Correction for Unequal Intensities of Left and Right Circularly Polarized Light in Steady-State and Lifetime-Resolved Fluorescence-Detected Circular Dichroism

1994 ◽  
Vol 48 (2) ◽  
pp. 167-175 ◽  
Author(s):  
Lei Geng ◽  
Linda B. McGown
Keyword(s):  
Circular Dichroism ◽  
Steady State ◽  
General Scheme ◽  
Multicomponent System ◽  
Polarized Light ◽  
Circularly Polarized Light ◽  
Circularly Polarized ◽  
Experimental Approaches ◽  
Total Absorbance ◽  
Circular Dichroïsm

A major difficulty in fluorescence-detected circular dichroism (FDCD) and lifetime-resolved fluorescence-detected circular dichroism (LRFDCD) is the generation of equal excitation intensities of left circularly polarized light (LCPL) and right circularly polarized light (RCPL). In the presence of unequal intensities, the observed FDCD signal of an optically active sample, or the resolved FDCD signals of a multicomponent system in the case of LRFDCD, will be contaminated by a factor that is the ratio of the two unequal intensities. For optically inactive samples, a sample-independent, artifactual, nonzero signal of constant magnitude is observed. A general scheme is presented for the correction of these inaccuracies caused by unequal intensities of LCPL and RCPL. Large differences between LCPL and RCPL excitation intensities were artificially introduced in steady-state FDCD measurements, and the artifact was accurately corrected by the scheme. Corrected results for the different experimental approaches that have been described for LRFDCD showed similarly good accuracy. In a related consideration, inclusion of the total absorbance and absorption circular dichroism of the sample in the calculation of the FDCD signal is shown to be essential for samples with high absorbances.

Philip John Stephens. 9 October 1940 — 31 July 2012

2013 ◽  
Vol 59 ◽  
pp. 359-382 ◽  
Author(s):  
Andrew J. Thomson ◽  
Laurence D. Barron
Keyword(s):  
Circular Dichroism ◽  
Density Functional ◽  
Magnetic Circular Dichroism ◽  
Polarized Light ◽  
Deep Understanding ◽  
Chiral Drugs ◽  
Widespread Application ◽  
Circularly Polarized Light ◽  
Circularly Polarized ◽  
Circular Dichroïsm

Philip J. Stephens was a theoretical chemist who brought to fruition two new forms of optical spectroscopy, using circularly polarized light, for the determination of electronic structure and molecular stereochemistry. The first was magnetic circular dichroism (MCD), the wavelength dependence of the differential absorption of left and right circularly polarized light induced by a magnetic field applied parallel to the light beam. Stephens established a methodology for extracting from MCD spectra the angular momentum characteristics of ground and excited electronic states and demonstrated applications to the assignment of the optical spectra of coordination complexes of transition metals and to metalloproteins. In the second half of his career Stephens led the field of vibrational circular dichroism (VCD), the measurement of the natural circular dichroism (CD) arising from the vibrational transitions of chiral molecules. He developed instrumental techniques to measure this weak dichroism over a wide frequency range with high sensitivity. Subsequently he developed a quantum-mechanical method that yielded reliable calculations of VCD spectra by using density functional theory. Thus absolute configurations of all the chiral centres in an organic molecule are readily established. Given the increasing importance of enantiomerically pure chiral drugs, VCD has found widespread application in the pharmaceutical industry. Philip had not only a deep understanding of chemical theory but also a thorough grasp of experiments. His lectures on theoretical topics were models of clarity. He was also an accomplished pianist in demand, when a student at Oxford University, as an accompanist and for chamber concerts.

scholarly journals The differential scattering of circularly polarized light by chloroplasts and evaluation of their true circular dichroism

1974 ◽  
Vol 142 (2) ◽  
pp. 193-201 ◽  
Author(s):  
Richard P. F. Gregory ◽  
Shirley Raps
Keyword(s):  
Circular Dichroism ◽  
Significant Interaction ◽  
Scattered Light ◽  
Polarized Light ◽  
Mutual Interference ◽  
Differential Absorption ◽  
Circularly Polarized Light ◽  
Circularly Polarized ◽  
Dilute Suspensions ◽  
Circular Dichroïsm

Chloroplasts isolated from pea leaves display an intense circular dichroism in the range 600 to 720nm. Circularly polarized light is also differentially scattered by chloroplasts, and this effect can be confused with circular dichroism. By using an instrumental modification it was possible to distinguish, and record separately, the ellipticities of the transmitted light (circular dichroism) and of the scattered light in the same c.d. instrument. By means of a light-scattering apparatus, the intensity of unpolarized light scattered by chloroplasts was measured as a function of wavelength and of angle. This measurement allowed the aforementioned ellipticities to be corrected for mutual interference. At a concentration of 4μg of chlorophyll/ml (the optimum practical concentration of chloroplasts at which there was no significant interaction of scattering and absorption effects) spectra of true circular dichroism (circular differential absorption) and circular differential scattering were obtained. The former showed maxima, positive at 688nm and negative at 676nm, with an intensity Δθ′8.3m°·litre·(mg of chlorophyll)-1·cm-1. The latter had a maximum at 683nm with an intensity of +47m° with respect to the solvent baseline; this value is independent of the concentration of chloroplasts in dilute suspensions. It is suggested that the intense circular dichroism of chloroplasts reflects specific chlorophyll–chlorophyll interactions in the light-harvesting pigment. The advantages of this method for determining the c.d. of scattering suspensions over those of other investigators are discussed.

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