Dichroism, circular

The application of circular dichroism, optical rotatory dispersion, and polarimetry in organic stereochemistry has been reviewed by Snatzke. The easily prepared, non-deliquescent n-propyl- and n-butyl-ammonium salts of d-lO-camphorsulphonic acid have been used to improve the precision of the calibration of circular dichro-meters.  

The absolute configuration of (-f )-2,3-dihydrotriquinacen-2-one, and of its tetra-hydro-derivative, have been established as 15 by chiroptical measurements of the long wavelength n-n transitions. The Py-double bond is a dominant chiroptical feature of this triquinacene derivative, and the positive Cotton effect of the enhanced n-Ti transition is consistent with the absolute configurational assignment indicated by (9 RR = O). These are the first members of the triquinacene family for which 

The signs of the Cotton effects near 255 and 315 nm observed in the c.d. spectra of the IV-salicylidene derivatives of a number of cyclic terpene amines correlate with the absolute configurations of the amines. The assignment of structure (lO), to one of a pair of diastereisomeric olefins formed in the reaction of d-( + )-camphor with TiCl3-LiAlH4, is consistent with the spectroscopic and chiroptical data and is in agreement with the olefin oetant rule of Seott and Wrixon. 

The resolution of 2,3 6,7-dibenzobicyclo[3,3 l]nona-2,6-diene-4,8-diol via the (-)-menthoxy acetate affords the ( —)-diol (11), from which several other derivatives were prepared, and chemical correlation with / )-( —)-3-phenyl butanoic acid established the absolute configuration in the series as IR, 5R. The (-I-)-diketone (12) exhibited a large positive rotational strength consistent with the assigned absolute configuration and with the predominance of the contribution of the 3y-unsaturation. The ap-unsaturation, which is expected from the octant rule to give rise to a small negative n-n Cotton effect, makes but a minor net contribution. References 522 and 582 are also pertinent to this section. 

Chromophores in chiral environments generate circular dichroism (CD) as a consequence of the absorption of light. This CD is portrayed as a spectrum by the CD 

The simplest CD spectrometers display the main features for the side-chains of coded aromatic a-amino acids, for example, since these features are within the easily accessible UV wavelength range. Flowever, the additional CD data obtained for peptides and proteins by penetration to shorter wavelengths (A 200 nm) calls for more sophisticated instrumentation and interpretations described later in this chapter would not have been possible without this penetration. 

The amine, amide and carboxy chromophores that are common to the general family of amino acids, peptides and proteins show absorption features in the short-wavelength part of the ultraviolet range to establish their associated CD features requires more sophisticated spectrometers. Much of the detailed conformational information gained from CD studies depends on data from this wavelength region. 

The phenomenon of differences between the absorption of left- and of right-circularly polarised light is not restricted to the visible and UV wavelength regions, so infrared and Raman CD are likely to yield even more sophisticated information 

CD spectra carry much more information than do UV spectra the intensity of the CD absorption is dependent upon the spatial relationship between the chromo-phore and groupings at the chiral centre and therefore there is no chromo-phore-intensity-of-absorption relationship such as that which exists for UV spectra (i.e. the Bouguer-Bccr Lambert law does not apply to CD spectra). Also, the sign of the CD feature can be positive or negative, unlike the isotropic absorption (i.e. the UV spectrum), which has no sign. 

Of all the physical methods commonly in use in chemistry and biochemistry, circular dichroism (CD) is probably the most underused. The method has great potential but probably due to the still-empirical nature of many of the rules, there tends to exist a large psychological barrier to its daily use. These empirical rules, the most well-known of which is the octant rule (274), are valid for particular sets of compounds and yield invaluable information (192) if applied with care. In the 

3 and 4. Strictly speaking it reflects the sense of screwness between the two La transition moments, but, because all ester bonds adopt an %-trans conformation (Fig. 2.57), the direction of the La transition roughly parallels that of the C-0 bond. Thus, despite the flexible conformation around the C - O bond, the chirality between the La transitions approximate the chirality between the two C-O bonds, which, in the example shown, is counterclockwise or negative. 

Chiral objects absorb left and right circularly polarized light to slightly different extents. Tltis phenomenon of circular dichroism [1] became the basis of the most widespread practical chiroptical method in the past few decades. There are some other chiroptical methods based on the interaction of chiral matter with circularly polarized light Optical rotatory dispersion is based on the analogous difference in refraction. Raman optical activity measures differences in scattered light, and circularly polarized luminescence deals with the difference in emission. 

Absorbance A v) = logio(/o//), h is the intensity of light entering the cell, / is the intensity of light leaving the cell dimensionless 

Circular dichroism AA(v) = j4i(v) — Ar(v), Al(v) and j4r(v) are absorbances for left and right circularly polarized light dimensionless 

Spectroscopy Electronic circular dichroism flbrafional circular dichroism ECD VCD 

The ratio of the circular dichroism signal and the corresponding absorption is called dissymmetry factor g  

UV (with band I at 712nm and band II at 529 nm) and CD spectra of [VO(naph-tyr)] (inset the L-isomer is shown). The scale for Ae is expanded 200-fold with respect to that for e. Reproduced from M. Ebel and D. Rehder, Irwrg. Chem. 45, 7083-7090. Copyright (2006), with permission from the American Chemical Society. 

At a farther distance from the absorption region, the rotation of the molecular system is expressed as a sum of the one-term Drude formula  

Expanding this expression in a series (A2 — A ) i, in which A, has to be defined we obtain  

The series converges rapidly if Aq A(. Then, Eq. (20) may be written in the form  

Experiments with poly-a-amino acids or proteins have been in perfect agreement with the above when A0 = 212 nm and b0 = —630. Equation (23) is also called Moffitt s equation or Moffitt-Yang s equation. 

Adding LCP and RCP components of different amplitudes, yields elliptically polarized light. The major axis of the ellipsoid is the sum of amplitudes AR and AL, and the minor axis is their difference, as shown in Fig. 5. The ellipticity 0 is defined as the arctangent of the ratio of the minor axis to the major axis  

The binding of a symmetrical molecule of PLP to the active site of tryptophanase generates so-called induced optical activity ( extrinsic Cotton effect), which was detected 

Breaking of the internal PLP-lysine aldimine bond by reaction with carbonyl reagents, hydroxylamine and aminooxyacetate was also accompanied by a strong diminution of the positive CD and by a reduction of the dissymmetry factor.44 It has been inferred that the 

A positive CD was found in the 500-nm quinonoid band which is formed on reaction of tryptophanase with L-alanine and oxindolyl-L-alanine (Fig. 9.10). The dissymmetry factor in this band is much smaller than in the absorption bands of the unliganded enzyme (Table 9.2). A negative 315-nm peak, which appears in the presence of L-alanine (Fig. 9.10), may be caused by interaction of an aromatic amino acid residue with the quinonoid coenzyme ring. 

On excitation of holotryptophanase in the absorption bands at 420 nm and 337 nm, the fluorescence is maximally emitted at about 500 and 385 nm, respectively46 (see Fig. 9.11). The fluorescence lifetime depends on the presence of monovalent cations.16  

When the holoenzyme was excited at 290 nm, two emission peaks were observed, one at 340 nm, arising from the tryptophan residues, and one at about 500 nm.46-47 The intensity of fluorescence of the holoenzyme at 340 nm was about 60% of that of the apoenzyme. Therefore, binding of PLP induces a marked quenching of tryptophan fluorescence. 

The related technlgues of optical rotation and optical rotatory dispersion (OkD) - that IS, optica rotation as a function of wavelength - are less frequei itly used nowadays lor studying biomolecules 

Light is said to be circularly polarized when the oscillating electric field vector rotaies-either to the left or to II lo right-about the propagation axis of the electromagnelic wave, so that the 

Chiral molecules may respond differently to left or right circularly polarized light. In particular, there may be slight differences in absorbance of UV/visible light these provide the basis for circular dichroism (CD) spectroscopy. 

Circular dichroism is defined as the normalized difference in molar extinction  

For historical reasons related to the phenomenon of optical nitary dispersion (ORD), it is conventional to describe CD in terms of the elliplicil . 6=32982xAe, in units of millidegrees. 

Lightner has reported the first simple cases of ketones with only a dissymmetric front octant perturber. The spiro-ketone (11) was made from (-f )-7,7-dimethoxybicyclo-[2,2,l]heptan-2-ol using a Trost spiroannelation procedure. The methyl group is subject to large downfield shifts in benzene or in presence of [Eu(dpm)3], confirming the relative stereochemistry. The ketone shows a strong positive Cotton effect near 

310 nm (and a second one at 190 nm) as predicted by the Octant Rule. The ketone (12) has been made from methyladamantanone of known absolute configuration and shows a strong negative Cotton effect as predicted. Both (13) and (14) would be expected to show positive Cotton effects however, in isopentane (13) shows a negative effect (Ae —0.15) whereas (14) has a value of + 0.60, both at 295 nm. In addition Cotton effects appear at 190 nm, of + 2.8 and —1.2 respectively. The anomalous result is explicable if the nodal surface between front and rear octants is curved so that the methyl in (13) is in front.  

A study of conformationally rigid halogeno-ketones shows that a- or y-fluoro-substituents produce a red shift in the n-% band, and P- or 5-fluorines a zero or blue shift, while an iodo-group produces red shifts when a, p, or 6 but blue when y. Explanations are tentatively offered in terms of alternating charge effects and conjugative through-bond interaction. Details have appeared of the preparation and circular dichroism of the almost symmetrical a-dione (15). 

The absolute stereochemistry of a series of 9,10-dihydro-9,10-ethanoanthracenes has been established. Those such as (17) show strong Cotton effects at ca. 270 nm (a negative Lj, transition of As ca. 3) and at ca. 215 nm (a positive band with Ae ca. 60). These result from the chirality of the substituted skeleton substituents on the ethano-bridge have small effects. The results accord completely with the recent theory of Hagashita. Those such as (18) show feeble Cotton effects which are difficult to interpret. The solvent dependence of the c.d. of a number of bicyclic terpenoid alkenes has been re-examined. Band positions were solvent dependent but could not be correlated with the solvent density as had been suggested in 1973. When perfluoro-alcohols are used as solvent, isomerization may occur. 

FIGURE 12.4 [m versus X in (1) native form and (2) denatured form. 

If the intensity of absorption (not the refractive index ri) is used as a function of the orientation of the plane of polarization, we have a phenomenon called circular 

The angle 0, written as [0], is called the molar ellipticity. The relationship between As and [0] is expressed in the equation 

The dimension of As is in (L/cm) moP, while that of [0] is in deg-cm /d mol. The molar ellipticity [0] is often considered to consist of possible electronic transitions in the molecules  

The wavelength at which the maximum ellipticity is located A° The half width of the dichroism band Ri The rotational strength of the ith transition 

As noted above, ZlAco for chiral molecules is generally rather small. For electronic transitions, AAcd is usually about 0.1% of the absorbance of the peak and is fairly easily measured. For example, ultraviolet-visible CD spectroscopy has been used for many years to study the conformation of biopolymers. For vibrational transitions, AAcd is rarely more than 0.01% of the peak absorbance and is often at least an order of magnitude smaller than this. Thus, the measurement of VCD spectra presents a real challenge from an instrumental viewpoint. 

It should also be noted that the time constant for the LIA must be quite short (less than 1 ms) to ensure that the Nyquist sampling criterion is fulfilled. 

The low-frequency interferogram that would be measured in the conventional FT-IR experiment, called Vdc( ) by Nafie [6], is given by (compare Eq. 2.30) 

The PEM introduces a wavenumber- and time-dependent phase retardation angle, a(v, f), on the beam. The magnitude of the intensity variation generated by the PEM depends on the sine of a(v, t), given by 

The first term in the summation, 2yi[ao( ], corresponds to the fundamental frequency of the PEM, so that the ac interferogram measured at the output of the LIA is given by 

Although the phenomenon of magnetically induced optical activity was discovered by Faraday in 1845, it has only been during the last 

The MCD properties of tryptophan and tryptophan containing model peptides have been extensively investigated (29, 30, 32,138, 190). The MCD spectra show two oppositely signed bands which are the 

Tryptophan is the only naturally occurring amino acid that gives positive MCD bands. Furthermore the 290 nm band is intense and almost completely free of overlapping contributions by other bands associated with chromophoric side chains of other amino acid residues. This fortuitous situation suggested the possibility of using the 290 nm MCD band as an analytical and non-destructive technique to determine the tryptophan content in proteins (see Section IV.). MCD can also monitor the consequences of certain chemical modifications of tryptophan which in turn can aid in studying the participation of this residue in catalysis and/or binding of substrates, inhibitors and coenzymes. 

The intensity of the 290 nm MCD band is a linear function both of concentration and magnetic field strength. Compared to its MCD, the CD of L-tryptophan is negligibly small. The D-isomer of tryptophan, tryptophan containing peptides, and other amino and carboxyl substituted derivatives of tryptophan display MCD characteristics closely similar to those of L-tryptophan itself, both in regard to sign, wavelength maxima and band intensities. 

Solvent perturbation and denaturation studies indicate that the environment of the indole chromophore minimally affects the intensity of the 290 nm MCD band but significantly shifts the wavelength maximum necessitating its identification in the quantitation process. 

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MCD Magnetic circular dichroism. See optical rotatory dispersion. 

CDAD Circular dichroism photo- Uses circularly polarized Orientation of adsorbed... 

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

Baudelet F, Odin S, Giorgetti C, Dartyge E, Itie J P, Polian A, Pizzini S, Fontaine A and Kappler J P 1997 PtFej Invar studied by high pressure magnetic circular dichroism J. Physique IV C7 441... 

Goldbeck R A, Kim-Shapiro D B and Kliger D S 1997 Fast natural and magnetic circular dichroism Annu. Rev. Rhys. Chem. 48 453-79... 

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... 

Xie X and Simon J D 1989 Picosecond time-resolved circular dichroism spectroscopy experimental details and applications Rev. Sol. Instrum. 60 2614-27... 

Xie X L and Simon J D 1990 Picosecond magnetic circular dichroism spectroscopy J. Rhys. Chem. 94 8014-16... 

Ihe rule-based approach to protein structure prediction is obviously very reliant on th quality of the initial secondary structure prediction, which may not be particularly accurate The method tends to work best if it is known to which structural class the protein belongs this can sometimes be deduced from experimental techniques such as circular dichroism... 

Molecular chirality is most often observed experimentally through its optical activity, which is the elfect on polarized light. The spectroscopic techniques for measuring optical activity are optical rotary dispersion (ORD), circular di-chroism (CD), and vibrational circular dichroism (VCD). 

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... 

This band is not seen in normal ultraviolet spectra but can be measured for circular dichroism of 3-R-A-4-thiazoline-2-thione. where R possesses an asymmetric center (74). Representative ultraviolet data are also eiven in Refs. l.S and 75. 

As in tic, another method to vaUdate a chiral separation is to collect the individual peaks and subject them to some type of optical spectroscopy, such as, circular dichroism or optical rotary dispersion. Enantiomers have mirror image spectra (eg, the negative maxima for one enantiomer corresponds to the positive maxima for the other enantiomer). One problem with this approach is that the analytes are diluted in the mobile phase. Thus, the sample must be injected several times. The individual peaks must be collected and subsequently concentrated to obtain adequate concentrations for spectral analysis. 

FiaaHy, the use of photoreversible change of the circular dichroism for optical data storage is of iaterest. This technique offers an advantage over photochromic materials ia that the data can be read ia a way that does not damage the stored information. These chirooptic data storage devices have been demonstrated with the example of chiral peptides with azobenzene side groups (155). 

G. Snat2ke, Optical Potatory Dispersion and Circular Dichroism in Organic Chemisty Sadder Research Laboratories, Inc., Philadelphia, Pa., 1967. 

Materials characterization techniques, ie, atomic and molecular identification and analysis, ate discussed ia articles the tides of which, for the most part, are descriptive of the analytical method. For example, both iaftared (it) and near iaftared analysis (nira) are described ia Infrared and raman SPECTROSCOPY. Nucleai magaetic resoaance (nmr) and electron spia resonance (esr) are discussed ia Magnetic spin resonance. Ultraviolet (uv) and visible (vis), absorption and emission, as well as Raman spectroscopy, circular dichroism (cd), etc are discussed ia Spectroscopy (see also Chemiluminescence Electho-analytical techniques It unoassay Mass specthot thy Microscopy Microwave technology Plasma technology and X-ray technology). 

Although the usual absorption and scattering spectroscopies caimot distinguish enantiomers, certain techniques are sensitive to optical activity in chiral molecules. These include optical rotatory dispersion (ORD), the rotation by the sample of the plane of linearly polari2ed light, used in simple polarimeters and circular dichroism (CD), the differential absorption of circularly polari2ed light. 

Circular dichroism employs standard dispersive or interferometric instmmentation, but uses a thermal source that is rapidly modulated between circular polari2ation states using a photoelastic or electro-optic modulator. Using phase-sensitive detection, a difference signal proportional to the absorption difference between left- and right-polari2ed light, AA is recorded as a function of wavenumber. Relative differential absorptions... 

N. Purdie and H. G. Bnttain, eds.. Analytical Applications of Circular Dichroism, Elsevier, Amsterdam, the Nethedands, 1994. 

G. D. Fasman, ed.. Circular Dichroism and the Conformational Analysis of Biomolecules, Plenum Press, New York, 1996. 

Other spectroscopic methods such as infrared (ir), and nuclear magnetic resonance (nmr), circular dichroism (cd), and mass spectrometry (ms) are invaluable tools for identification and stmcture elucidation. Nmr spectroscopy allows for geometric assignment of the carbon—carbon double bonds, as well as relative stereochemistry of ring substituents. These spectroscopic methods coupled with traditional chemical derivatization techniques provide the framework by which new carotenoids are identified and characterized (16,17). 

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. 

Optical properties of cyanines can be usefiil for both chiral substituents/environments and also third-order nonlinear optical properties in polymer films. Methine-chain substituted die arbo cyanines have been prepared from a chiral dialdehyde (S)-(+)-2-j -butylmalonaldehyde [127473-57-8] (79), where the chiral properties are introduced via the chiral j -butyl group on the central methine carbon of the pentamethine (die arbo cyanine) chromophore. For a nonchiral oxadicarbocyanine, the dimeric aggregate form of the dye shows circular dichroism when trapped in y-cyclodextrin (80). Attempts to prepare polymers with carbocyanine repeat units (linked by flexible chains) gave oligomers with only two or three repeat units (81). However, these materials... 

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). 

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