Oriented circular dichroism

Membrane proteins often contain a-helical sections. We have developed a method called oriented circular dichroism (OCD see reference 1), which can be used to determine the orientation of a-helices with respect to the plane of the membrane. This method is simple and easy to use compared with, for example, the NMR method, which requires isotope labeled samples. Indeed, it is the ease of this method that allowed us to examine alamethicin in many different chemical conditions and that resolved a controversial question about the nonconducting state of alamethicin and subsequently led to the discovery of a new phenomenon of amphiphilic helical peptides (2). 

In the next section, uniformly aligned multilayers of membranes are described. The third section illustrates the theoretical basis and applications of the method of oriented circular dichroism (OCD). The OCD study of alamethicin is discussed in the fourth section and the last section is devoted... 

CDAD Circular dichroism photo- Uses circularly polarized Orientation of adsorbed... 

Ever since Pasteur s work with enantiomers of sodium ammonium tartrate, the interaction of polarized light has provided a powerful, physical probe of molecular chirality [18]. What we may consider to be conventional circular dichroism (CD) arises from the different absorption of left- and right-circularly polarized light by target molecules of a specific handedness [19, 20]. However, absorption measurements made with randomly oriented samples provide a dichroism difference signal that is typically rather small. The chirally induced asymmetry or dichroism can be expressed as a Kuhn g-factor [21] defined as ... 

It may be worthwhile to compare briefly the PECD phenomenon discussed here, which relates to randomly oriented chiral molecular targets, with the likely more familiar Circular Dichroism in the Angular Distribution (CDAD) that is observed with oriented, achiral species [44 7]. Both approaches measure a photoemission circular dichroism brought about by an asymmetry in the lab frame electron angular distribution. Both phenomena arise in the electric dipole approximation and so create exceptionally large asymmetries, but these similarities are perhaps a little superficial. 

G. Schonhense and J. Honnes, Photoionization of oriented systems and circular dichroism. In U. Becker and D. A. Shirley (eds.), VUVand Soft X-Ray Photoionization, Chapt. 17, Plenum, New York, 1996, pp. 607-652. 

Optically active chromophores show different absorption for left and right circular polarized light (where the orientation of the polarized light changes periodically). These substances modify a circular polarized beam in such a way that the light is elliptically polarized after leaving the sample, an effect called circular dichroism. 

A Marietta, D Gongalvcz, ON Oliveira Jr., RM Faria, and FEG Guimaraes, Circular dichroism and circularly polarized luminescence of highly oriented Langmuir-Blodgett films of poly(/j-phenylene vinylene), Synth. Met., 119 207-208, 2001. 

Photochemically Triggered Induced Circular Dichroism in Liposomes When an optically inactive chromophore is subject to the effect of optically active environment, optical activity may be induced at the absorption wavelength of the optically inactive chromophore. This phenomenon of induced circular dichroism(ICD) is often observed in polypeptides bearing various achiral chromophores on the side chain( ). The strong chiral environment caused by the peptide helix structure is responsible for this. Distance from, and orientation to, the chiral field decide the degree of ICD appearing on the achiral chromophore. 

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]. 

This is the form of the scattering matrix for any medium with rotational symmetry even if all the particles are not identical in shape and composition. A collection of optically active spheres is perhaps the simplest example of a particulate medium which is symmetric under all rotations but not under reflection. Mirror asymmetry in a collection of randomly oriented particles can arise either from the shape of the particles (corkscrews, for example) or from optical activity (circular birefringence and circular dichroism). 

As chiral molecules, CD hosts are one of the best chiral selectors [23]. They can also induce circular dichroism signal in an achiral guest. As shown by Zhang and Nau [24] for the complexes of p-CDwith bicyclic azaalkanes 290 or 291, this effect may allow one to determine the orientation of the latter molecule in the CD cavity. An interesting example of the influence of 11 on the guest conformation was reported by Brett and coworkers [25]. They have shown that p-amino-/ -nitrobiphenyl 292, which is planar in the solid state, becomes nonplanar in the solid state complex with 3-CD. 

What we have shown in this brief review is that a combination of denaturation experiments with circular dichroism, air oxidation, and redox equilibrium experiments can lead to an understanding of the specificity of coiled-coil formation (hetero- versus homodimer formation or parallel versus antiparallel chain orientation). 

Zhang, X. and Nau, W.M. (2000) Chromophore Alignment in a Chiral Host Provides a Sensitive Test for the Orientation - Intensity Rule of Induced Circular Dichroism, Angew. Chem., Int. Ed. Engl. 39, 544-547. 

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