolved on the Chiralpak AD CSP in LC. Pressure-related effects on stereoselectivity were observed by Bargmann-Leyder et al. on a Chiralpak AD CSP [58]. No corresponding effect of pressure on selectivity was observed with a Chiralcel OD CSP. The authors speculated that the helical conformation of the amylose-based CSP is more flexible than that of the cellulose-based CSP. 

Fig. 12. Schematic representation of the a-helix-inducing effect of specific bound anions like CIO4 or SCN" in the case of N-co-trialkylaled BPAA the extended helical conformation contracts by the insertion of the anions. The water molecules of the hydration shells are omitted 1091...

As Fig. 16 shows, the preferential binding of DMSO, DMF and NMF from aqueous solution to (Lys HBr)n at low contents of the organic solvent x increases with its concentration. However, at approximately x3 = 0,2 a maximum is reached and then preferential hydration between x3 = 0,3 and 0,5 occurs. No preferential binding was observed for NMP, EG or 2 PrOH, however increasing hydration occured with x3. Only in 2 PrOH at x3 > 0,3 a-helix formation occured. Furthermore binding parameters for the systems NMP + DMSO, EG + DMSO and DMF + DMSO have been determined. An initial preferential binding of DMSO by (Lys HBr)n, a maximum and a subsequently inversion of the binding parameter was also observed in these mixtures. The order of relative affinity is DMSO > DMF > EG > NMP. In DMF/DMSO-mixtures (Lys HBr) attains an a-helical conformation above 20 vol.- % DMF and in 2-PrOH/water above 70 vol.- % 2 Pr-OH. 

A. Side view of channel spanning the lipid layer of a planar lipid bilayer, The structure is comprised of two monomers, each in a left-handed, single stranded p -helical conformation, and joined together at the head or formyl end by means of six, intermolecular hydrogen bonds. The two formyl protons are seen at the center of the structure in this view. Replacement of these protons by methyls destabilizes the conducting dimer as shown with N-acetyl desformyl Gramicidin A (Fig. 3D). 

Exopolysaccharides in solution have an ordered helical conformation, which may be single, double or triple for example, xanthan forms a double or triple helix (Figure 7.3c). These are stabilised by intermolecular hydrogen bonds. The helical conformation makes the exopolysaccharide semirigid and the molecules can move large volumes of solution. These volumes overlap even at low concentrations of exopolysaccharide, giving rise to relatively high viscosities. 

Many features are common to all polymer electrolytes. The helical conformation... 

Hydrophobicity plots of AQPs indicated that these proteins consist of six transmembrane a-helices (Hl-H6 in Fig. la) connected by five connecting loops (A-E), and flanked by cytosolic N- and C-termini. The second half of the molecule is an evolutionary duplicate and inverse orientation of the first half of the molecule. Loops B and E of the channel bend into the membrane with an a-helical conformation (HB, HE in Fig. lb) and meet and each other at their so-called Asn-Pro-Ala (NPA) boxes. These NPA motifs are the hallmark of AQPs and form the actual selective pore of the channel, as at this location, the diameter is of that of a water molecule (3 A Fig. la and b). Based on the narrowing of the channel from both membrane sides to this small... 

Fig. 1 a, b. Projection along the chain axis and side view of models of syndiotactic polystyrene in the a) trans-planar conformation b) s(2/l)2 helical conformation... 

Fig. 2 a, b. Side view and projection along the chain axis of models of isotactic polystyrene in the a) s(3/l) helical conformation b) nearly /raw-planar conformation, proposed for the crystalline gels [12]... 

In other cases, the transition induced by thermal treatments are from a metastable to a stable form and are therefore irreversible. This is observed, for instance, for the transition from the y form of s-PS (with helical conformation of the chain) toward the a form (having a trans-planar conformation of the chain) which has been shown to occur, in reasonable times, only above T 180 °C [8,9],... 

It is hence easy to detect by this technique different polymorphic forms having different chain conformations. For instance, the a or p forms of s-PS (tram-planar chain conformations) present only a single methylene resonance at 48.1 ppm (vs.TMS), while the y form (helical conformation) presents two methylene resonances at 37.3 and 47.3 ppm (Fig. 20) [114]. 

The secondary structure of a protein is the shape adopted by the polypeptide chain—in particular, how it coils or forms sheets. The order of the amino acids in the chain controls the secondary structure, because their intermolecular forces hold the chains together. The most common secondary structure in animal proteins is the a helix, a helical conformation of a polypeptide chain held in place by hydrogen bonds between residues (Fig. 19.19). One alternative secondary structure is the P sheet, which is characteristic of the protein that we know as silk. In silk, protein... 

Fig. 18. The dimeric boronic acid anhydride 68 can potentially exist in a face-to-face or a helical form. X-ray crystallography proves the helical conformation...

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. 

Peptoids based on a-chiral aliphatic side chains can form stable helices as well [43]. A crystal of a pentameric peptoid homooligomer composed of homochiral N-(1-cyclohexylethyl)glycine residues was grown by slow evaporation from methanol solution, and its structure determined by X-ray crystallographic methods. In the crystalline state, this pentamer adopts a helical conformation with repeating cis-... 

F. E.Chiral N-substituted glycines can form stable helical conformations. Fold. Design 1997, 2, 369-375. 

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