Propeller-like structure

Folding motifs form a propeller-like structure in neuraminidase... 

Figure 5.9 The six four-stranded motifs in a single subunit of neuraminidase form the six blades of a propeller-like structure. A schematic diagram of the subunit structure shows the propeller viewed from its side (a). An idealized propeller structure viewed from the side to highlight the position of the active site is shown in (b). The loop regions that connect the motifs (red in b) in combination with the loops that connect strands 2 and 3 within the motifs (green in b) form a wide funnel-shaped active site pocket, [(a) Adapted from P. Colman et ah, Nature 326 358-363, 1987.]...

The second protein in the membrane of influenza vims, neuraminidase, does not belong to any of these three groups of barrel structures. Instead, it forms a propeller-like structure of 24 p strands, arranged in six similar motifs that form the six blades of the propeller. Each motif is a p sheet of 4 up-and-down-connected p strands. The enzyme active site is formed by loop regions on one side of the propeller. 

By ionizing the respective alkenediol, Olah and co-workers403 were able to prepare the (hexaphenyltrimethylene)methane dication 188 [Eq. (3.47)] 403 The dication possesses C3 symmetry and adopts a propeller-like structure, but no evidence for Y-aromatic stabilization was found. Furthermore, the phenyl groups themselves are twisted, which prevents the optimum overlap between the 7t-system and the vacant p-orbitals at the three C(2) carbon atoms. Calculated charge density distributions and the 13C NMR data show that C(l) is not significantly deshielded for an sp2 atom, indicating that the three C(2) carbon atoms and their phenyl substituents bear the charge of dication 188 similar to the trityl cation 135. 

Complexes of mono-valence cations (Na+, K+) sandwiched between two T tetrads (also U tetrads) have been found to adopt an Ss-symmetry [31]. It is interesting to note that intercalating a cation between two T tetrads does not alter the propeller-like structure of T tetrad. Similarly, U tetrads in the cation sandwiched U tetrads complex adopt the bowl-like structure. 

The stabilization of benzhydryl 31 and triphenylmethyl 2 is less than additive, as expected for the non-planar propeller-like structures of these radicals, which do not allow the development of full conjugation. The angle of twist is probably very similar in benzhydryl and trityl radicals 73) and one is tempted to attribute to each twisted phenyl an additive stabilization of 6 kcal mol"1. On the other hand two cyano groups in 32 likewise stabilize a radical less than additively. Phenyl and cyano (33) and phenyl and methoxy (34) show additive stabilization. For one cyclopropyl group in 35 a little more than 1 kcal mol-1 stabilization can be counted and additivity follows consequently for 36. The captodative radical 37 is stabilized according to additivity... 

X-ray study of 6 revealed a propeller -like structure (Fig. 5), in which three SiMe3 groups are in a cis position with respect to the chlorine atom. The other three myrtanyl groups are arranged on the opposite side of the chlorine. The distances between the benzamidinate moiety or the chloride atom and the Zr center are normal as compared to nonchiral Group 4 and actinide benzamid-... 

Beta propellers. Another major folding pattern is a circular array of four to eight "blades" that form a propeller-like structure. Each blade is a small, roughly triangular four-stranded antiparallel P sheet (See Figs. 11-7 and 15-23). Sequences that fold into these blades can often, but not always, be recognized as WD repeats. These are typically 44- to 60-residue sequences that have the sequence GH (Gly-His) about 11-24 residues from the N terminus and WD (Trp-Asp) at the C terminus.This repeat sequence encodes the... 

A point that might be raised is that if the three NO2 groups in C1-C(N02)3 were all coplanar with the C-Cl bond, the Oupper—Cl separations would be less and the interactions stronger. This is true, as can be seen by comparing the O— Cl distances in C1-C(N02)3 with those in CI-CH2NO2 and C1-CH(N02)2, each of which has an NO2 coplanar with C-Cl. However Gobel et al. [36] showed that such a structure would increase the total energy of C1-C(N02)3, because it interferes with the favorable O—close contacts and increases Oiower—Oiower and N—N repulsions. The beauty of the propeller-like structure is that it finds the optimum balance between all of these factors. 

We know that a phenyl grou p stabilizes an adjacent radical due to resonance (benzyl, for example), and three phenyls should provide more stabilization than two. This is indeed the case trityl radical is certainly stabilized. However, it is not possible for all three aromatic rings of trityl to be in perfect conjugation with the radical center. Simple model building will convince you that a fully planar structure would experience severe steric clashes. The trityl radical adopts the nonplanar, propeller-like structure shown in Figure 2.5. In this structure, each ring stabilizes the radical considerably, but the overall stabilization is not three times the benzyl stabilization. 

Ethylene 1 displays a single sharp line in its p.m.r. spectrum (S = 2-32 p.p.m., external TMS), indicating that rotation about the central C—bonds is rapid on the n.m.r, time scale. The requirement of a center of symmetry imposed by the lack of C=C stretching absorption in the infrared spectrum of 1 (strong Raman band at 1630 cm ) ° is, of course, consistent with a propeller-like structure as well as a planar one, and the steric barrier to coplanarity is considerable. Bond-order calculations, from Hiickel theory and vibrational spectra, indicate that nearly all of the twist in a non-planar structure would involve the C—N bonds rather than the G—G bond. ... 

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