Dichroism Spectra

The first reported optically active PAn s were the PAn/(+)-HCSA and PAn/(-)-HCSA salts (HCSA = 10-camphorsulfonic acid), prepared in our laboratories by the electropolymerization of aniline in the presence of either (+)- or (-)-HCSA.68 69 The optically active dopant anions are believed to induce a preferred one-handed helical arrangement in the PAn chains in these emeraldine salts 4, giving rise to intense CD bands in the visible region. Recent studies by Li and Wang70 have shown that PAn/(+)-HCSA films of exceptionally high optical activity can be electrochemically deposited by polymerizing the aniline monomer in the presence of small amounts of aniline oligomers. 

Optically active films of PAn/(+)-HCSA and POM A/(+)-HCSA may be dedoped in 1 M NH4OH to give the corresponding optically active EBs,69,76 and the oxidation (with S2082-) and reduction (with N2H4) of such films have also been shown to yield optically active PB and LEB films, whose chiroptical properties have been exhibited 

FIGURE 5.8 CD spectra of spun-cast 1 1 PMAS/(+)-PhEA and PMAS/(-)PhEA films. 

PAn structures and chiral recognition properties has been increasingly reported by other research groups over the last few years, as summarized in the following text. 

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Benedetti M, Biscarini P and Brillante A, The effect of pressure on circular dichroism spectra of chiral transition metal complexes Physica B 265 1... 

The unmodified and complementary oligonucleotides were also synthesized, in order to detect thermodynamic and spectroscopic differences between the double helices. Circular dichroism spectra revealed that the covalently bound anthracene does not stack in the centre of the DNA double helix. Mutagenic activity by intercalative binding of the anthracene residue is thus unlikely. Only in vitro and in vivo replication experiments with site-specifically modified... 

The secondary stmcture of the plasminogen molecule, as determined by circular dichroism spectra, is 80% random coil, 20% beta-stmcture, and 0% alpha-helix. Electron microscopy has demonstrated the tertiary stmcture of plasminogen to be a 22- to 24-nm long spiral filament with a diameter of 2.2 to 2.4 nm. 

Data on the protonation sites for a variety of azaindolizines have been summarized in the previous survey (76AHCS1, p. 536).Tlie measured magnetic circular dichroism spectra for 14 azaindolizines confirmed with some corrections the protonation sites of polyazaheterocycles previously reported and also established the conjugated acid structures of additional compounds (85JOC302).Tlie data are summarized in Scheme 1. 

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")... 

Fig. 4.—Difference-Circular Dichroism Spectra or-D-manno-Heptulose minus...

Spectrum (- -), and Fragment Circular Dichroism Spectra (—) for the Corresponding d-Xylose and the Addition of a Hydroxymethyl Group at C-S (a) a-o-Glucose (b) /3-D-Glucose (c) Methyl a-o-Glucopyranoside and (d) Methyl /8-D-Glucopyranoside. (Redrawn from Ref. 

Fig. 7.—Predicted (- -) and Fragment (—) Circular Dichroism Spectra /3-L-Arabinose Calculated from (a) a-D-Xylose, and (b) Methyl /3-L-Aiabinoside a-L-Arabinose Calculated from (c) j3-D-Xylose, and (d) Average Calculated for /3-L-Arabinose Methyl a-L-Arabinoside Calculated from (e) Methyl /3-D-Xyloside, and (f) Methyl /3-L-Arabinoside. (Redrawn from Ref. 6.)...

Fig. 9.—Comparison of the Circular Dichroism Spectra Observed for Amylose ( ). Cyclomaltohexaose (-), and Methyl a-o-GIucopyranoside (- - -) (From Ref. 19.)...

Fig. 10.—Circular Dichroism Spectra of Amylose (-), Maltohexaose (—), Maltotetraose...

Fig. 27.—Circular Dichroism Spectra of Galactopyranuronic Acid for d.p. = 2 (First and Third Panels) and d.p. = 340 (Second and Fourth Panels) as a Function of Percent of Neutralization with NaOH (First and Second Panels) and Ca(OH)2 (Third and Fourth Panels) 0%...

Stephens, P. J., Devlin, J. F., Chabalowski, C. F., Frisch, M. J, 1994, Ab Initio Calculations of Vibrational Absorption and Circular Dichroism Spectra Using SCF, MP2, and Density Functional Theory Force Fields , J. Phys. 

Optical Rotatory Dispersion and Circular Dichroism Spectra. 181... 

The photochemical formation and the analysis of the absorption and magnetic circular dichro-ism spectra of the anion radical of zinc phthalocyanine were carried out. A complete assignment of the optical spectrum of the anion radical was proposed.834 Similarly, spectroelectrochemical cells have been used to record absorption and magnetic circular dichroism spectra of zinc phthalocyanines and a band assignment scheme proposed.835... 

The redox potentials of zinc-substituted phthalocyanines are shown to be linearly dependent on the total Hammett substituent constant.837 In 1987, Stillman and co-workers used the absorption and magnetic circular dichroism spectra of the zinc phthalocyanine and its 7r-cation-radical species to assign the observed bands on the basis of theoretical calculations. The neutral and oxidized zinc phthalocyanine complexes with cyanide, imidazole, and pyridine were used with the key factor in these studies the stability of the 7r-cation-radical species.838 The structure of zinc chloro(phthalocyaninato) has been determined and conductivity investigated.839... 

Natural circular dichroism (optical activity). Although circular dichroism spectra are most difficult to interpret in terms of electronic structure and stereochemistry, they are so very sensitive to perturbations from the environment that they have provided useful ways of detecting changes in biopolymers and in complexes particularly those remote from the first co-ordination sphere of metal complexes, that are not readily apparent in the absorption spectrum (22). It is useful to distinguish between two origins of the rotational strength of absorption bands. 

Optical activity in metal complexes may also arise either if one of the ligands bound to the metal in the first co-ordination sphere is itself optically active or if the complex as a whole lacks a centre of inversion and a plane of symmetry. Thus all octahedral cts-complexes of the tris-or bis-chelate type have two isomeric forms related by a mirror plane, the d- and /-forms. These species have circular dichroism spectra of identical intensities but opposite in sign. The bands in the circular dichroism spectrum are, of course, modified if ligand exchange occurs but they are also exceedingly sensitive to the environment beyond the first co-ordination sphere. This effect has been used to obtain association constants for ion-pair formation. There also exists the possibility that, if such compounds display anti-tumour activity, only one of the mirror isomers will be effective. 

UNFOLDED PEPTIDES AND PROTEINS STUDIED WITH INFRARED ABSORPTION AND VIBRATIONAL CIRCULAR DICHROISM SPECTRA... 

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. 

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

The enzymatic activities of O -chymotrypsin in solution and adsorbed at the different surfaces are presented in Fig. 11, where the specific enzymatic activity (defined as activity per unit mass of protein) is plotted as a function of temperature. The enzyme loses activity due to adsorption. On the hydrophobic Teflon and PS surfaces, the activity is completely gone, whereas on the hydrophilic silica surface, or-chymotrypsin has retained most of its biological function. These differences are in agreement with the adsorption isotherms and the circular dichroism spectra. The influence of the hydrophobicity of the sorbent surface on the affinity of the protein for the sorbent surface, as judged from the rising parts of the adsorption isotherms (Fig. 8), suggests that the proteins are more perturbed and, hence, less biologically active when adsorbed at hydrophobic surfaces. Also, the CD spectra indicate that adsorption-induced structural perturbations are more severe at hydrophobic surfaces. 

Magnetic circular dichroism spectra of some 3-arylfervenulins and 3-aryltoxoflavins were studied (78JHC615). Hammett plots showed that substituents in both compounds exerted opposite effects on the magnetic CD spectra. The difference was explained by the contribution of the 1,5-dipolar structure of 3-aryltoxoflavins. 

Although, as just pointed out, a number of the properties of Co(n) cage and non-cage metal complexes may be very different, similarities between related complexes also occur. For example, the magnetic and visible spectral properties of [Co(l,2-diaminoethane)3]3+ and [Co(sepul-chrate)]2+ are essentially the same, although some differences in their circular dichroism spectra are apparent. Differences also occur in the electrochemical behaviour of cage and non-cage species but further mention of these is deferred until Chapter 8. 

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