Other Structural Considerations

Crystal Structure of a Seven-Residue Peptide from Sup35p 

Recently, a seven-residue peptide from the Sup35p prion domain has been analyzed by X-ray crystallography (Nelson et al., 2005). Although there is, in general, little reason to suppose that a short peptide will assume the same structure in a crystal as it will in the context of a folded protein containing it, these crystals seem to be related to amyloid fibrils of the same 

In many amyloid systems, filament polymorphism has been observed by EM (e.g., Goldsbury et al., 1997, 2000). Structural variations may be expressed in terms of long-range axial repeats (Goldsbury et al, 2005 Jimenez et al., 2001), diameter (Louis et al., 2005), and/or number of protofilaments (Jimenez et al., 2002). Solid-state NMR has also been used to detect slight structural differences in Alzheimer s /1-peptide filaments 

We infer that the spread of repeat lengths appears to be determined primarily by the region from residue 1 to 65. Although the linker between that region and the appended GFP moiety has some influence, it is not the principal structural determinant. With an axial rise per subunit of 0.47 nm (see above), these repeats translate into twist angles per subunit for short repeats of about 40 nm, 4—5° per subunit for the longest repeats of about 150 nm, -1.0° per subunit. The commonest repeat of —100 nm corresponds to a twist angle of -1.8° per subunit. 

Polymorphism also seems to influence the growth rate of filaments. In their AFM study of Sup35pNM filaments involving a compilation of the growth rates measured on many in dividual filaments, DePace and Weissmann (2002) found different classes, for example, fast on both ends, slow at one end, and so on (Section V.A Fig. 8). They were able to show in several rounds of growth analysis that growth rate is an intrinsic property for each class of filaments. 

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In the case of TGT structures which are acyclic or which contain isolated rings, the disconnection of non-ring bonds must be examined to identify those disconnections which may be most effective on topological grounds. However, for such acyclic disconnections the topological factors may be overshadowed by other structural considerations. For instance, if a powerful stereosimplifying disconnective transform, such as stereospecific organometallic addition to carbonyl... 

The two exchangeable Primary Amide protons resonate at low field as either one or two very broad bands. The table of chemical shifts provided below indicates that the aliphatic Primary Amides resonate at slightly higher field than the aromatic compounds. The chemical shift(s) of these protons vary over a relatively wide range of values due to their sensitivity to the concentration of the sample solution, the solvent employed and the temperature at which the solution was examined, in addition to any possible hydrogen bonding effects and other structural considerations. 

Spectra—Structure Correlations and Other Structural Considerations... 

The next chapter explores how to analyze and design for the loads and other structural considerations that underlie such external coatings. 

Nuclear magnetic resonance spectra of all four parent compounds have been measured and analyzed.The powerful potentialities of NMR as a tool in the study of covalent hydration, tautomerism, or protonation have, however, as yet received no consideration for the pyridopyrimidines. NMR spectra have been used to distinguish between pyrido[3,2-d]pyrimidines. and isomeric N-bridgehead compounds such as pyrimido[l,2- ]pyrimidines and in several other structural assignments (cf. 74 and 75). 

Galvanic corrosion of magnesium, i.e. the enhanced corrosion to which the anodic member of a pair of metals in contact is subject to when both are in contact with a common electrolyte, is of considerable practical importance, since magnesium is anodic to all other structural metals in most electrolytes. 

Table XIV lists comparative SD and /values for fittings of all the sets of Table Xlll with each of the scales of Table V, the FandR values of Swain, and with the single substituent parameter treatment, po y These statistics, coupled with structural considerations, we believe support the usefulness and uniqueness of a scale of limited generality. In general, the / values of Table XIV for the Or scale are smaller than those of the other scales by factors of from 2 to 10. The root-mean-square F values for the other scales are from 2.25 (< j (BA)) to 3 to 4 (S L,, cr (yv)) times that for. Because this analysis has demonstrated that Swain s F and R are generally inferior for the discriminating data for all four types, there appears little to encourage proliferation of these parameters.

In Section III, a distinction is made between experimental conformation and folded conformation. Thus, the most populated conformation of a peptide is identified as the folded conformation, irrespective of experimental or secondary-structure considerations. The folded state is then associated to the folded or most populated conformation, while the unfolded state embodies all other conformations and includes any other substates. 

In this category, the units giving rise to the liquid-crystalline behavior can be in the backbone (as already discussed), in the side chains, or in both [172], Of considerable interest is the orientation of the mesogenic groups and the chain backbones to which they are attached [173,174], Frequently-studied backbones include siloxanes and acrylates [175,176], but a variety of other structures have also been studied, including amphiphilics [177]. 

According to reports Po et al. [31] and Amoco [32], the reaction rate of PEN is lower than other polyesters. Considerations about this fact lead to the assumption that the structure-dependent reactivities of the acid and glycol components and their mobilities are responsible for the individual reaction rates of these polymers. Based on unpublished data, rigid or voluminous co-monomers result in reduced reactivities during melt polycondensation and SSP. The mobility of the component, as a result of its structure and stiffness, seems to explain this observation. 

Variations in musculature are not as interesting from the standpoint of biochemistry as those involving other structures, but there are some available facts regarding variation in this area which are of considerable general interest. Because they emphasize the general importance of individuality, they support the specific theme of this bookthe importance of biochemical individuality. 

The replacement of the oxygen atom in sulfoxides by nitrogen leads to a new class of chiral sulfur compounds, namely, sulfimides, which recently have attracted considerable attention in connection with the stereochemistry of sulfoxide-sulfimide-sulfoximide conversion reactions and with the steric course of nucleophilic substitution at sulfur. The first examples of chiral sulfimides, 88 and 89, were prepared and resolved into enantiomers by Phillips (127,128) by means of the brucine and cinchonidine salts as early as 1927. In the same way, Kresze and Wustrow (129) were able to separate the enantiomers of other structurally related sulfimides. 

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