Optical rotatory dispersion properties

The UV spectrum of a complex conjugated molecule is usually observed to consist of a few broad band systems, often with fine structure, which may be sharpened up in non-polar solvents. Such a spectrum can often be shown to be more complex than it superficially appears, by investigation of the magnetic circular dichroism (MCD) spectrum, or by introduction of dissymmetry and running the optical rotatory dispersion (ORD) or circular dichroism (CD) spectrum. These techniques will frequently separate and distinguish overlapping bands of different symmetry properties <71PMH(3)397). 

Chirooptical properties give more subtle information on the conformational behavior of biopolymers and peptides in solution. In early experiments, optical rotation and optical rotatory dispersion (ORD) have been recognized as valuable techniques, followed more recently by significant progress and refinements in the equipment which have resulted in the routine measurements of applied circular dichroism (CD). 

Werbowyj and Gray (79) examined the relationships between the cholesteric pitch and optical properties of HPC in water, CH3COOH and CH3OH. The reciproc pitch varied as the third power of the HPC concentration. Optical rotatory dispersion results show HPC has a right-handed superhelicoidal structure regardless of structure. As will be discussea below, a change in solvent can reverse the handedness of other cellulose derivatives. 

The physical and chemical properties of the AChR have been elucidated. Optical rotatory dispersion measurements indicate that the receptor consists of about 34% helix and 28-30% P-sheet structure—a high proportion of ordered secondary structure. Some carbohydrates are part of the molecule. The DNA encoding the receptor has been cloned and sequenced, revealing the complete amino acid sequence of the subunits. 

The inherent difficulty in analyzing enantiomers arises from the well-known fact that apart from their chiroptical characteristics, optical isomers have identical physical and chemical properties in an achiral environment (assuming ideal conditions). Therefore, methods of distinguishing enantiomers must rely on either their chiroptical properties (optical rotation, optical rotatory dispersion, circular dichroism), or must employ a chiral environment via diastereomer formation or interaction. Recently, it has become increasingly clear that such diastereomeric relationships may already exist in nonracemic mixtures of enantiomers via self-association in the absence of a chiral auxiliary (see Section 3.1.4.7.). 

Polarimetry, circular dichroism (CD) and optical rotatory dispersion (ORD) are the most important tools for the study of properties arising from optical activity. Although many chiral thiophenes have been prepared, there is no secure basis for a systematic discussion of the special effects of thiophene or annelated thiophene rings. For the purpose now at hand it is more expedient to discuss three different areas in which thiophene containing molecules and the related chiroptical techniques are central features. 

As discussed, optical rotatory dispersion (ORD) is determined by the unequal indices of refraction for left- and right-circularly polarized light in a chiral medium, within an absorption band, the ORD spectrum exhibits anomalous dispersion, which is referred to as a Cotton effect. Full understanding of ORD and anomalous dispersion requires a more detailed examination of the properties associated with refractive indices. 

Recently, another red protein Buchanan, Lovenberg, and Rabinowitz 32) Mortenson 72) has been isolated in crystalline form from C. pasteurianum and certain of its properties determined Lovenberg and Sobel 67)). Lovenberg and Sobel 67) named it rubredoxin because of its color and properties of an electron carrier. They showed rubredoxin differed from ferredoxin in absorption spectrum, composition and redox potential. Rubredoxin contained no inorganic sulfide the recent demonstration 49) of the similarity of the optical rotatory dispersion spectra of rubredoxin and bacterial ferredoxin makes a further comparison of the properties of these proteins particularly interesting. 

The configuration of the localized iron binding area in adrenal and testis non-heme iron proteins could be extensively studied by measuring the optical rotatory dispersion (ORD) and circular dichroism (CD). The ORD properties of various non-heme iron containing proteins were reported by Vallee and Ulmer (67). 

Both Haworth and Hirst believed in team-work, and this was seen to advantage in the research that settled the notable controversy with C. S. Hudson over the structure of D-mannose. This was concluded at a famous meeting with Haworth, Hudson, and Purves in 1930, which was fully described by Hirst in The Hudson Memorial Lecture [/ Chem. Soc., 4042-4058 (1954)]. Hirst always had great admiration for Hudson s researches on the optical properties of carbohydrates, and conducted extensive personal experimental work (with C. E. Wood) on optical rotatory dispersion. 

In modem organic chemistry optical-rotatory dispersion, the variation with wavelength of optical-rotatory power (and certain related properties), is used in molecular-structure investigation as a means of identifying and characterizing chromophore groups. Automatic polarimetric spectrophotometers of high complexity have been developed for this purpose. 

The ability of chemical compounds to change the plane of polarization of polarized light compounds may be dextrorotatory (d) or levorotatory (/). These descriptions have, in part, been replaced with R and S to indicate right and left, respectively. The optical activity of a chemical compound is a chemical property and an index of stereochemical purity. Optical rotatory dispersion and circular dichroism are measurements of optical activity. 

A solvent which has been foimd to be of great interest in connection with protein conformation studies is ethylene glycol. Sage and Singer (1958, 1962) have investigated in some detail the properties of RNase in pure ethylene glycol, containing added neutral electrolyte. They examined the ultraviolet absorption spectrum, the ionization behavior of the tyrosine residues by spectrophotometric titration experiments, and the optical rotatory dispersion of the system. 

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