Folds symmetrical

While crystal structures of rubredoxins have been known since 1970 (for a full review on rubredoxins in the crystalline state, see Ref. (15)), only recently have both crystal and solution structures of Dx been reported (16, 17) (Fig. 3). The protein can be described as a 2-fold symmetric dimer, firmly hydrogen-bonded and folded as an incomplete /3-barrel with the two iron centers placed on opposite poles of the molecule, 16 A apart. Superimposition of Dx and Rd structures reveal that while some structural features are shared between these two proteins, significant differences in the metal environment and water structure exist. They can account for the spectroscopic differences described earlier. 

The l3C NMR spectrum of the C4H7+ cation in superacid solution shows a single peak for the three methylene carbon atoms (72) This equivalence can be explained by a nonclassical single symmetric (three-fold) structure. However, studies on the solvolysis of labeled cyclopropylcarbinyl derivatives suggest a degenerate equilibrium among carbocations with lower symmetry, instead of the three-fold symmetrical species (13). A small temperature dependence of the l3C chemical shifts indicated the presence of two carbocations, one of them in small amounts but still in equilibrium with the major species (13). This conclusion was supported by isotope perturbation experiments performed by Saunders and Siehl (14). The classical cyclopropylcarbinyl cation and the nonclassical bicyclobutonium cation were considered as the most likely species participating in this equilibrium. 

The scheme of cluster condensation or cluster fragment condensation leads eventually to structures observed in bulk metals. Particularly through extensive condensation of tetrahedral and octahedral clusters, arrangements closely related to the hexagonal and cubic close-packed structures can be obtained. Condensation also of icosahedral five-fold symmetrical clusters may be related to crystalline and quasicrystalline metallic structures. 

FigurelO. LEEM images of 2-D gratings following annealing at 980C and at 1060C. The980C sample shown in a) is symmetrical with respect to maxima and minima and the saddle point features( bow-tie shapes) are approximately 4-fold symmetric hypocycloids. In the 1060C sample the minima have extensive (001) facets and the saddle point features are elongated in the directions between maxima note also that a particular reconstructed domain is preferred at the saddle points[31,34].

Fig. 1.15. Bending of the DNA in the CAP protein-DNA complex. The CAP protein ( . coli) binds as a dimer to the two-fold symmetric operator sequence. The DNA is bent nearly 90deg in the complex. The turns are centered around two GT sequences (shown in black) of the recognition element.

Fig. 1.19. Tetramerization of the Lac repressor and loop formation of the DNA. The Lac repressor from E. coli binds as a dimer to the two-fold symmetric operator seqnence, whereby each of the monomers contacts a half-site of a recognition sequence. The Lac operon of E. coli possesses three operator sequences Of, 02 and 03, aU three of which are required for complete repression. Of and 03 are separated by 93 bp, and only these two sequences are displayed in the figure above. Between Of and 03 is a binding site for the CAP protein and the contact surface for the RNA polymerase. The Lac repressor acts as a tetramer. It is therefore assumed that two dimers of the repressor associate to form the active tetramer, whereby one of the two dimers is bound to 03, the other dimer binds to Of. The intervening DNA forms a so-caUed repression loop. After Lewis et al., 1996.

The HREs of the steroid hormone receptors posses a palindromic structure, comparable to the DNA binding elements of procaryotic repressors (see fig. 4.7a). The glucocorticoid receptor, for example, binds as a homodimer to the two-fold symmetrical recognition sequence, whereby the receptor is already dimerized in solution. In complex with the DNA each subimit of the dimer contacts one half-site of the HRE. As a consequence of the two-fold repeat of the recognition sequence, a high affinity binding of the receptor dimer results (compare 1.2.4). 

The orbit tiling consists of a 1-gon (the image of a 5-gon with a 5-fold symmetry axis), a 5-gon (the image of an asymmetric 5-gon), and a 4-gon (the image of a 2-fold symmetric 8-gon). Exactly one degree 1 vertex and no half-edge occurs, because there are 2-fold symmetric, but no 3-fold symmetric, faces. 

There are comparatively few molecules possessing four-fold and five-fold symmetrical rotational barriers. Six-fold barriers are more common and for them only V6, V12,...,etc,apply. 

Over the past several years, Gruen and coworkers have examined the SH response from iron electrodes in alkaline solutions [45, 53, 172]. In their work on polycrystalline iron, they concluded that the potential dependent SH response which was observed during surface oxidation could be attributed to two intermediate phases on the electrode surface between the passive film at oxidative potentials and the reduced metal at hydrogen evolution potentials [53]. They have recently extended this work to Fe(110). In this study [172], they examined the SH rotational anisotropy from this crystal under ambient conditions. They found that the experiments reveal the presence of both twofold and threefold symmetric species at the metal/oxide interface. When their data is fit to the theory of Tom et al. [68], they conclude that the measured three-fold symmetric oxide is found to be tilted by 5° from the Fe(110) plane. The two-fold symmetric structure is aligned with the Fe(110) surface. 

We also systematically measured the fs-resolved fluorescence transients with the addition of KC1, and the results are nearly the same as those obtained with KI. The anion CP and P make no difference to the solvation dynamics observed here. As a comparison, we also studied another crown ether supramolecule with only one indole sidearm. With and without KI, the results are similar to those obtained for BI18C6 with two indole sidearms. This observation is consistent with the folded symmetric structure (Fig. 22b and c), and either of two indole rings gives similar solvation behaviors. However, in solvent CH3OH, the solvation dynamics, decay in 0.95-16.3 ps and rise in 0.55-12 ps, are similar in the presence and absence of K+ with slightly faster processes for the former. Both results have the similar trend as tryptophan in CH3OH. 

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