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Results for “Circular dichroism spectroscopy”

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156 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

X-ray dichroism as a probe of orbital quenching and unquenching induced by optically excited Jahn-Teller modes

The use of soft-X-ray spectroscopy to probe changes in the ground-state due to vibronic excitations is studied. The changes in L-edge X-ray absorption on divalent cobalt in the presence of a vibrating Jahn-Teller distortion are calculated. For linear dichroism, a strong dependence on the distortion is obtained reflecting the change in the local quadrupole moment due to the changes in the xy versus yz/zx hole densities during the oscillation. Here, in the presence of a magnetic moment, significant variations in the circular dichroic signal are obtained related to the quenching and unquenching of the orbital angular momentum.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Copper Reductase Activity and Free Radical Chemistry by Cataract-Associated Human Lens γ-Crystallins

Cataracts are caused by high-molecular weight aggregates of human eye lens proteins that scatter light, causing lens opacity. Metal ions have emerged as important potential players in the etiology of cataract disease, as human lens γ-crystallins are susceptible to metal-induced aggregation. Here, the interaction of Cu 2+ ions with γD-, γC-, and γS- crystallins, the three most abundant γ-crystallins in the lens, has been evaluated. Cu 2+ ions induced non-amyloid aggregation in all three proteins. Solution turbidimetry, SDS-PAGE, circular dichroism and differential scanning calorimetry showed that the mechanism for Cu-induced aggregation involves: i) loss of beta-sheet structure in the N-terminal domain; ii) decreased thermal and kinetic stability; iii) formation of metal-bridged species; and iv) formation of disulfide-bridged dimers. Electron paramagnetic resonance (EPR) revealed two distinct Cu 2+ binding sites in each protein. Spin quantitation demonstrated reduction of γ-crystallin-bound Cu 2+ ions to Cu + under aerobic conditions, while X-ray absorption spectroscopy (XAS) confirmed the presence of linear or trigonal Cu + binding sites in γ-crystallins. Our EPR and XAS studies revealed that γ-crystallins’ Cu 2+ reductase activity yields a protein-based free radical that is likely a Tyr-based species in human γD-crystallin. This unique free radical chemistry carried out by distinct redox-active Cu sites in human lens γ-crystallins likely contributes to the mechanism of copper-induced aggregation. Finally, in the context of an aging human lens, γ-crystallins could be acting, not only as structural proteins, but also as key players for metal and redox homeostasis.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Charge Delocalization and Vibronic Couplings in Quadrupolar Squaraine Dyes

Squaraines are prototypical quadrupolar charge-transfer chromophores that have recently attracted much attention as building blocks for solution-processed photovoltaics, fluorescent probes with large two-photon absorption cross sections, and aggregates with large circular dichroism. Their optical properties are often rationalized in terms of phenomenological essential state models, considering the coupling of two zwitterionic excited states to a neutral ground state. As a result, optical transitions to the lowest S 1 excited state are one-photon allowed, whereas the next higher S 2 state can only be accessed by two-photon transitions. A further implication of these models is a substantial reduction of vibronic coupling to the ubiquitous high-frequency vinyl-stretching modes of organic materials. Here, in this study, we combine time-resolved vibrational spectroscopy, two-dimensional electronic spectroscopy, and quantum-chemical simulations to test and rationalize these predictions for nonaggregated molecules. We find small Huang–Rhys factors below 0.01 for the high-frequency, 1500 cm –1 modes in particular, as well as a noticeable reduction for those of lower frequency modes in general for the electronic S 0 → S 1 transition. The two-photon allowed state S 2 is well separated energetically from S 1 and has weak vibronic signatures as well. Thus, the resulting pronounced concentration of the oscillator strength in a narrow region relevant to the lowest electronic transition makes squaraines and their aggregates exceptionally interesting for strong and ultrastrong coupling of excitons to localized light modes in external resonators with chiral properties that can largely be controlled by the molecular architecture.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Orientation-Driven Chirality Funnels in Chiral Low-Dimensional Lead-Halide Perovskite Heterostructures

Chiral hybrid metal-halide perovskites show low-symmetry crystal structures, large Rashba splitting, spin-filtering, and strong chiroptical activity. Circular dichroism and circularly polarized photoluminescence have been investigated in chiral perovskites with increasingly distorted chiral structures. Here, we report the fabrication of chiral (R/S)-EBAPbI3 (EBA = α-ethylbenzylamine) single crystals, which possess highly distorted octahedral structures with a high angle variance value of ∼68 degree2. Using control in the fabrication conditions, we transfer chiral single crystals to thin films and achieve different crystal orientation preferences that induce tunable chiroptical properties to their heterostructures with PbI2 nanodomains, which we characterize with in situ X-ray diffraction and grazing-incidence wide-angle X-ray scattering measurements. Using transient chiroptical spectroscopies, we resolve photoexcited charge carrier dynamics and chirality transfer processes in such heterostructures down to cryogenic temperatures. We observe rapid carrier transfer along the in-plane (002) facets in chiral perovskite phases to PbI2 nanostructures within the initial few picoseconds, while carrier transfer along the out-of-plane (002) facets occurs at a slower rate. This fast transfer process leads to high photoluminescence intensities and large degrees of circular polarization in the emission from PbI2 nanodomains at cryogenic temperatures. Our findings report a multidimensional chiral-achiral heterostructure which takes advantage of controllable chirality transfer and offers new routes for future spintronic and chiroptical applications.

Liu, Shangpu↗
NASA NTRS1985

Magnetic optical activity in intense laser fields. I - Self-rotation and Verdet constant

The paper is concerned with the Faraday rotation and circular dichroism of dilute gases in longitudinal magnetic fields subject to strong radiation fields. In particular, magnetic fields are considered which produce Zeeman shifts much smaller than inverse collision times but which may be greater or smaller then the multipole relaxation times. Doppler broadening is considered, and explicit expressions are given for dipole transitions in a two-level system with arbitrary angular momentum for strong and weak magnetic fields. The same system is then studied as a function of the external magnetic-field strength with a fixed laser frequency. It is shown that this experimental arrangement, referred to as forward scattering, has certain advantages that are not present in the more usual magnetic rotation spectroscopy.

Giraud-Cotton, S.↗
DOE OSTI2024

X-ray circular dichroism measured by cross-polarization x-ray transient grating

Measuring natural circular dichroism in the x-ray regime to extract stereochemical information from chiral molecules in solution remains a challenge. This is primarily due to technical limitations of the existing synchrotron sources, which hinder access to measurements of local chirality by exploiting core hole electronic transitions. In response to this challenge, we propose an alternative approach: utilizing XFEL-based cross-polarization x-ray transient grating (XTG). This method provides an indirect means to measure x-ray circular dichroism (XCD). Notably, our findings reveal that the signal emerges only once the excited cores have undergone dephasing through relaxation. XTG is now routinely measured in the XUV regime and has recently been made available for hard x-rays. Free electron lasers now offer polarization controls, and XTG can be extended to various polarization states for the two pump beams, making XCD measured by XTG feasible with the current state-of-the-art technology.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2023

ATCUN-like Copper Site in βB2-Crystallin Plays a Protective Role in Cataract-Associated Aggregation

Cataract is the leading cause of blindness worldwide, and it is caused by crystallin damage and aggregation. Senile cataractous lenses have relatively high levels of metals, while some metal ions can directly induce the aggregation of human γ-crystallins. Here, for this work, we evaluated the impact of divalent metal ions in the aggregation of human βB2-crystallin, one of the most abundant crystallins in the lens. Turbidity assays showed that Pb 2+ , Hg 2+ , Cu 2+ , and Zn 2+ ions induce the aggregation of βB2-crystallin. Metal-induced aggregation is partially reverted by a chelating agent, indicating the formation of metal-bridged species. Our study focused on the mechanism of copper-induced aggregation of βB2-crystallin, finding that it involves metal-bridging, disulfide-bridging, and loss of protein stability. Circular dichroism and electron paramagnetic resonance (EPR) revealed the presence of at least three Cu 2+ binding sites in βB2-crystallin, one of them with spectroscopic features typical for Cu 2+ bound to an amino-terminal copper and nickel (ATCUN) binding motif, which is found in Cu transport proteins. The ATCUN-like Cu binding site is located at the unstructured N-terminus of βB2-crystallin, and it could be modeled by a peptide with the first six residues in the protein sequence (NH 2 -ASDHQF-). Isothermal titration calorimetry indicates a nanomolar Cu 2+ binding affinity for the ATCUN-like site. An N-truncated form of βB2-crystallin is more susceptible to Cu-induced aggregation and is less thermally stable, indicating a protective role for the ATCUN-like site. EPR and X-ray absorption spectroscopy studies reveal the presence of a copper redox active site in βB2-crystallin that is associated with metal-induced aggregation and formation of disulfide-bridged oligomers. Our study demonstrates metal-induced aggregation of βB2-crystallin and the presence of putative copper binding sites in the protein. Whether the copper-transport ATCUN-like site in βB2-crystallin plays a functional/protective role or constitutes a vestige from its evolution as a lens structural protein remains to be elucidated.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

A versatile X-ray phase retarder for lock-in XMCD measurements

X-ray magnetic circular dichroism (XMCD) is a technique commonly used to probe magnetic properties of materials with element and orbital selectivity, which requires the use of circularly polarized (CP) X-rays. It is possible to accomplish XMCD experiments with fixed CP and alternating the magnetic field orientation, but most reliable data are obtained when alternating the magnetization orientation and the polarization between right and left helicities. A versatile strategy has been developed to perform XMCD experiments using a hard X-ray quarter-wave plate, at both polychromatic dispersive and conventional monochromatic optics, in combination with synchronous data acquisition. The switching frequency waveform is fed into a lock-in amplifier to detect and amplify the XMCD signal. The results on a reference sample demonstrate an improvement in data quality and acquisition time. The instrumentation successfully generated 98% of CP X-rays switching the beam helicity at 13 Hz, with the possibility of faster helicity switching once it is installed at the new Brazilian fourth-generation source, SIRIUS.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2025

Strong NIR II Chiroptical Response and Magnetic Anisotropy via Modular Installation of Chiral Capping Ligands on a Light-Emitting Diradicaloid Scaffold

Low-energy molecular lumiphores have seen increased interest due to potential imaging and communications applications. Specifically, molecules that emit in the near-infrared (NIR, 700–1700 nm) or telecom (∼1260–1625 nm) regions, where attenuation is minimized in biological tissue and optical fibers, respectively, can drastically improve image resolution and depth penetration; however, bright low-energy emission is rare due to exponentially decreasing quantum yields in this region. Chiral molecules exhibiting strong NIR or telecom absorption/emission would be of particular interest due to advanced security and spintronics applications, but these compounds remain scarce and are currently restricted to lanthanide or nanoparticle-based systems. Here, we report the synthesis of a chiral organic-based NIR emitter, enabled by simple peripheral installation of chiral capping ligands. These ligands twist the achiral NIR-emissive core, breaking inversion symmetry. Furthermore, this twisting enables strong chiroptical responses observed via static and transient circular dichroism and increased magnetic anisotropy through electron paramagnetic resonance (EPR) measurements, establishing this strategy as a promising method for the development of new chiral emitters and sensors.

Electron paramagnetic resonance spectroscopy↗
DOE OSTI2020

Amorphous nonstoichiometric oxides with tunable room-temperature ferromagnetism and electrical transport

Material functionalities strongly depend on the stoichiometry, crystal structure, and homogeneity. Here we demonstrate an approach of amorphous nonstoichiometric inhomogeneous oxides to realize tunable ferromagnetism and electrical transport at room temperature. In order to verify the origin of the ferromagnetism, we employed a series of structural, chemical, and electronic state characterizations. Combined with electron microscopy and transport measurements, synchrotron-based grazing incident wide angle X-ray scattering, soft X-ray absorption and circular dichroism clearly reveal that the room-temperature ferromagnetism originates from the In 0.23 Co 0.77 O 1-v amorphous phase with a large tunable range of oxygen vacancies. The room-temperature ferromagnetism is tunable from a high saturation magnetization of 500 emu cm -3 to below 25 emu cm -3 , with the evolving electrical resistivity from 5 × 10 3 μΩ cm to above 2.5 × 10 5 μΩ cm. Inhomogeneous nano-crystallization emerges with decreasing oxygen vacancies, driving the system towards non-ferromagnetism and insulating regime. Our work unfolds the novel functionalities of amorphous nonstoichiometric inhomogeneous oxides, which opens up new opportunities for developing spintronic materials with superior magnetic and transport properties.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
NASA NTRS1992

Isolation, characterization, and amino acid sequences of auracyanins, blue copper proteins from the green photosynthetic bacterium Chloroflexus aurantiacus

Three small blue copper proteins designated auracyanin A, auracyanin B-1, and auracyanin B-2 have been isolated from the thermophilic green gliding photosynthetic bacterium Chloroflexus aurantiacus. All three auracyanins are peripheral membrane proteins. Auracyanin A was described previously (Trost, J. T., McManus, J. D., Freeman, J. C., Ramakrishna, B. L., and Blankenship, R. E. (1988) Biochemistry 27, 7858-7863) and is not glycosylated. The two B forms are glycoproteins and have almost identical properties to each other, but are distinct from the A form. The sodium dodecyl sulfate-polyacrylamide gel electrophoresis apparent monomer molecular masses are 14 (A), 18 (B-2), and 22 (B-1) kDa. The amino acid sequences of the B forms are presented. All three proteins have similar absorbance, circular dichroism, and resonance Raman spectra, but the electron spin resonance signals are quite different. Laser flash photolysis kinetic analysis of the reactions of the three forms of auracyanin with lumiflavin and flavin mononucleotide semiquinones indicates that the site of electron transfer is negatively charged and has an accessibility similar to that found in other blue copper proteins. Copper analysis indicates that all three proteins contain 1 mol of copper per mol of protein. All three auracyanins exhibit a midpoint redox potential of +240 mV. Light-induced absorbance changes and electron spin resonance signals suggest that auracyanin A may play a role in photosynthetic electron transfer. Kinetic data indicate that all three proteins can donate electrons to cytochrome c-554, the electron donor to the photosynthetic reaction center.

NASA Discipline Exobiology↗
DOE OSTI2025

Ferroaxial phonons in chiral and polar Ni⁢Co 2 ⁢Te⁢O 6

Perfect circular dichroism has been observed in Raman scattering by optical phonons in single chiral domain NiCo 2 TeO 6 crystals. The selection rules for optical phonons are determined by the combination of the chiral structure C and electric polarization P along the c axis. These two symmetry operations are equivalent to the ferroaxial order (C·P) = A, so the observed optical phonons are referred to as ferroaxial. For a given Raman scattering geometry, the observed effect may also be described as a perfect nonreciprocal propagation of the ferroaxial optical phonons, whose preferable q-vector direction is determined by the sign of A. Here, the combination of Raman scattering and polarization plane rotation of the transmitted white light allows for identification of the direction of electric polarization P in monodomain chiral crystals. In addition to the ferroaxial selection rules for optical phonons, a weaker ferroaxial dichroism was observed for the magnon peak at 18 cm −1 .

Altermagnets↗