Circular dichroism Ellipticity

Figure 23-8 Resolution of the visible circular dichroism (ellipticity) spectrum (A) and absorption spectrum (B) of the Pseudomonas blue protein into series of overlapping Gaussian hands (—). The numbers 1 to 6 refer to hands of identical position and width in both spectra. Absorption envelopes resulting from the sum of the set of overlapping Gaussian bands (—) correspond within the error of the measurement to the experimental spectra. The dashed part of the CD envelope above 700 nm was completed by a curve fitter with the use of a band in the position of hand 1 of the absorption spectrum. From Tang et al.68...

Racemization constitutes a special case of opposing first-order reactions. The equilibrium constant is unity, and the opposing rate constants are equal to one another. Racemization can be followed by polarimetry (monitoring the angle of optical rotation) or by circular dichroism (monitoring the ellipticity). The kinetic analysis can be done by either Eq. (3-15) or (3-16). The rate constant for racemization is krac = ke/2. 

Fig. 2.22 Circular dichroism spectra for/ -peptide 105 (0.1 mM, 25°C) in MeOH (solid line) and water (dashed line). The vertical axis is the mean-residue ellipticity [123]. Reprinted with permission from [123]. American Chemical Society (2001)...

Amino acid sequences of eleven homologous sea anemone polypeptides have been elucidated. All possess three disulfide bonds. The six half-cysteine residues always occur in the same positions (7,8). Initial studies concerning the toxin secondary and tertiary structures relied upon circular dichroism, laser Raman, and, to a lesser extent, fluorescence spectral measurements (15—18). The circular dichroism spectra of the four toxins so far examined are essentially superimpos-able and thus indicate a common secondary structure. The only peak observed, a negative ellipticity at 203 nm, largely results from a non-regular ("random")... 

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. 

Figure 5.32 Circular dichroism spectra of tubules made from mixtures of opposite enantiomers of DCg.gPC in 80 20 methanol-water. Spectra shown correspond to mixtures of (a) 100 0, (b) 75 25, (c) 50 50, (d) 25 75, and (e) 0 100 L-/D-DC89PC. Inset shows dependence of peak molar ellipticity on enantiomeric excess along with linear fit of data. Reprinted with permission from Ref. 125. Copyright 1998 by the American Chemical Society.

The circular dichroism spectra (Figure 1) was obtained on a 0.0155 mg/ml solution in methanol with a Cary Model 61 Circular Dichroism Spectrophotometer. The following molar ellipticity values were obtained ... 

Optical activity also manifests itself in small differences in the molar extinction coefficients el and er of an enantiomer toward the right and left circularly polarized light. The small differences in e are expressed by the term molecular ellipticity [9 J = 3300(el — r). As a result of the differences in molar extinction coefficients, a circularly polarized beam in one direction is absorbed more than the other. Molecular ellipticity is dependent on temperature, solvent, and wavelength. The wavelength dependence of ellipticity is called circular dichroism (CD). CD spectroscopy is a powerful method for studying the three-dimensional structures of optically active chiral compounds, for example, for studying their absolute configurations or preferred conformations.57... 

Ae Dielectric constant Ellipticity in circular dichroism n.i Statistical factor for ligand-i binding at n sites on a macromolecule... 

Protein concentrations were approximately 0.4%. Circular dichroism data were presented as mean residue ellipticity [0], in degrees X cm x decimole . The same mean residue weight 112.4 was employed for native lysozyme and its ozonized products, since the deviation caused by the ozonolysis of few amino acid residues of lysozyme is far less than the experimental error. 

Figure 9.19 Temperature dependent circular dichroism spectra of 1.2 x 10 " M melittin in a 43% (w/w) l-palmitoyl-2-linoleoyl-L-3-phosphatidylcholine(PLPC), cubic phase (10 irtM tris-HCl buffer, pH 7.4). Spectra taken during a heating cycle (1, 5 °C 2, 15 °C 3, 25 °C 4, 35 °C 5,45 °C) [0] is the mean residue ellipticity. (Adapted from Landau and Luisi, 1993.)...

Fig. 51a. Circular dichroism spectra for hydrated lipid 27 (0.24 mM). b Temperature dependencies of [01 values for hydrated lipid 27(0.24 mM). The circles and triangles show the molecular ellipticity at 246 and 231 nm, respectively [367]...

A mixture consisting of aniline ( 0.2 g) and (lS)-(+) camphorsulfonic acid (3.48 g) was dissolved in 10 ml of water and then treated with five separate portions of 0.1 g of ammonium peroxydisulfate dissolved in 1 ml water. Each successive portion was added when the solution turned from blue to green while the reaction mixture was maintained at 20°C. After the additions were completed the mixture was centrifuged and the product washed with water. The circular dichroism spectrum of the product suspensed in water indicated a molar ellipticity of about 90 x 103 deg-cm2/dmol. Transmission electron micrographs showed that the product had a nanofibrous structure with fiber diameters from 30 to 70 nm and had a length of several hundred nanometers. 

Transmission electron micrographs showed that the precipitate was in the form of particles, although some nanofibrous structures were observed. The circular dichroism indicated a molar ellipticity of 5 x 103 deg-cm2/dmol. 

Circular dichroism arises from the same optically active transitions responsible for the Cotton effects observed in ORD curves, but unlike ORD it is an absorption, not a dispersion, phenomenon. Hence, the CD effect is restricted to the region of the transition and can be interpreted more straightforwardly. Both ORD and CD can best be understood if one imagines the incident plane-polarized beam resolved into two in-phase circularly polarized beams whose vectors rotate in opposite directions. A difference in index of refraction between the left and right circularly polarized beams results in rotation of the transmitted plane polarized beam while differential absorption of the two circularly polarized beams results in depolarization of the transmitted beam, so that an incident plane-polarized beam whose frequency is within that of an optically active absorption band becomes both rotated and elliptically polarized upon passage through the sample. This depolarization effect is CD, and the measured parameter is (et — er), the difference in extinction coefficient between the left and right circularly polarized beams. The data is usually recorded as the specific ellipticity, defined as ... 

The dichrograph gives a direct measure of Ae. A circular dichroism (CD) spectrum often resembles an absorption spectrum, the peaks coming at the same positions as the peaks in the absorption spectrum of the same sample. However, the CD can be either positive or negative and may be positive for one transition and negative for another (Fig. 23-8). It is most convenient to plot Ae directly as a function of wavelength or wave number. However, much of the literature makes use of the molar ellipticity (Eq. 23-8) ... 

Figure 17.1 Thermal unfolding of bamase measured by calorimetry and spectroscopy. The heat capacity of bamase (trace A) was measured using differential scanning calorimetry with a baseline (trace B) of buffer versus buffer Tm is 310.9 0.01 K, A D-N(cai) = 98.4 0.2kcal/mol, and A//D N(vh) = 98.1 0.3 kcal/mol. The ellipticity at 230 nm in the circular dichroism (trace C) under identical conditions fits Tm = 310.5 0.1 K, and A//D N(vh) = 93 3 kcal/mol (equation 17.5).

Figure B3.5.3 The relation of ellipticity to the differential absorption of circularly polarized radiation. The oscillating radiation sine wave, 01, is proceeding out of the plane of the paper towards the viewer. (A) Plane-polarized radiation is made up of left- and right-handed circularly polarized components, OL and OR, respectively. Absorption by a chromophore in a nonchiral environment results in an equal reduction in intensity of each component, whose resultant is a vector oscillating only in the vertical plane—i.e., plane-polarized radiation. (B) Interaction of the radiation with achiral chromophore leads to unequal absorption, so that combination of the emerging vectors, OL and OR, leads to a resultant that describes an elliptical path as it progresses out of the plane of the paper. The ratio of the major and minor axes of the ellipse is expressed by tan 0, thus defining ellipticity. The major axis of the ellipse makes an angle (q) with the original plane, which defines the optical rotation. This figure thus demonstrates the close relation between optical rotation and circular dichroism.

Fig. 18 Circular dichroism spectra comparing (a) Trl and (b) Tr4. Titration of pDNA with Trl results in minimal change in molar ellipticity representative of B-form DNA. Tr4 elicits a shift in ellipticity to a modified B-form, suggesting interaction with DNA base pairs by the polymer. Figure adapted with permission from [156], 2008 American Chemical Society...

Upon binding of cisplatin to DNA in vitro (either ss or ds) one may monitor the differences in spectroscopic properties. A very sensitive technique has been shown to be circular dichroism spectroscopy70,103 In fact, early observations103,104) already dealt with the enhancement of the ellipticity at 275 nm, indicating the occurrence of intrastrand cross linking. Gur results on the decanucleotide (see above) show the same CD effects (Fig. 18) after platination, suggesting similar distortions in the decamer and in DNA. 

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