Dichroism linear

A classic application of linear dichroism to study molecular orientations and motions in a complex biological system was R. Cone s study of induced dichroism in retinal rod outer segements [14]. Rhodopsin, the light-sensitive pigment-protein complex of the retina, contains 11-cw-retinal attached covalently to a protin (opsin) by a Schiff base linkage (Fig. 4.13A, B). Its transition dipole is oriented 

In another application, Junge et al. [16,17] measured the rate of rotation of the Y subunit of the chloroplast ATP synthase when the enzyme hydrolyzes ATP. Rotation of the y subunit relative to the a and p subunits appears to couple transmembrane movement of protons to the synthesis or breakdown of ATP. 

LD spectroscopy is a technique based on the interaction between linearly polarized radiation and chemical species [3]. Such a technique has no relation with molecular chirality and circular dichroism. It relies on the fact that the probability of inducing an electronic transition with electromagnetic radiation is proportional to the cosine square of the angle between the transition dipole moment and the polarization direction of the radiation (which coincides with the direction of oscillation of the electric field). Therefore, if a linearly polarized radiation is employed, the probability of a transition to occur is maximum when the corresponding dipole moment is parallel to the polarization direction, while the probability is zero when the transition dipole moment is oriented perpendicularly to the polarization direction. The dependence of the absorption of a sample on the polarization direction of the light is called linear dichroism. 

In an isotropic sample (e.g., a solution) the molecules, and hence the transition moments, can exhibit all possible orientations. In order to observe linear dichroism the sample molecules must be all or in part oriented along a certain direction, as it can happen for instance in crystals, polymers, thin films, gels and liquid crystals. In some cases, the molecules of an isotropic sample can be oriented by applying an electric field. 

The linear dichroism spectrum is a diagram reporting the difference between the absorption of the radiation with parallel polarizahon (An) and that of the radiation with perpendicular polarization (Ax) with respect to the direction of preferenhal orientation of the molecules, as a function of wavelength. 

The measurement of the hnear polarization of the light emitted by a sample constitute the subject of the luminescence anisotropy technique that will be discussed in Sect. 6.3. As discussed above for CPL and CD spectroscopies, luminescence anisotropy can be considered as the emission equivalent of the linear dichroism technique. 

An intriguing report by Steinke et al. [ 17] describes the creation of molecularly imprinted anisotropic polymer monoliths . Optically transparent blocks of MIP using either methacrylic acid (MAA) or 2-(acrylamido)-2-methylpropanesulphonic acid (AMPSA) as functional monomers and TRIM (trimethylolpropane trimethacrylate 2-ethyl-2-(hydroxymethyl)-l,3-propanediol trimethacrylate) as the cross-linker were synthesised using the photoactive template Michler s ketone 

These monoliths were then irradiated with linearly polarised light, causing only those template molecules having a transitional dipole moment aligned with that of the light source to photoreact with the polymer backbone within the MIP binding site. This is shown schematically in Fig. 20.3. 

The resulting monoliths displayed dichroism, with a maximum coinciding with that of the maximum of the absorption band of the (unreacted) template molecule. The observed anisotropy of the monoliths (Ah. = h — h, ca. 0.02 AU) was comparable with that of common oriented polymer films. 

Subjecting the resulting monolithic MIPs to either heating or extended solvent extraction annulled the anisotropy, presumably due to changes in the orientation of the polymer backbone. The number of analytes to which this technique can be applied is rather limited but it is, nonetheless, an interesting concept. 

The most outstanding feature of this work is that it describes the synthesis of MIPs which possess not only the attributes of typical imprinted polymers (number and strength of binding sites, pore size distribution) but that they are also transparent It is rather surprising that this has not been further exploited. Such MIPs could conceivably be used for the determination of any analyte possessing a suitable chromophore. Alternatively, they could be used, in conjunction with a chromogenic reporter, in competitive assays. 

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Che D P, Shapiro D B, Esquerra R M and Kliger D S 1994 Ultrasensitive time-resolved linear dichroism spectral measurements using near-crossed linear polarizers Chem. Rhys. Lett. 224 145-54... 

Alison Rodger and Bengt Norden, Circular dichroism and linear dichroism. Oxford University Press, Oxford, UK, 1997. 

We now report that in the region of the absorption band the flow linear dichroism of a solution of 1 is positive (Fig. 3). Assuming that the nature of the flow orientation is of the usual kind, i.e., that the polymer chains in a random coil conformation which dominates in solution (34) tend to align with the flow direction, this observation provides additional support for the absolute assignment of the transition moment direction along the chain direction, even in solution. Similar conclusions based on polarization studies on a stretched film of poly(di-n-hexyl silane) have recently been reported (36). 

Figure 3. Room-temperature flow linear dichroism of 1 in cyclohexane.

N soukpoe-Kossi, Ch., J. Sielewiesiuk, R.M. Leblanc, R.A. Bone, and J.T. Landrum. 1988. Linear dichroism and orientational studies of carotenoid Langmuir-Blodgett films. Biochim. Biophys. Acta 940 255-265. 

Jones MA, Bohn PW (2001) Total internal reflection fluorescence and electrocapillary investigations of adsorption at the water-dichloroethane electrochemical interface. 2. Fluorescence-detected linear dichroism investigation of adsorption-driven reorientation of di-N-butylaminonaphthylethenylpyridiniumpropylsulfonate. J Phys Chem B 105(11) 2197-2204... 

I. Noda, A.E. Dowrey and C. Marcott, Characterization of polymers using polarization-modulation infrared techniques Dynamic infrared linear dichroism (DIRLD) spectroscopy. 

Benninger, R. K. P., Onfelt, B., Neil, M. A. A., Davis, D. M. and French, P. M. W. (2005b). Fluorescence imaging of two-photon linear dichroism Cholesterol depletion disrupts molecular orientation in cell membranes. Biophys. J. 88, 609-22. 

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... 

When there is only one oriented chromophore present, it is important to note that the magnitude of AA is proportional to A. In such cases, the linear dichroism spectra resemble the absorption spectra 05,.6), i.e. AA 0CA. 

Two types of DNA binding sites. Two different spectroscopically distinct types of binding sites have been identified utilizing absorption, fluorescence and linear dichroism data on non-covalent (6), and covalent (7) pyrene-like metabolite model compound-DNA complexes. 

Figure 2. Typical linear dichroism spectra of non-covalent (solid lines) and covalent (dashed lines) DNA complexes (data of M. Shahbaz).

Linear Dichroism. The AA spectra of covalent adducts derived from the binding of racemic anti-BaPDE and of the enantiomer (+)-anti-BaPDE to DNA are positive in sign and similar in shape (5,31) this is expected since the (+) enantiomer binds more extensively to DNA than the (-) enantiomer (15). These covalent adducts are therefore of the site II type. 

Similar experimental results on the linear dichroism of covalent adducts derived from the covalent binding of the two enantiomers of anti-BaPDE to DNA have also been published recently(54). 

The non-covalently bound BPDEs to DNA formed initially appear to be intercalation complexes (1 6,52-55) Meehan et al. (1 6) report that the BPDE intercalates into DNA on a millisecond time scale while the BPDE alkylates DNA on a time scale of minutes. Most of the BPDE is hydrolyzed to tetrols (53-56). Geacintov et al. (5l ) have shown with linear dichroism spectral measurements that the disappearance of intercalated BPDE l(+) is directly proportional to the rate of appearance of covalent adducts. These results suggest that either there may be a competition between the physically non-covalently bound BPDE l(+) and an externally bound adduct or as suggested by the mechanism in the present paper, an intercalative covalent step followed by a relaxation of the DNA to yield an externally bound adduct. Their results for the BPDE i(-) exhibit both intercalative and externally bound adducts. The linear dichroism measurements do not distinguish between physically bound and covalent bound forms which are intercalative in nature. Hence the assumption that a superposition of internal and external sites occurs for this isomer. 

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