Optical Activity Measurements Research Articles

Multiple reflections in a photoelastic modulator: errors in polarization measurement

The use of a coherent light source (laser) can lead to significant errors when measurements of optical activity, magneto optical Kerr rotation, dichroism or ellipsometric parameters are down with a photoelastic modulator. In particular, a phenomenon of interferences occurs between beams arising from multiple reflections in the modulator. These interferences give rise to parasitic effects which depend on the one hand on the characteristics of the modulator and on the other hand on the wavelength of the light. A variation of temperature causes a modification of those artefacts. These have been noticed experimentally and their amplitude is in good agreement with theoretical predictions, based on a calculation of interferences. The amplitude of an artefact may reach one degree of angle in case of optical activity and is equal to five thousandth in case of measurement of a dichroism. We have shown experimentally that these effects can be cancelled by inclining the modulator with respect to the axis of the light beam or by using a new modulator with a trapezoidal section.

Measurement of Dispersive Vibrational Circular Dichroism: Signal Optimization and Artifact Reduction

Artifacts, or spurious signals, are common problems in the measurement of optical activity. Until recently they have limited the applicability of vibrational circular dichroism (VCD) in cases of weak ellipticity—for example, in structural studies of small peptides. The artifacts in dispersive VCD to be discussed here can be from the optical and electronic hardware as well as from the data processing methods in the software. The data acquisition and processing methods reported here reduce system artifacts in VCD measurements, particularly for samples with weak ellipticity.

Critical point for the blue-phase-III-isotropic phase transition in chiral liquid crystals.

The highly chiral compound S,S-4\ensuremath{''}-(methylbutyl)phenyl-4\ensuremath{'}-(methylbutyl) biphenyl carboxylate (S,S-MBBPC) undergoes a continuous supercritical evolution from the isotropic (I) phase to the third blue phase (BPI III). Mixtures of S,S-MBBPC and its racemate have been studied with high-resolution calorimetry capable of quantitative latent heat determinations and with optical activity measurements. Both experiments indicate that the first-order BP III--I transition line ends at a critical point when the chiral mole fraction ${\mathit{X}}_{\mathit{c}}$\ensuremath{\simeq}0.45. Analysis of ${\mathit{C}}_{\mathit{p}}$ and optical activity data for the near-critical mixture with X=0.45 indicates mean-field behavior instead of the theoretically predicted Ising fluctuation behavior, which would be analogous to that at the liquid-gas critical point of a simple fluid. It is speculated that the Ginzburg criterion can explain this mean-field behavior since the critical regime may be too small for experimental observation, as is the case for almost all Smectic-A--Smectic-C transitions. \textcopyright{} 1996 The American Physical Society.

Improved differentiation between luminescence decay components by use of time-resolved optical activity measurements and selective lifetime modulation

The analysis of luminescence decay experiments from proteins is typically modeled as a combination of independent first-order decay functions. However, Poisson noise in the photon counting experiment limits the ability of this approach to resolve decay components from separate lumiphores with similar lifetimes. To provide further differentiation, we incorporate time-resolved circular polarization of luminescence, an additional independent observable, into the analysis. In the simplest case, for example, each lumiphore's chirality is assumed to be time independent and is determined by the position of the lumiphore with respect to the surrounding chiral environment within the protein. In this paper, we describe the analysis of simultaneously recorded time-resolved luminescence and circularly polarized luminescence data to obtain improved temporal resolution. When combined with selective dynamic luminescence quenching, in a model system comprising a mixture of Tb/transferrin and Tb/conalbumin, we demonstrate resolution between two decay components with a lifetime difference of 7% and a difference in emission anisotropy of 5 X 10(-2). Evidence for the improved discrimination is further demonstrated by the increase in curvature of the chi 2 surface that results from the additional information.

A simple and rapid polarimetric method for quantitative determination of castor oil

AbstractA simple and rapid polarimetric method is developed for quantitation of adulteration of castor oil in edible oils such as cottonseed, coconut, mustard, olive, palm, peanut, rice bran, safflower, soybean, and sunflower. The method is based on the optical activity of ricinoleic acid (12‐hydroxy octadecenoic acid), a major constituent of castor oil. There is a good correlation between optical rotation and castor oil content in admixtures above 5%. Highly colored and viscous oils interfere in the measurement of optical activity. The method is highly specific and cost‐effective. No solvents and chemicals are required for the analysis because no sample processing is involved in the present method.

Vibrational Raman optical activity of α-helical and unordered poly(L-lysine)

Measurements of the vibrational Raman optical activity (ROA) spectra of poly(L-lysine) in H2O and D2O solutions, under conditions said to generate model α-helical and unordered conformations, are reported. These provide new insights into the conformational elements actually present. In addition to several signatures characteristic of α-helix, the α-helical state (pH 11.0, 2 °C) of a sample with text-decoration:overlineMw= 26 000 shows a strong sharp positive ROA band at ca. 1340 cm–1, previously assigned to rigid loop structure with local order possibly that of 310-helix, which suggests that the α-helical sections are connected by segments of the corresponding loop. On increasing text-decoration:overlineMw to 268 000 this loop signature grows to dominate the ROA spectrum with a concomitant decrease in the α-helix bands. The unordered state (pH 3.0, 20 °C) shows clear signatures of several distinct conformational elements, including α-helix, β-strand and, possibly, left-handed helix. Under conditions which tend to disrupt secondary structure (5 mol 1–1 NaCl solution heated to 50 °C) the ROA spectrum of the unordered state shows a decrease in the intensity of the signature of the putative left-handed helix and an increase in bands assigned to α-helix.

Following the polyisocyanate helix reversal from dilute solution through the liquid crystal and into the solid state

AbstractThe absence of a predetermined helical sense in the polyisocyanates causes the formation of helix reversals which enforce a strong cooperativity for the amplification of chiral effects. The helix reversals though also place a limit on this amplification, a limit which would be eliminated by reducing the helix reversal population. Approaching the liquid crystal state of the polyisocyanates in both dilute and concentrated solutions appears to cause the exclusion of the helix reversals which is consistent with theoretical expectations.Since the linear birefringence of the polyisocyanate solid state precludes optical activity measurements associated with the polymer, we have synthesized hydrogen bonding side chain adapted polyisocyanates which appear by DSC and infra‐red criteria to form equilibrium molecular composites with copolymers of vinyl phenol and styrene. The optical activity properties of these composites may offer a new way to probe polymer motion in the solid state.

Vibrational Raman Optical Activity of α-Lactalbumin: Comparison with Lysozyme, and Evidence for Native Tertiary Folds in Molten Globule States

Proteins in aqueous solution are now accessible to Raman optical activity (ROA) measurements, which provide an incisive new probe of secondary and tertiary structure illustrated here by a study of bovine α-lactalbumin. The room-temperature ROA spectrum of native bovine α-lactalbumin is similar to that of native hen egg-white lysozyme except for features attributable to differences in the loop regions: in particular, a positive ROA band at ∼1338 cm −1assigned to conformationally homogeneous loop structure, possibly with local order corresponding to 3 10-helix, has more than double the intensity in α-lactalbumin compared with lysozyme. This is consistent with the two proteins having similar secondary structure but different local details in the tertiary fold. ROA measurements on α-lactalbumin at pH 2.0 over a range of temperatures have provided a new perspective on the molten globule state. Thus at 35°C ROA reveals the presence of some secondary structure but an almost complete loss of the tertiary loop structure; whereas at 2°C the ROA spectrum is almost identical with that of the native protein, which is strong evidence that virtually all of the secondary structure and the tertiary backbone fold persist, albeit within a looser framework associated with increased solvent exposure and change of environment of many of the side-chains as evidenced by an increase in noise and bandwidth of some of the ROA signals together with aromatic fluorescence and near-UV circular dichroism signals characteristic of the molten globule state. Our sample of acid α-lactalbumin at 2°C therefore appears to be an archetypal example of Ptitsyn's "native-like" molten globule, having a fixed native-like tertiary fold but with loss of tight packing of the side-chains; whereas at 35°C it is a "disordered" molten globule. At 20°C the acid molten globule appears to retain highly native-like secondary structure but with most of the tertiary fold already lost. A calcium-free sample of α-lactalbumin at neutral pH displayed a broad cooperative transition between native and molten globule states at ∼15 °C, with the latter state showing similar but somewhat degraded tertiary loop ROA signatures to the native protein. In both the acid and apo molten globule states the ROA signatures of the secondary structure and the tertiary loops showed a gradual change with temperature.

Uniaxial Stress Apparatus for Measurements of Optical Activity

A device is described with which the optical activity of crystalline materials under stress can be measured. The sample is not moved, but a three-axis rotation is possible.

Anionic polymerization of optically active oxiranes

AbstractThe anionic bulk polymerization of optically active (2R, 3S)‐3,4‐epoxy‐1,2‐O‐isopropylidenebutane‐1,2‐diol (1) and its (2 S, 3 S) diastereomer 2 was studied. Molecular weights and optical activity measurements as well as carbon and proton NMR spectra are reported. The polymers show solvent dependent inversion of the sign of optical rotation. The NMR spectra are consistent with isotactic polymer chains.

Determination of enantiomeric excess using Raman optical activity

AbstractA simple method for the determination of enantiomeric excess is described. It is based on depolarized Raman optical activity (ROA) measurements in right‐angle scattering employing the incident ciruclar polarization (ICP) modulation approach. The accuracy of this method was assessed utilizing α‐pinene as a chiral test compound. The four α‐pinene ICP ROA signals associated with the Raman bands observed at ca. 772, 787, 887 and 929 cm−1 were selected to establish a linear relationship between ROA signal strength and enantiomeric composition for a set of calibration mixtures containing both α‐pinene enantiomers. Using this relationship, a (+)‐α‐pinene sample content of 16.3% was determined for a blind mixture containing 83.6% (1S)‐( — )‐α‐pinene and 16.4% (1R)‐(+)‐α‐pinene. Similary, utilizing additional spectral subtraction procedures, a (+)‐α‐pinene sample content of 1.5% was estimated for a test mixture consisting of 50.1% (1S)‐( — )‐α‐pinene and 49.9% (1S)‐( — )‐trans‐pinane.

Vibrational Raman optical activity of biopolymers

Vibrational Raman optical activity measurements can now provide a wealth of new information about the structure and conformation of biopolymers in aqueous solution. Typical results are exemplified here for proteins by bovine serum albumin in H2O and D2O solution, and for polysaccharides by laminarin.

Instrumentation for Raman optical activity measurements

We describe the latest versions of two current multichannel Raman optical activity (ROA) instruments from our laboratory. Both are based on an astigmatic single-grating spectrograph equipped with a holographic notch filter and a charge coupled device detector and employ the incident circular polarization (ICP) modulation strategy for ROA measurements, one in backscattering and the other in right-angle scattering. A selection of ICP ROA spectra is presented which demonstrates the excellent performance characteristics of both instruments, especially in the lower wavenumber region.

Optical-activity measurements with bihelicoidal laser eigenstates

A novel, doubly differential method for the measurement of optical activity that uses the so-called bihelicoidal eigenstates of a laser is demonstrated. An experimental realization is presented, and prospects are discussed.

Vibrational Raman optical activity of lysozyme: hydrogen–deuterium exchange, unfolding and ligand binding

Measurements of the vibrational Raman optical activity (ROA) spectra of hen egg white lysozyme are reported which show that ROA is a useful new probe of protein secondary and tertiary structure and dynamics. ROA spectra can be measured just as easily in D2O as in H2O and a comparison of the two gives information about the relative exchange rates of the amide hydrogens in the peptide backbone for the various types of secondary and tertiary structure in lysozyme. Unfolded lysozyme shows a large conservative ROA couplet in the amide III region which might facilitate the identification of signatures in the ROA spectra of native proteins from irregular structures with the same type of conformational heterogeneity as that of an unfolded protein. The ROA spectrum of lysozyme bound to a saccharide inhibitor shows evidence for an increase in rigid loop content.

Signal and noise analysis of non-modulated polarimeters using Mueller calculus simulations

Two non-modulated polarimeters have been modelled using the Mueller optical calculus. The effects of polarizer extinction ratio, orientation of the polarizer, source shot noise, 1/f noise and polarization ratio, on the signal-to-noise ratio of the measurement of optical activity, are evaluated. It is shown that the performance of a conventional polarimeter consisting of a crossed polarizer pair is strongly influenced by the arrangement of the optics and the type of dominant noise. The quality of the optics used is also important, if the measurement is source 1/f noise limited. The other polarimeter modelled, consists of a polarizer in combination with a polarizing prism, which resolves the light into orthogonally polarized components. Numerical simulation of this design shows that its performance is not degraded by the use of inferior optics, provided that the optics are positioned appropriately. The optimum arrangement is influenced by the dominant noise type, and in some instances, the polarization ratio and the quality of the optical components. However, the optimum arrangement may prove difficult to achieve experimentally. This design is also shown to be superior to the other scheme because of its ability to discriminate against source noise effectively.

固体の光学活性の測定法の開発と応用 ハウプの機能

The measurements of optical activity of solids were unfeasible for more than 170 years since its discovery in 1811, due to coexisting large birefrinigences. We developed an optical apparatus named HAUP (high accuracy universal polarimeter), which enabled us to measure simultaneously optical activity, birefringence and rotation of indicatrices of any crystals, even belonging to the monoclinic and triclinic systems. The outstanding features of the HAUP are explained first. The indispensable utility of optical activity in researches in material sciences is demonstrated by showing our results obtained by the HAUP.

Coherent spectroscopy of isotropic noncentrosymmetric media based on measurement of the frequency dispersion of a nonlinear optical activity

A phenomenological analysis (in terms of the nonlinear optical susceptibility) is given of a novel form of nonlinear optical spectroscopy based on measurement of a nonlinear optical activity (NOA) under the conditions of coherent anti-Stokes Raman scattering (CARS) in noncentrosymmetric media (nonracemic solutions and mixtures of mirror-asymmetric molecules). The dependence of the NOA on the state of polarisation and directions of propagation of the interacting waves is determined. The conditions are identified for discrimination of a quadrupole NOA—CARS signal from the background of the much stronger dipole CARS and optical-Kerr-effect signals. An estimate is obtained of the role of cascade processes due to lower-order (quadratic) nonlinearities of the investigated medium. A comparison is made of the NOA—CARS and Raman optical activity spectroscopy methods in the case of spontaneous Raman scattering. The intensities of the expected signals are estimated. The proposed nonlinear spectroscopy method is shown to be promising for the study of bioorganic molecules in aqueous solutions.

Optical activity measurements in solids 7. Polylactides and poly(β‐hydroxybutyrates)

AbstractThe optical activities of poly‐(R)‐lactide, poly‐(S)‐lactide, poly(β‐hydroxybutyrate) and two β‐hydroxyvalerate copolymers were measured in solution, as solid powders in suspension, and where possible, as films. Poly‐(+)‐3‐methyl‐1‐pentene was also reinvestigated. In some cases the specific rotation values of powder samples showed significant differences from the values of the solution measurements. The discrepancies of the data observed seem to reflect the local environment of the polymer chains in supermolecular assemblies and consequently the solid state structure(morphology)of the polymers.The circular dichroism (CD) spectra of the polymers were also measured in solution and in the form of their films. For comparison, the CD spectra of the naturally occurring protein casein and of the synthetic polypeptide poly‐(L)‐proline were also measured.

Instrument for natural and magnetic Raman optical activity studies in right‐angle scattering

AbstractA simple computer‐controlled instrument for natural and magnetic incident circular polarization Raman optical activity (ICP ROA) measurements in right‐angle scattering is described. The key spectral elements consist of a fast single‐stage monochromator equipped with a holographic notch filter, a highly efficient holographic diffraction grating and a thinned back‐illuminated, thermoelectrically cooled charge‐coupled device (CCD) detector. A temperature‐stabilized longitudinal electro‐optic modulator is employed to switch between orthogonal circular polarization states in the incident laser radiation. A polaroid film together with a mica half‐wave retardation plate are used to select particular linear polarization states in the Raman scattered radiation with the same instrumental response. High‐quality depolarized and polarized Stokes and depolarized anti‐Stokes ICP ROA spectra of (R)‐(−) and (S)‐(+)‐phenyloxirane and depolarized magnetic resonance ICP ROA spectra of copper(II) tetrabromide are presented as typical examples of the excellent performane characteristics.