Measurement circularity

Lewis J W, Tilton R F, Einterz C M, Milder S J, Kuntz I D and Kliger D S 1985 New technique for measuring circular dichroism changes on a nanosecond time scale. Application to (carbonmonoxy)myoglobin and (carbonmonoxy)hemoglobin J. Rhys. Chem. 89 289-94... 

Fig. 6.—Measured Circular Dichroism Spectrum (-----), Calculated Circular Dichroism...

Circular Dichroism Measurements. Circular dichroism measurements were carried out by using a Cary model 6002 spectro-polarimeter calibrated with d-10 camphor sulfonic acid. All measurements were run at room temperature in the same 1 cm, quartz cell over the near-ultraviolet region (250-330 nm). 

Experimental Considerations and Apparatus for Measuring Circular Dichroism... 

Figure 3. Integrated system of Bruhat for measuring circular dichroism and optical rotation. From ref. [4].

Birefringence can be produced by mechanical stress exerted on optically isotropic media like glass and cubic crystals. Devices based on this phenomenon are used for polarization modulators. The so called photoelastic modulators (PEM) are widely used for measuring circular dichroism. The mechanical stress in the PEMs is produced by... 

There are two basic types of chiral detectors for LC, those that measure optical rotation and those that measure circular dichroism. At the time of writing this book, the only commercially available chiral detectors are those that measure optical rotation. Nevertheless, a detector that measures circular dichroism and utilizes a diode array sensor system is thought to be in the design stage and will be briefly described later. 

Another measurement, circular dichroism, CD, is caused by a difference in the absorption of right and left circularly polarized light, defined by the equation... 

Raman optical activity has only been measured so far in pure liquids and strong solutions. Crystals and powders are harder to study crystals must be polished and oriented carefully to eliminate artefacts, whereas multiple scattering in powders depolarizes the incident light. It would be of great interest to measure pure rotational, and rotational-vibrational, ROA in gases, but insufficient scattered intensity has so far prevented this. An additional complication in resonance scattering is that circular dichroism of the incident beam can contribute to the measured circular intensity difference. 

Figure 4.13 The Gecor 6 linear polarization meter with sensor controlled guard ring for corrosion rate measurement (circular sensor A) and concrete cover resistance/resistivity head (tubular sensor B). Courtesy James Instruments USA and Geocisa SA, Spain.

The basic apparatus that is used for measuring circular dichroism is shown in figure 1.2 It consists of a light source, a linear polarizer, a Fresnel rhomb that converts the linear polarized light to circularly... 

The Basic Apparatus for Measuring Circular Dichroism The modem form of the CD spectrometer is shown in Figure 1.3. 

Fig. 3. Schematic diagram for inslmmeiitation used to measure circularly polarized luminescence.

Recent papers on the relationship between optical rotatory dispersion and circular dichroism of optically active systems have stressed the superiority of the latter in interpreting the data. Unfortunately, circular dichroism measurements have been either difficult or expensive to obtain. Recently available instruments are costly and the reliability of the data these instruments afford certainly has not reached its maximum. Universal polarimeters have been described, but they require either major changes in existing insttuments or special construction of new instruments. This paper describes a method of modifying the RUDOLPH Spectropolarimeter, Model 200 AS, to measure the ellipticity of elliptically polarized light and thus indirectly measure circular dichroism. This modification requires no major changes and virtually no expense. 

Specialized instruments measuring circular dichroism are also available and have a wavelength range of 185-600 nm. Recently composite CD/ORD measuring instruments have also become available and offer a wavelength ran of 185-800 nm. 

For example, in the liquid crystal, cholesteryl 2-2-ethoxy-ethoxy ethyl carbonate (CEEC), at A = 650 nm the ORP = 285°/mm, which from (5.60) gives that I n, - ni I = 1.03 x 10". This shows that optical rotation is an extremely sensitive way to measure circular double refraction. 

We want to be able to deduce the conformation of a polymer in solution from its measured circular dichroism spectrum. For a linear polymer each ordered conformation must be a helix. The limiting cases for a helix are a straight line and a circle. A helix of identical residues is characterized by very few parameters. These are the radius of the helix, the rise per residue (the distance along the helix axis) and the number of residues per turn. A few examples will make this clear. A planar, all trans polyene will have a radius of zero (the helix is a straight line), a rise of 1.44 A and one residue per turn. A polypeptide a-helix has a radius of 2.28 A at the a-carbon, a rise per residue of 1.50 A and 3.6 residues per turn. One strand of a polynucleotide helix in the Watson-Crick B-form of DNA has a radius of 5.72 A at Cr of deoxyribose, a rise per residue of 3.4 A and 10 residues per turn. 

Jordanov, B., and D. Tsankov. 1984. A method for measuring circular dichroism of induced cholesteric solutions in the infrared region. J. Mol. Struct. 117 261-264. 

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