Expressions for the interpretation of circular intensity differential scattering of chiral aggregates

The derivation of compact expressions of the circular intensity differential scattering (CIDS) of chiral molecules is presented in the first Born approximation of the fields. The expressions derived are valid for a suspension of scattering chiral particles free to adopt any orientation in solution. The connection is established between the preferential scattering cross section for right- vs left-circularly polarized light for a given scattering angle and the geometrical parameters of the molecule. As observed experimentally, the equations predict that the circular differential scattering patterns must show as a function of the scattering angle a series of lobes of alternating sign. In between these lobes, zeros in the differential scattering cross section occur. For the case of two dipole moments arranged in chiral fashion, an expression is derived that shows how the relative arrangement of the dipoles and their separation relative to the wavelength of light control the number and the position of the zeros. A compact expression predicting the CIDS of a sample for very small angles of scattering is derived for a system of helices whose dimensions are small compared with the wavelength of light. Finally, the presence of CIDS in a sample is related to the appearance of anomalous signals in the CD spectrum of chiral systems. Expressions and computations of the magnitudes and sign of the anomalies are presented. The expressions obtained confirm the main features of the experimental CIDS patterns of chiral molecules previously published.     

Helically organized macroaggregates of pigment-protein complexes in chloroplasts: evidence from circular intensity differential scattering

Angle dependence of circular intensity differential scattering (CIDS) and of nonpolarized scattering was determined in isolated spinach chloroplasts at 514.5 nm. CIDS between 0 degrees and 170 degrees was independent of the nonpolarized scattering and showed intense lobes of alternating signs, exhibiting the negative and positive maxima around 15 degrees and 70 degrees, respectively. These results provide experimental evidence for the existence of large helically organized macroaggregates of pigment-protein complexes in thylakoid membranes. Modeling of the CIDS data by a simple helical array of uniaxial polarizable groups suggests that the chiral structure is left-handed with pitch and radius of the order of 385 nm.     

Circular dichroism studies on single Chinese hamster cells

The circular dichroism (CD) and circular intensity differential scattering (CIDS) contributions to the CD of single Chinese hamster (CHO) cells have been measured as a function of the position in the cell cycle. The data are analyzed in three main spectral regions: (1) the region above 290 nm (scattering region), (2) the 250-290-nm regions (nucleic acid absorption region), and (3) the region below 240 nm (protein absorption region). The results show that CD/CIDS microspectrophotometry is a good indicator of the cell cycle phase. The results are consistent with the view that chromatin is organized in chiral superstructures which differentially scatter circularly polarized light. These structures appear highly specific and repeatable as the cell passes through its cycle.     

Differential scattering of circularly polarized light by chromatin modeled as a helical array of dielectric ellipsoids within the born approximation

Theoretical predictions within the Born approximation of the expected differential light scattering of circularly polarized light (CIDS) were made for the 300-Å chromatin fiber, modeled as a helical array of dielectric ellipsoids. Computed CIDS values were strongly dependent on the exact geometry of the solenoid model, depending particularly on parameters relaed to the chiral nature of the fiber and the orientation of the nucleosomes within the helix, in contrast to the values of the total light scattering, which mainly probed size and shape. In particular, both a superbead model and a strict linear 110-Å nucleosome filament would be predicted as giving rise to zero CIDS (in disagreement with the finite values observed). At the same time, helical models in which the normal vectors to the nucleosome faces were exactly parallel to the helical axis also yielded zero CIDS. Confirming earlier expectations, CIDS values were significantly less dependent on helical length than total light scattering. Finally, comparison of these calculated results from those extrapolated from available experimental data indicates that predicted CIDS values, based on currently accepted models of solenoid structure, are within an order of magnitude of those experimentally observed. Together, these results indicate the potential of differential light scattering measurements as a probe of chromatin higher-order structure, complementary to existing scattering measurements.     

Model computations on the differential scattering of circularly polarized light (CIDS) by dense macromolecular particles

Circular intensity differential scattering (CIDS) patterns have been calculated for two general types of chiral arrangements. The calculations were done using the second Born approximation. The geometries considered are (a) a linear array of anisotropic scattering groups with a spiraling orientation (a twisted ladder) and (b) a helical array of scattering groups with arbitrary anisotropy and orientation. The CIDS expressions for the first of these models (twisted ladder) reduces to a simple analytical form and clearly illustrates many properties of CIDS patterns. The second geometry (helix) is more general. Calculations were done for oriented scatterers and for rotationally averaged scatterers as would be found in solution.     

基于旋转偏振角的线偏振扫描成像方法研究

提出一种基于旋转偏振角的新的偏振光成像方法:改变入射线偏振光偏振角和检偏角,采集样品图像系列,总结出背向散射光2个正交偏振分量的光强差关于入射线偏振光偏振角和检偏角的函数关系式.通过对模拟散射介质,猪肉脂肪,猪肉骨骼肌和牛肉骨骼肌等样品进行实验,论证了偏振差函数式中各个参数与样品光学特性之间的联系,并从中提取出2个相互独立的参数,分别表征样品的纤维方向角和光学各向异性度,从而实现对样品浅表层光学特性进行定量测量.和普通光强图像相比,用这些独立的光学信息生成不同基色的图像,能更直观明了地区分组织结构差异,具有潜在的临床医学应用价值,如成为一种皮肤疾病、皮肤损伤的检测方法.     

Circular intensity differential scattering and chromatin-DNA structure. A combined theoretical approach

Circular intensity differential scattering (CIDS) has been proven a powerful method in determining the higher-order structure of large biopolymers, such as chromatin. Theoretical predictions of the expected differential light scattering of circularly polarized light have previously been made for chromatin, either within the Born approximation, treating nucleosomes as noninteracting, oblate ellipsoids, or within a multiple dipole approximation, treating nucleosomes as interacting spheres. In order to conduct a meaningful interpretation of the CIDS signal in terms of given geometric parameters of the chiral structure, we have in this paper combined the two approaches considering the mutual interactions of ellipsoidal nucleosomes. In the process we have also found a confirmation for the validity of the Born approximation itself.     

Differential scattering of circularly polarized light as a unique probe of polynucleosome superstructures. A simulation by multiple scattering of dipoles

A theoretical approach to modeling Circular Intensity Differential Scattering (CIDS) of native chromatin as multiple scattering of dipoles is discussed without the Born approximation. The model can explain the experimental data in the literature. It is shown that CIDS contains more structural information than does total light scattering and to a good approximation is independent of the length of the scattering molecules. Finally, CIDS in conjunction with traditional light scattering measurements should aid in discriminating between various alternative models of higher order chromatin structure now being proposed. Generalization of this theoretical study to other complex biomolecular structures, is also briefly discussed.     

Daunomycin inverts the long-range chirality of DNA condensed states

The effect of daunomycin upon DNA condensed states induced by poly(ethylene glycol) (PEG) was studied by circular dichroism (CD) and circular intensity differential scattering (CIDS). The CD spectra of these aggregates showed psi-type anomalies and intensities 10-100 times greater than those obtained with the dispersed DNA solutions in the absence of PEG. Increasing concentrations of daunomycin, added to the DNA solution prior to its aggregation, led, in the presence of PEG, to CD and CIDS signals which gradually decreased in magnitude and eventually inverted sign. The coincidence of the transition point of both signals and a careful characterization of the CD spectrum at the transition point clearly indicated that the inversion observed corresponds to an inversion of the handedness of the aggregates. The latter result suggests that the structure of the aggregates at the inversion point should resemble that of a nematic liquid-crystalline structure. The characteristic B-DNA spectrum obtained in this case further suggests that the packing process does not affect the secondary structure of the DNA molecules and that small changes in their local structure can induce dramatic changes in their long-range tertiary packing. The results obtained in this study represent a confirmation of a recent theory of psi-type CD in which the anomalous signals are interpreted as a manifestation of the long-range chirality of the aggregates.     

Circular intensity differential scattering and chromatin-DNA structure

Circular intensity differential scattering (CIDS) has been proven a powerful method in determining the higher-order structure of large biopolymers, such as chromatin. Theoretical predictions of the expected differential light scattering of circularly polarized light have previously been made for chromatin, either within the Born approximation, treating nucleosomes as noninteracting, oblate ellipsoids, or within a multiple dipole approximation, treating nucleosomes as interacting spheres. In order to conduct a meaningful interpretation of the CIDS signal in terms of given geometric parameters of the chiral structure, we have in this paper combined the two approaches considering the mutual interactions of ellipsoidal nucleosomes. In the process we have also found a confirmation for rthe validity of the Born approximation itself.     

Differential scattering of circularly polarized light as a unique probe of polynucleosome superstructures

A theoretical approach to modeling Circular Intensity Differential Scattering (CIDS) of native chromatin as multiple scattering of dipoles is discussed without the Born approximation. The model can explain the experimental data in the literature. It is shown that CIDS contains more structural information than does total light scattering and to a good approximation is independent of the length of the scattering molecules. Finally, CIDS in conjunction with traditional light scattering measurements should aid in discriminating between various alternative models of higher order chromatin structure now being proposed. Generalization of this theoretical study to other complex biomolecular structures, is also briefly discussed.     

Differential polarization imaging. I. Theory

A theory of differential polarization imaging is derived using Mueller calculus. It is shown that, for any arbitrary object, 16 images (in general different) can be obtained by combining different incident polarizations of light and measuring the specific polarization components transmitted or scattered by the object. These are called the Mueller images of the object. Mathematical expressions of these images for an object of arbitrary geometry are derived using classical vector diffraction theory and the paraxial and thin lens approximations. The object is described as a collection of point polarizable groups. The electromagnetic fields are calculated using the first Born-Approximation, but extension of the theory to higher-order approximations is shown to be straightforward. These expressions are obtained for the transmission, or bright-field, geometry, and the scattering, or dark-field, configuration. In both cases, the contributions of scattering, absorption, and background illumination to the Mueller images are characterized. The contributions of linear dichroism, circular dichroism, and linear and circular intensity differential scattering to certain Mueller images are established. It is shown that the Mueller images represent a complete two-dimensional mapping of the molecular anisotropy of the object.     

Circular differential scattering and circular differential absorption of DNA-protein condensates and of dyes bound to DNA-protein condensates

DNA-protein condensates that give positive and negative psi-type circular dichroism (CD) spectra (psi condensates) bind intercalative and nonintercalative dyes. CD depends both on circular differential scattering and on circular differential absorption; scattering-corrected CD measurements are approximations to circular differential absorption. The circular differential scattering and scattering-corrected CD patterns observed in the DNA absorption band of psi condensates are mimicked in the induced CD band of intercalators bound to psi condensates. The induced scattering-corrected CD and circular differential scattering patterns of the groove-binding dye Hoechst 33342 bound to psi condensates are the inverse of the patterns seen with intercalative dyes, whereas the groove-binding dye manganese(III) meso-tetrakis(4-N-methylpyridyl)porphine [MnIIITMpyP-4] shows no significant induced CD patterns. The large circular differential scattering and scattering-corrected CD bands are interpreted as resulting from long-range chiral packing, rather than near-neighbor short-range interactions. Dyes intercalated into the DNA of the psi condensates have the same type of long-range chiral packing as the DNA bases. Therefore, the psi-type CD spectra seen in the UV spectra originating from the long-range packing of the DNA bases are also observed in the visible spectra when dyes are intercalated in the DNA of the psi condensates. Our interpretation comes from the observation that the induced circular differential scattering and circular differential absorption of the dye bound to the psi condensates depend only upon the sign of the circular differential absorption and the pattern of the circular differential scattering of the psi condensates without bound dye.(ABSTRACT TRUNCATED AT 250 WORDS)     

A Universal Plasmonic Polarization State Analyzer

We propose a universal plasmonic polarization state analyzer consisting of rectangular holes arranged along an Archimedes spiral in silver film. The analyzer can detect different polarization states of light including linear, circular, radial and azimuthal polarizations. The theoretical analysis of its transmitted field is performed on the basis of the dipole radiations, and the analytic expressions of the electric field distributions under different polarized illuminations are provided. The numerical simulations of the near-field transmissions are also conducted to verify the analytic results. The significant differences between the field distributions predict the practicability of the universal plasmonic polarization state analyzer in determining the incident light polarization states.     

Differential polarization imaging. V. Numerical aperture effects and the contribution of preferential scattering and absorption to the circular dichroism images.

Chiral objects which scatter and absorb preferentially left versus right circularly polarized light give rise to bright-field circular dichroism (CD) images containing contributions from both these two phenomena. These contributions are separated and characterized mathematically, and the effect of the dimensions of the chiral object on their relative magnitude is discussed. CD images of the long-range chiral organization of the thylakoid membranes in chloroplasts are obtained at two different wavelengths to illustrate the diverse wavelength dependence of the preferential absorption and scattering contributions to the images. The bright field CD images not only depend on the magnitude and sign of the preferential scattering and preferential absorption contributions, but also on the numerical aperture of the lens used. This dependence is obtained formally and a method to extract the angle dependent preferential scattering contributions to the images is presented. The validity of this method is confirmed experimentally.     

Chiral Biases in Solids by Effect of Shear Gradients: A Speculation on the Deterministic Origin of Biological Homochirality

We present an experimental approach to the study of the chirality of three CM2 meteorite solid samples by direct measurement of the optical activity (circular birefringence; CB). The measurements are based on transmission two modulator generalized ellipsometry in conjuction with microscope optics to map the CB of the samples. In spite of the complexity of such optical analysis, these first results indicate the presence of optically active areas in the meteorite solid matrix. In the case of the Murchison sample the statistics of the CB mapping shows a bimodal distribution with a bias to negative CB values. The composition of the active areas probably corresponds to serpentines and other poorly identified phyllosilicate phases. The results are compatible with the hypothesis that in a mineral-based scenario for the origin of life a CB sign bias in the chiral fractures originated by mechanical and flow shear gradients on clays could be later transferred to the reactions of the absorbed organic compounds.     

Gastric cancer detection based on blood plasma surface-enhanced Raman spectroscopy excited by polarized laser light

We have recently applied surface-enhanced Raman spectroscopy (SERS) for blood plasma analysis for non-invasive nasopharyngeal cancer detection and obtained good preliminary results. The aim of this study was to develop a more robust SERS spectroscopy based blood plasma analysis method for non-invasive gastric cancer detection. The effect of different laser polarizations (non-polarized, linear-polarized, right-handed circularly polarized, and left-handed circularly polarized) on blood plasma SERS spectroscopy was explored for the first time. Silver nanoparticles as the SERS-substrate were directly mixed with blood plasma to enhance the Raman scattering of various biomolecular constituents. High quality SERS spectra were obtained using a fiber optic probe and a dispersive type near infrared Raman system. Blood plasma samples from gastric cancer patients (n=32) and healthy subjects (n=33) were analyzed. The diagnostic performance for differentiating gastric cancer plasma from normal plasma was evaluated. Principal component analysis combined with linear discriminant analysis (LDA) of the obtained spectral data was used to develop diagnostic algorithms. Classification results obtained from cross-validation of the LDA model based on the four spectral data sets of different laser polarizations demonstrated different diagnostic sensitivities and specificities: 71.9% and 72.7% for non-polarized laser excitation, 75% and 87.9% for linear-polarized laser excitation, 81.3% and 78.8% for right-handed circularly polarized laser excitation, 100% and 97% for left-handed circularly polarized laser excitation. The results from this exploratory study demonstrated that plasma SERS spectroscopy with left-handed circularly polarized laser excitation has great promise of becoming a clinically useful diagnostic tool for non-invasive gastric cancer detection.     

Theoretical Study of the Local Surface Plasmon Resonance Properties of Silver Nanosphere Clusters

The local surface plasmon resonance properties in systems consisting of silver nanosphere clusters are studied by Green’s function. The extinction, absorption, and scattering efficiencies band of two, three, and more silver nanospheres clusters are discussed in detail. The clusters show new types of the local surface plasmon resonances compared with single silver nanosphere. Our results suggest that the resonances depend strongly on individual particles’ characteristics such as their shapes, gap distances, directions and polarizations of incident light waves, and the number of clusters. The spectrum shows that equilateral triangle nanospheres has a good absorption peak, while the better red-shifted with three aligned nanospheres. In addition, the distributions of electric field intensity for three and four touched silver nanospheres are also investigated. The study is useful to broaden the application scope of Raman spectroscopy and nanooptics.     

Large Circular Dichroism in MDM Plasmonic Metasurface with Subwavelength Crescent Aperture

A metal–dielectric–metal planar chiral plasmonic metasurface is proposed and its circular dichroism (CD) property is numerically studied using finite difference time domain computation. The unit cell of planar plasmonic metasurface consists of crescent apertures that are arranged in a particular orientation. The proposed structure exhibits multiband circular dichroism at near-infrared wavelengths. By changing the orientational symmetry, the structure shows a drastic reduction in the circular dichroism. Passive controlling of orientational symmetry shows a systematic change in the sign of the CD. High incident angular tolerance of the planar chiral plasmonic metasurface (PCPM) to about 15° suggests the proposed structure might be useful for CD spectroscopy.