Electric linear dichroism

The alignment of 4 -chloro-4-stilbenecarboxylic acid (CSC) intercalated in Mg/Al LDHs has been investigated [197] by electric linear dichroism, which indicated that the guest anions were intercalated as a double layer, with the tilt of the molecular plane being 40°. 

The B-DNA structure given by Amott and Hukins [171] is taken as initial conformation for the DNA fragment as this structure is close to the native DNA at physiological conditions. According to electric linear dichroism measurements [154], there are several possibilities for the mutual geometrical arrangement of the components of the physical complex. Hence, different initial orientations and conformations of PAH metabolites were considered with the aim to find the most stable structure of the intercalated complex. 

In addition to measuring solid samples by the methods just described, the orientation of dyes in suspension has been investigated using electric dichroism (199-205). Electric linear dichroism (ELD) measures the change in the absorption of incident light linearly polarized parallel and perpendicular to the applied electric field direction, as schematically shown in Figure 19. This technique has been applied extensively for dye-polyelectrolyte solutions. For cationic dye-clay systems, the roll and tilt angles of dyes at the surface of clays have been derived by detailed analysis of the ELD. It should be noted that electric dichroism can be performed only in solution of low ionic conductivity. IntCTcalation compounds... 

Linear Dichroism. In this technique, the DNA molecules are aligned either by an applied electric field pulse or in a flow gradient. The orientation of the aromatic residues of the metabolite model compounds bound to the DNA (either covalently or non-covalently) relative to the orientation of the DNA bases is probed utilizing linearly polarized light. The linear dichroism A A can be either negative or positive, and is defined as... 

Linear dichroism data with DNA oriented by an electric field [53, 54] or a linear flow [55, 56], under linearly polarised light, lead to determinations of the angle between the absorbing transition dipole moment of the chromophore in the molecule and the DNA helix axis conclusions concerning intercalation may thus be drawn from this technique. Finally, with chiral compounds, circular dichroism is also an attractive method to determine the enantioselectivity in the binding of the molecule [48, 57,58]. 

If peptide chains can be oriented in a regular fashion, it may be useful to measure infrared linear dichroism.24 25 Absorption spectra are recorded by passing plane polarized light through the protein in two mutually perpendicular directions, with the electric vector either parallel to the peptide chains or perpendicular to the chains. Such a pair of spectra is shown in Fig. 23-3A for oriented fibrils of insulin. In this instance, the insulin molecules are thought to assume a P conformation and to be stacked in such a way that they extend transverse to the fibril axis (a cross-P structure). When the electric vector is parallel to the fibril axis, it is perpendicular to the peptide chains. Since the amide I band is dominated by a carbonyl stretching motion that is perpendicular to the... 

A / is the absorbance of the sample when the light is polarized parallel to a reference axis, and Aj is the absorbance of light which is polarized perpendicular to this axis. The strength of the absorption depends on the orientation of the electric field vector of the light and the transition moment of the chromophore - parallel orientation results in maximum absorption whereas perpendicular orientation leads to zero absorption. By dividing the LD value by the absorbance of the unoriented sample under isotropic conditions (Aiso), the reduced linear dichroism (LDr), i.e. the wavelength-dependent LD, is obtained (Eq. 7) [36]. 

Infrared linear dichroism measurements may be performed on unidirectionally oriented samples. In the case of DNA, the favored axis of orientation is the axis of the double helix. Films are prepared by unidirectional stroking while the sample is gently dried. Two spectra are recorded with the electric field of the incident light oriented parallelly... 

FIGURE 11. Linear dichroism (LD) absorbance curves for anthraUn (28A) partially aligned in uni-axiaUy stretched polyethylene . Eu and Ev denote absorbance curves measured with the electric vector of the hnearly polarized light parallel (U) and perpendicular (V) to the stretching direction. The regions 1480-1430, 1380-1350 and 740-700 cm were blocked by strong polyethylene absorption... 

In a static linear dichroism experiment, one measures the sample absorbance using light propagating along the laboratory z axis and polarized along either the x or y axis. For a single electric-dipole electronic transition from state 0 to state n, the parallel and perpendicular absorbance components are then proportional to... 

Unlike the static linear dichroism, the anisotropy function r(t) exhibits nonzero values even in randomly oriented samples, because polarized excitation creates an anisotropic excited state population through the electric dipole orientational factor (x (if. However, polarized fluorescence experiments in oriented samples are potentially far more informative than in isotropically random samples (Section II,A). 

Dielectrochemical affinity in the presence of electric fields Absorbance (per centimeter) at wavelength X Electric-field-induced absorbance change Absorbance change at the light polarization angle a Linear dichroism... 

When optical changes are induced by the electric fields, light transmission and fluorescence emission appear to cover, in general, both concentration changes and rotational contributions in anisotropic systems. The linear dichroism seems to yield maximum information on molecular shape or chromophore position relative to rotation axis. " ... 

Note that the linear dichroism is given by the measured absorbance changes in the electric field. 

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