Temperature-Induced Unfolding Pathway of Staphylococcal Enterotoxin B:Insights from Circular Dichroism and Molecular Dynamics Simulation

In this study,circular dichroism(CD)and molecular dynamics(MD)simulation were used to investigate the thermal unfolding pathway of staphylococcal enterotoxin B(SEB)at temperatures of 298–371 and 298–500 K,and the relationship between the experimental and simulation results were explored.Our computational findings on the secondary structure of SEB showed that at room temperature,the CD spectroscopic results were highly consistent with the MD results.Moreover,under heating conditions,the changing trends of helix,sheet and random coil obtained by CD spectral fitting were highly consistent with those obtained by MD.In order to gain a deeper understanding of the thermal stability mechanism of SEB,the MD trajectories were analyzed in terms of root mean square deviation(RMSD),secondary structure assignment(SSA),radius of gyration(R_(g)),free energy surfaces(FES),solvent-accessible surface area(SASA),hydrogen bonds and salt bridges.The results showed that at low heating temperature,domain Ⅰ without loops(omitting the mobile loop region)mainly relied on hydrophobic interaction to maintain its thermal stability,whereas the thermal stability of domain Ⅱ was mainly controlled by salt bridges and hydrogen bonds.Under high heating temperature conditions,the hydrophobic interactions in domain Ⅰ without loops were destroyed and the secondary structure was almost completely lost,while domain Ⅱ could still rely on salt bridges as molecular staples to barely maintain the stability of the secondary structure.These results help us to understand the thermodynamic and kinetic mechanisms that maintain the thermal stability of SEB at the molecular level,and provide a direction for establishing safer and more effective food sterilization processes.

Second-harmonic generation circular dichroism spectroscopy from tripod-like chiral molecular films

The second-harmonic generation （SHG） circular dichroism in the light of reflection from chiral films of tripod-like chiral molecules is investigated. The expressions of the second-harmonic generation circular dichroism are derived from our presented three-coupled-oscillator model for the tripod-like chiral molecules. Spectral dependence of the circular dichroism of SHG from film surface composed of tripod-like chiral molecules is simulated numerically and analysed. Influence of ehiral parameters on the second-harmonic generation circular dichroism spectrum in chiral films is studied. The result shows that the second-harmonic generation circular dichroism is a sensitive method of detecting chirality compared with the ordinary circular dichroism in linear optics. All of our work indicates that the classical molecular models are very effective to explain the second-harmonic generation circular dichroism of chiral molecular system. The classical molecular model theory can give us a clear physical picture and brings us very instructive information about the link between the molecular configuration and the nonlinear processes.

Enhanced Circular Dichroism of Z-Shaped Nanorod in Rectangular Nanohole Arrays

Chiral nanostructures have attracted much attention due to the valuable applications in biochemistry, medicine industries, and photonic devices. In this study, we propose an ease-of-fabrication planar nanostructure that consists of rectangular nanohole arrays in which the Z-shaped nanorod is arranged. Theoretically, such chiral nanostructure supports significant absorption circular dichroism (CD) compared with the Z-shaped nanorod because charge distributions are tuned after the introduction of the rectangular frame. Meanwhile, the Z-shaped nanorod directs the flow of current on the rectangular frame, thereby generating the effective quadruple electron oscillation in the Z-shaped nanorod. A novel mode also emerges when an identical Z-shaped nanorod is added into the rectangular hole. The studies will provide a novel approach to enhance the CD effect of planar structures. .

Anomalous scattering-induced circular dichroism in continuously shaped metasurface

The interconnection of polarization and phase in electromagnetic scattering process has attracted both fundamental and practical interest.Here we propose the principle and experimental demonstration of a scatteringcontrol mechanism based on the simultaneous control of polarization and phase via a continuously shaped planar metasurface.Under circularly polarized illumination,the scattering is redirected to off-specular direction,leading to significant reduction of the scattering cross section.Theoretical investigation shows that the underlying physical mechanism is the spin-dependent phase modulation in the anisotropic scattering process.Our approach would provide valuable guidance for the full control of electromagnetic wave for diverse applications.

Effect of microwave irradiation on secondary structure of α-amylase by circular dichroism

Based on the starch hydrolysis reaction accelerated by microwave irradiation with α-amylase, the circular dichroism （CD） and secondary structure changes of α-amylase under the condition of microwave irradiation and water bath were studied by circular dichroism spectra. The results showed that, both the peak heights （at 2=193 nm） of the CD spectra of the samples treated by microwave irradiation and water bath reduced. The reduced rate by microwave irradiation ranged from 140% to 220%, while the reduced rate by water bath ranged from 60% to 140%. The peak of the sample treated by microwave irradiation for 60 min disappeared at λ=193 nm, while the sample showed a wake peak by water bath. The peak position by microwave irradiation emerged a blue shift in the range of 5-8 nm at λ=204 nm and λ=220 nm, while it emerged in the range of 3-5 nm by water bath. With time going on, the microwave irradiation and water bath have prompted the secondary structure of α-helix, β-sheet, β-turn and the mutual transformations of random coil, but the trends were different.

Enhanced circular dichroism based on the dual-chiral metamaterial in terahertz regime

The obvious circular dichroism（CD） and optical activity can be obtained based on the chiral metamaterial due to the plasmon-enhanced effect, which is very attractive for future compact devices with enhanced capabilities of light manipulation. In this paper, we propose a dual-chiral metamaterial composed of bilayer asymmetric split ring resonators（ASRR）that are in mirror-symmetry shape. It is demonstrated that the CD can get enhancement in the terahertz regime. Moreover,the CD can be further improved by modulating the asymmetry of ASRR. The enhanced CD effect in the terahertz regime has great potential applications in sensing, biomedical imaging, and molecular recognition.

Enhanced and tunable circular dichroism in the visible waveband by coupling of the waveguide mode and local surface plasmon resonances in double-layer asymmetric metal grating

Circular dichroism(CD)has shown very interesting possibilities as a means to characterize the chiral signal of a chiral structure.Here,we theoretically demonstrated enhanced and tunable CD in the visible light regime using a composite structure consisting of a double-layer metal grating gaped by a dielectric waveguide layer.Based on the coupling of the waveguide modes and the localized plasmonic resonances,the CD could reach a maximum value as high as 0.52 at635 nm,which is four times higher than the CD value obtained in a conventional double-layer grating without the waveguide coupling effect.Furthermore,the spectral positions of the enhanced CD bands could be easily tuned by controlling the structural parameters.The proposed hybrid double-grating and waveguide structures could have potential applications in chiral selective imaging,sensing and spectroscopy,especially where the transmission measurement is required.

Circular dichroism spectra of α-lactose molecular measured by terahertz time-domain spectroscopy

Researchers have long studied circular dichroism(CD) for its enormous prospects in life sciences. Many biomolecules have vibration modes in the terahertz region, and terahertz CD spectra are robust to detect biomolecular structures. However,few studies explore the terahertz CD spectra on even natural materials due to technical challenges in both fields. Here, we report a setup of home-built terahertz time-domain spectroscopy to measure the polarization states of terahertz waves.By carefully measuring the transmission Jones matrix, we obtain terahertz CD spectra of α-lactose tablets and D-glucose tablets. Our results show that the terahertz CD spectra are sensitive to vibrational motions in biochemical compounds,which will find wide applications in biosensing and biomedical diagnostics.

Investigation of binding behaviour of procainamide hydrochloride with human serum albumin using synchronous,3D fluorescence and circular dichroism

Interaction of procainamide hydrochloride（PAH） with human serum albumin（HSA） is of great significance in understanding the pharmacokinetic and pharmacodynamic mechanisms of the drug. Multi-spectroscopic techniques were used to investigate the binding mode of PAH to HSA and results revealed the presence of static type of quenching mechanism. The number of binding sites, binding constants and thermodynamic parameters were calculated. The results showed a spontaneous binding of PAH to HSA and hydrophobic interactions played a major role. In addition, the distance between PAH and the Trp–214 was estimated employing the F？rster＇s theory. Site marker competitive experiments indicated that the binding of PAH to HSA primarily took place in subdomain IIA（Sudlow＇s site I）. The influence of interference of some common metal ions on the binding of PAH to HSA was studied. Synchronous fluorescence spectra（SFS）, 3D fluorescence spectra and circular dichroism（CD） results indicated the conformational changes in the structure of HSA.

The thermal and storage stability of bovine haemoglobin by ultraviolet-visible and circular dichroism spectroscopies

The effects of temperature,pH and long-term storage on the secondary structure and conformation changes of bovine haemoglobin（bHb） were studied using circular dichroism（CD） and ultraviolet-visible（UV-vis） spectroscopies.Neural network software was used to deconvolute the CD data to obtain the fractional content of the five secondary structures.The storage stability of bHb solutions in pH 6,7 and8 buffers was significantly higher at 4 ℃ than at 23 ℃ for the first 3 days.A complete denaturation of bHb was observed after 40 days irrespective of storage temperature or pH.The bHb solutions were also exposed to heating and cooling cycles between 25 and 65 ℃ and structural changes were followed by UVvis and CD spectroscopies.These experiments demonstrated that α-helix content of bHb decreased steadily with the increasing temperature above 35 ℃ at all pH values.The loss in a-helicity and gain in random coil conformations was pH-dependent and the greatest under alkaline conditions.Furthermore,there was minimal recovery of the secondary structure content upon cooling to 25 ℃.The use of bHb as a model drug is very common and this study elucidates the significance of storage and processing conditions on its stability.

Enhanced circular dichroism of plasmonic system in the strong coupling regime

The circular dichroism(CD) signal of a molecule is usually weak,however,a strong CD signal in optical spectrum is desirable because of its wide range of applications in biosensing,chiral photo detection,and chiral catalysis.In this work,we show that a strong chiral response can be obtained in a hybridized system consisting of an artificial chiral molecule and a nanorod in the strong coupling regime.The artificial chiral molecule is composed of six quantum dots in a helix assembly,and its CD signal arises from internal Coulomb interactions between quantum dots.The CD signal of the hybridized system is highly dependent on the Coulomb interactions and the strong coupling progress through the electromagnetic interactions.We use the coupled oscillator model to analyze strong coupling phenomenon and address that the strong coupling progress can amplify the CD signal.This work provides a scenario for designing new plasmonic nanostructures with a strong chiral optical response.

Enhanced circular dichroism of TDBC in a metallic hole array structure

We investigate the enhanced chirality of chiral molecular J-aggregates(TDBC)by the propagating surface plasmons(PSPs)in the metallic hole array structure filled with TDBC.The two ends of the hole in the metal film form a low quality factor Fabry-Perot(FP)cavity,and this cavity confines PSPs.The resonant wavelength of the metallic hole array is tuned by the lattice constant and the size of the hole.Both the resonant wavelength of Ag hole array and the volume ratio of TDBC in the hybridized structure influence on the enhancement of the circular dichroism(CD)spectrum.The curve of CD spectrum shows Fano-like line-shape,due to the interaction between the non-radiative field in the FP cavity and the radiative field in chiral TDBC.The maximum of the CD spectrum of the hybridized structure is 0.025 times as the one of the extinction spectrum in a certain structure,while the maximum of the CD spectrum of TDBC is 1/3000 times as the one of the extinction spectrum.The enhanced factor is about 75.The resonant wavelength of the metallic hole array can be tuned in a large wavelength regime,and the chirality of a series of molecular J-aggregates with different resonant wavelengths can be enhanced.Our structure provides a new method to amplify the chirality of molecular J-aggregates in experiments.

Circular Dichroism and Fluorescence Spectroscopic Study of RNA-protein Folding Patterns in Human hnRNP A3 and Their Implications in Human Autoimmune Diseases

In human cells, the heterogeneous nuclear ribonucleoproteins (hnRNP) are represented by a group of polypeptides, with various molecular properties, comprizing the most abundant constituents of the cell nucleus. Autoantibodies to hnRNPs have been reported in patients suffering from different rheumatic dieseases since 1980s. Experimental evidence indicates that hnRNP complexes undergo substantial structural changes during mRNA formation and export. However, how this contributes to disease development still has to be elucidated. Here some preliminary physicochemical features of RNA-protein folding and stability patterns of newly characterized hnRNP A3 with further functional implications in development of systemic human autoimmune states are reported.

Circular Dichroism and Derivative Spectra Study of the Excitonic Aggregation of Pinacyanol by Aerosol-OT

Different structures are proposed for the complexes formed from the interaction between the cationic cyanine dye 1, 1’-diethyl-2, 2’-carbocyanine chloride (Pinacyanol chloride) and the anionic surfactant sodium bis (2-ethylhexyl) sul- fosuccinate (Aerosol-OT) in ethanol-water solutions. UV/vis and electronic circular dichroism (CD) spectra provide tools to study the concentration and solvent dependence of this interaction. The aggregation spectra of the dye are dominated by a strongly blue-shifted, sharp and single visible band, which appears at concentrations much below the critical micelle concentration (cmc) of Aerosol-OT. Above the cmc, the spectra in pure aqueous solution indicate disso- ciation into monomer and dimer species, and the system becomes completely empty of chirality. Two different CD spectra with a distinct isosbestic point are observed for complexes with different surfactant to dye ratios. Both the addi- tion of ethanol (more than 7.5%) and increasing the temperature retards the metachromatic process between the dye and Aerosol-OT. By using the peakFit program, the two overlapping excitonic absorption bands together with the optically inactive absorption band for one spectrum (the most optimal one) at a specific Aerosol-OT concentration were sepa- rated.

Interaction of bovine serum albumi with two alkylimidazolium-based ionic liquids investigated by microcalorimetry and circular dichroism

The interactions of bovine serum albumin (BSA) with two alkylimidazolium-based ionic liquids, 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]BF4) and 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim]PF6), in buffer solutions at pH 7.0 were investigated by isothermal titration calorimetry (ITC) and circular dichroism (CD). CD spectra showed that the two ionic liquids changed the secondary structure of BSA. Data process was based on the supposition that there were several independent types of binding sites on each BSA molecule for the two ligand molecules. The results obtained by using this supposition combined with Langmuir adsorption model showed that there were two types of such binding sites. One was the high affinity binding site, and the other was the low affinity binding site. The binding constants, changes in enthalpy, entropy and Gibbs free energy for the two types of binding were obtained, which showed that the two types of binding were driven by a favorable entropy increase. Furthermore, for either the ionic liquids, the number of the high affinity binding sites is much smaller than that of the low affinity ones. These results were interpreted with the molecular structure of BSA and the different substituent groups on imidazole ring of the two ionic liquid molecules.

Absolute configuration of Buagafuran:An experimental and theoretical electronic circular dichroism study

Buagafuran is a novel drug candidate derived from natural product.Its absolute configuration has been confirmed by electronic circular dichroism combined with modern quantum-chemical calculation using time-dependent density functional theory.The predicted UV absorbance peak is underestimated by several nanometers compared with the experimental data.The applicability of empirical rule for the C=C-C-O system in Buagafuran has also been discussed.Our results show that electronic circular dichroism could be a useful tool for the absolute configuration assignment of chiral drugs,especially for the oily or semisolid substances,whose crystal structures are impossible to obtain.

Novel Secondary Structure of Calcitonin in Solid State as Revealed by Circular Dichroism Spectroscopy

The solid-state circular dichroic study reveals that salmon calci-toninpresents a typical α-helical structure while human calci-tonin appears to form a β-sheet in solidstate, although both of them adopt random coil structures in aqueous solution.

Construction of Chiral One-dimensional Chains via Mononuclear Chiral Precursors and Circular Dichroism Properties

One-dimensional(1D)chiral chain complexes[CuLSCu(Pydc)](2S)and[CuLRCu(Pydc)](2R)(LS=(E)-3-(((1S,2S)-2-(((E)-3-oxo-3-(4-pyridin-4-yl)phenyl)propylidene)amono)-1,2-diphenyl)imino)-1-(4-(py-ridi-4-yl)phenyl)butan-1-one)and LR=(E)-3-(((1R,2R)-2-(((E)-3-oxo-3-(4-pyridin-4-yl)phenyl)propyli-dene)amono)-1,2-diphenyl)imino)-1-(4-(pyridi-4-yl)phenyl)butan-1-one and Pydc=2,6-pyridinedicarboxylic acid)have been synthesized vis mononuclear chiral enantiomer precursors CuLS(1S)and CuLR(1R).Their different chiral configurations of 1S,1R,2S and 2R were determined by single-crystal X-ray diffraction analyses and further characterized by elemental analyses,infrared spectra(IR),powder X-ray diffraction(PXRD),thermal gravimetric analyses(TGA)and circular dichroism spectra(CD).

Circular dichroism of tail-to-tail bisbenzylisoquinoline and the absolute configuration of daurisoline isolated from Menispermum dauricum

Since two asymmetric centres are usually present in the bisbenzylisoquinolines,the circular dichroism is known to be an important method to elucidate their stereochemistry.We studied the relationships of the CD spectra with the stereochemistry of synthetic RR-(1),SS-(2),RS-daurisoline(3) and SR-daurisoline(4).By comparision of the CD spectrum of natural daurisoline(1)from Menis- permum dauricum(Menispermaceae)with those of synthetic enantiomers,the absolute configuration was established as RR-daurisoline(1).