Optical Rotatory Dispersion and Circular Dichroism Spectra

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

The preceding discussion illustrates the continuing uncertainty as to the nature of the Ls and ligands. The properties of low-spin ferricata-lase derivatives are consistent with the presence of histidine at Ls but, seemingly, the characteristics of the free enzyme and of the low-spin ferrocatalase-nitric oxide complex are not. It could be argued, of course, that the orientation of L, or even its identity, is not invariant in different derivatives, but is mobile or interchangeable. Indeed, Samejima and Kita 103) studied the optical rotatory dispersion and circular dichroism spectra of the free enzyme and its cyanide and azide derivatives and concluded that these compounds had different amounts of helicity. 

All optical rotation measurements, optical rotatory-dispersions, and circular dichroism spectra were determined on a Cary-60 spectropolarimeter with circular dichroism attachment. In addition, some optical rotations viere determined on a Perkin-Elmer Model 141 photoelectric polarimeter. Only reagent grade chemicals were used in the syntheses mentioned below. 

The absorption data and yields for the nine complexes are reported in Table VI. The specific and molar rotations at 546 nm and 365 nm are given in Table VII. The optical rotatory dispersion and circular dichroism spectra (in aqueous solution) are given in Tables VIII and IX. The ORD and CD spectra were obtained on modified computerized Perkin Elmer Model 241 and Cary Model 61 Spec-tropolarimeters, respectively.1314... 

Fig. 3.—Ultraviolet, Optical Rotatory Dispersion, and Circular Dichroism Spectra of Compound 159.

The electronic spectra of a number of heterohelicenes have been discussed. The optical rotatory dispersion and circular dichroism spectra of the resolved one were measured. Calculation on a model compound was performed to establish the absolute configuration. The right-hand chirality was assigned to the (+)-heterohelicenes. An. ST-ray analysis of (444)... 

If two different three-dimensional arrangements in space of the atoms in a molecule are interconvertible merely by free rotation about bonds, they are called conformations if not, configurations.l85 Configurations represent isomers that can be separated, as previously discussed in this chapter. Conformations represent conformers, which are rapidly interconvertible and thus nonseparable. The terms conformational isomer and rotamer are sometimes used instead of conformer. A number of methods have been used to determine conformations.186 These include x-ray and electron diffraction, ir, Raman, uv, nmr,187 and microwave spectra,188 photoelectron spectroscopy,189 supersonic molecular jet spectroscopy,190 and optical rotatory dispersion and circular dichroism measurements.191 Some of these methods are useful only for solids. It must be kept in mind that the conformation of a molecule in the solid state is not necessarily the same as in solution.192 Conformations can be calculated by a method called molecular mechanics (p. 149). 

Optical rotatory dispersion and circular dichroism (83, 84) can often be of great value, and the spectra are particularly sensitive to the conformation of the protein. Much work remains to be done in this field before the results can be definitely interpreted in terms of the electronic structure of the metal. Magneto-optical rotation and magnetic circular dichroism (MOR and MCD), which are beginning to be applied to porphyrins and haemoproteins, offer much greater promise (30, 85). 

FIGURE 9-20 The Cotton Effect in ORD and CD, (a) Idealized optical rotatory dispersion (ORD) and circular dichroism (CD) curves at an absorption peak, with a positive Cotton effect, (b) Structures of tris-(S-alaninato) cobalt(IIl) complexes, (c) Absorption and circular dichroism spectra of the compounds in (b). (Data and structures in (b) adapted with permission from R. G. Denning and T. S. Piper, Inorg. Chem., 1966, 5, 1056. 1966 American Chemical Society. Curves in (c) adapted with permission from J. Pujita and Y. Shimura, Optical Rotatory Dispersion and Circular Dichroism, in K. Nakamoto and P. J. McCarthy, eds.. Spectroscopy and Structure of Metal Chelate Compounds, John Wiley Sons Inc., New York, 1968, p. 193. 1968 John Wiley Sons, Inc. Reprinted by permission of John Wiley Sons, Inc.)... 

As for equilibrium studies, UV-absorption spectra as well as optical rotatory dispersion and circular dichroism of various ligands and their complexes have also proved useful for kinetic investigations103, U3 Detailed analyses of relaxation effects have actually led to a deeper insight into the mechanism of complex formation and hence has provided a better understanding of the reasons underlying selectivity. 

In addition to the intensity, other properties such as polarization are concerned in optical rotatory dispersion and circular dichroism. The various processes give rise to different spectroscopic methods, as summarized in the Table 7.1. Various spectra (UV, IR, NMR and MS) of simple biomolecules can be accessed from Spectral Database Systems (SDBS) of the National Institute of Material and Chemical Research, Japan at http //www.sist.go.jp/RIODB/SDBS/menu-e.html. 

Their characteristic optical rotatory dispersion or circular-dichroism curves, and their infrared spectra, rich in characteristic frequencies, may be useful. Paper chromatography permits preliminary identification of the glycosyl phosphate or monosaccharide resulting after degradation, and the specific enzymic reactions of these products are widely used to provide additional evidence. 

The method and accuracy of proving the presence of a chiral stmcture in a polymer vary depending on the types of study and the stmcture of the polymer. Stmctural questions can be addressed by (1) various methods based on computer calculations or observations of molecular models, (2) achiral spectroscopic evidence (nuclear magnetic resonance (NMR) spectra, absorption spectra, fluorescence spectra, Raman spectra, X-ray diffraction (XRD), and so on), (3) viscosity or light scattering data giving information on the shape and size of an entire molecule, (4) chiroptical properties (optical activity, optical rotatory dispersion, electronic circular dichroism... 

Our contribution is concerned with transitions between the folded, native and the randomly coiled, denatured conformations. Conformational transitions of proteins may be observed by a variety of methods, measurements of changes in optical rotatory dispersion, in circular dichroism, in (ultraviolet) absorption and in fluorescence spectra generally giving the most precise results. The experimental results and their interpretation have recently been reviewed very extensively by Tanford (81,82 and we shall (in Section II) only discuss those observations and conclusions which are of immediate importance to our argument The single... 

The primary motivation for the development and application of vibrational optical activity lies in the enhanced stereochemical sensitivity that it provides in relation to its two parent spectroscopies, electronic optical activity and ordinary vibrational spectroscopy. Over the past 25 years, optical rotatory dispersion and more recently electronic circular dichroism have provided useful stereochemical information regarding the structure of chiral molecules and polymers in solution however, the detail provided by these spectra has been limited by the broad and diffuse nature of the spectral bands and the difficulty of accurately modeling the spectra theoretically. 

Fig. 2.—Ultraviolet Absorption Spectra, Optical Rotatory Dispersion Spectra, and Circular Dichroism Spectra of Methyl (Benzyl 2,3-Di-0-benzyl-4-deoxy-/3-L-threo-hex-4-enopyranosid)uronate (------) and its a Anonier (--------).

A. Circular Dichroism and Optical Rotatory Dispersion Spectra. 236... 

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