Orientation selectivity

Fig. 3.4 Polyamide-DNA binding motifs with equilibrium association constants K,). Hairpin amino-substitution at the a-position of the y-turn residue leads to enhanced binding affinity (10-fold) without loss of specificity, and with higher orientational selectivity and offers potential for further substitution. Cycle Cyclic polyamides show higher affinity than analogous hairpin molecules with the same number of cationic groups and eliminate all possibility of extended 1 1 binding. H-pin and U-pin compared to their non-linked analogs, H-pins and U-pins exhibit higher binding affinity. The black and open circles represent Im and Py rings, respectively diamonds repre-...

There is also the normal dipole selection rule in operation, as illustrated in Figure 5.48, due to Liith (1981). Any dipole at a surface induces an image charge within the surface. If the dipole orientation is normal to the surface, the effect is enhanced by the image dipole. If, however, the orientation is parallel to the surface, the effect is annihilated by the image dipole. This orientation selection rule thus strongly favours normally oriented dipoles. 

In powder samples with broad EPR lines, large Zeeman modulation amplitudes have to be applied to improve the sensitivity. Such amplitudes often produce microphonic noise in the cavity and cause an uncertainty in the orientation selection in single crystal-like ENDOR spectra (Sect. 4.1). A modulation technique which avoids these problems in powder ENDOR studies has been proposed by Hyde et al.32). In this scheme the Zeeman modulation is replaced by a 180° modulation of the phase of the microwave signal. 

Orientation Selection in Powders, Frozen Solutions and Nematic Glasses... 

Orientation selection 4.1 Determination of hf and quadrupole coupling constants from single crystal-like ENDOR spectra... 

Furthermore, the method of orientation selection can only be applied to systems with an electron spin-spin cross relaxation time Tx much larger than the electron spin-lattice relaxation time Tle77. In this case, energy exchange between the spin packets of the polycrystalline EPR spectrum by spin-spin interaction cannot take place. If on the other hand Tx < Tle, the spin packets are coupled by cross relaxation, and a powder-like ENDOR signal will be observed77. Since T 1 is normally the dominant relaxation rate in transition metal complexes, the orientation selection technique could widely be applied in polycrystalline and frozen solution samples of such systems (Sect. 6). 

In many planar metal complexes it is not possible to record an ENDOR spectrum which only contains contributions from Bo orientations in the complex plane. This is due to the fact that in the powder EPR spectrum the high- or low-field turning points may arise from extra absorption peakssl which do not correspond to directions of the principal axes. ENDOR spectra observed near the in-plane region of such a powder EPR spectrum are due to molecules oriented along a large number of B0 directions (in- and out-of-plane), so that the orientation selection technique is no longer effective. 

For cylindrical or disk-like shaped metal complexes the orientation selection may be improved using nematic glasses as host compounds. It is well established that the molecules of a liquid crystal in the nematic phase temperature region are oriented in a... 

Fig. 12 a, b. Orientation selection in ENDOR. a) Powder EPR spectrum of Co(salen)py. Arrow indicates EPR observer b) Single crystal-like ENDOR spectrum of the pyridine nitrogen with B0 along g . (From Ref. 80)... 

ENDOR on VO(II) complexes is facilitated for several reasons289 (a) the EPR transitions can easily be saturated (ENDOR signals are often observed at 100 K), (b) the large anisotropy of the Av tensor allows for a high orientation selectivity in powder samples, and (c) the V=0 bond may be used as internal reference axis. Moreover, the g tensor is nearly isotropic so that contributions to the hf interactions from an unquenched orbital moment may be neglected (Sect. 5.1). 

ENDOR experiments can be performed in liquid solution, in which only the isotropic hfc s (Ajso) are detected. They are proportional to the spin density at the respective nucleus. Erom the assigned isotropic hfc s a map of the spin density distribution over the molecule can be obtained. In frozen solutions and powders the anisotropic hf interactions can also be determined. Eurthermore, the method allows the detection of nuclear quadrupole couplings for nuclei with 1 1. For dominant g anisotropy as found in many metal complexes the external magnetic field can be set to several specific g values in the EPR, thereby selecting only those molecules that have their g tensor axis along the chosen field direction. In such orientation-selected spectra only those hf components are selected that correspond to this molecular orientation ( single crystal-like ENDOR ). 

Gessner, C., Stein, M., Albracht, S. P. and Lubitz, W. (1999) Orientation-selected ENDOR of the active center in Chromatium vinosum [NiFe] hydrogenase in the oxidized ready state. /. Biol. Inorg. Chem., 4, 379-89. 

Substrate-induced orientation selectivity has also been observed for thin films of FieZnPc grown in UHV from ML coverages to average thickness of about 20 nm. 

Zircon belongs to the tetragonal system and is a positive uniaxial. The typical form shows the ill and the 110 planes. The two orientations selected for luminescence polarization study were the (110) plane, parallel to the basal section and the [100] row. In such cases the axis perpendicular to the (110) plane will be called X. The orientation notation is made according to the so-called Porto notation (Porto et al. 1956). The Xi(ZX2)Xi orientation means that the laser light entered parallel to the Xi axis of the crystal and is polarized in the Z direction, while the emission is collected along the Xi axis with X2 polarization. By polarization spectroscopy with a high spectral resolution (less then 0.1 nm) six lines are observed for the Dq- Fi transition of the Eu-II center instead of the maximum three allowed for an unique site (Fig. 5.12). In Z(XX)Z geometry which corresponds to observation of a-polarized luminescence we... 

A model-free approach to analysis of DEER data in the absence of orientation selection was proposed based on shell factorization.22 The decay curves are simulated as the products of orientationally averaged thin shells of interacting electrons. The dipolar time-evolution data can be separated into a linear contribution and a non-linear contribution from background. The linear contribution can be converted to a radial distribution function for spin-spin interaction. 

Four-pulse DEER measurements were performed on a dimer of copper-substituted azurin molecules with a Cu(II)-Cu(II) distance of 26 A.27 Experiments were performed at 10 K with pulse lengths of 16 ns for ji/2 and 32 ns for p pulses and a 75 MHz difference between the frequencies of the pump and observe pulses. Analysis of the dipolar frequencies required consideration of orientation selection in both the pump and observe pulses because only a subset of the Pake pattern is represented in the Fourier transform of the experimental data. For this sample the orientation of the interspin vector relative to the g matrices of the two centres was known from high-field EPR. Dipolar modulation could not be detected for a second dimer with a copper-copper distance of 14.6 A.27... 

The presence of the four minima in the conformational map reported by Mark and Goodman for poly(W-methyl-L-alanine) (Biopolymers 1967, 5, 809) Is confirmed. The conformational map reported by Tanaka and Nakajlma for polysarcosine (Polymer J. 1970, 1, 71 see also Polymer J. 1971, 2, 717), however, Is found to be incorrect due to their faillure to consider several crucial interatomic contacts. The conformational maps for both polysarcosine and polyW-methyk-alanlnel are found to be sensitive to the orientation selected for the methyl groups. 

IETS is well suited for investigating the adsorption of organofunctional-alkoxy silanes on alumina. Surface sensitivity at the sub-monolayer level, and orientational selectivity makes this technique particularly attractive, especially when used in conjunction with other techniques such as FT-IR, and SER spectroscopies. 

The width of an ocular dominance column is approximately 0.4 mm (Hubei and Wiesel 1977). Two such columns containing the data for both eyes amount to roughly 1 mm. The orientation-selective cells are not arranged randomly. If a penetration is made horizontally to the cortical surface, recording the response of the cells, it is found that the optimal or preferred orientation to which the cell responds changes continuously. Roughly, a 1-mm displacement corresponds to a 180° change in orientation. In other words, a 2 x 2 mm2... 

The structure of layer 2 and 3 creates two new pathways the P-B (parvocellular-blob pathway) and the P-I (parvocellular-interblob pathway). The cells located inside the blobs of layer 2 and 3 are either color or brightness selective. They are not orientation selective. 

VI mainly connects to area V2, which surrounds VI (Tov6e 1996). Area V2 seems to be organized into three types of stripes, the so-called thick, thin, and interstripes. The stripes seem to be used to process visual orientation (thick stripes), color (thin stripes), and retinal disparity (interstripes). Adjacent stripes respond to the same region of the visual field. Neurons of layer 4B of V1 connect to the thick stripes. Cells found inside the thick stripes are selective for orientation and movement. Many of the cells also respond to retinal disparity. The neurons of the blobs are connected to the thin stripes. These cells are not orientation selective. More than half of these cells respond to color. Most show a double opponent characteristic. The cells of the interblob region connect to the interstripes. Neurons of the interstripe region respond to different orientations but neither to color nor to motion. A condition know as chromatopsia is caused by damage to certain parts of VI and V2. Individuals who suffer from chromatopsia are not able to see shape or form. However, they are still able to see colors. 

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