Resonant and off-resonant Raman Optical Activity signals in the X-ray regime (XROA) are predicted. XROA is a chiral-sensitive variant of the spontaneous Resonant Inelastic Scattering (RIXS) signal. Thanks to the highly localized nature of core excitations, these signals provide a direct probe of local chirality with high sensitivity to the molecular structure. We derive sum-over-states expressions for frequency domain XROA signals and apply them to tyrosine at the nitrogen and oxygen K-edges. Time-resolved extensions of ROA made possible by using additional pulses are briefly outlined.
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Hyper-Raman optical activity of biologically relevant chiral molecules
The optical activity of Raman scattering provides insight into the absolute configuration and conformation of chiral molecules. Applications of Raman optical activity (ROA) are limited by long integration times due to a relatively low
sensitivity of the scattered light to chirality (typically 10^-3 to 10^-5). We apply ROA techniques to hyper-Raman scattering using incident circularly polarized light and a right-angle scattering geometry. We explore the sensitivity of hyper-
Raman scattering to chirality as compared to spontaneous Raman optical activity. Using the excitation wavelength at around 532 nm, the photobleaching is minimized, while the hyper-Raman scattering benefits from the electronic
resonant enhancement. For S/R-2-butanol and L/D-tartaric acid, we were unable to detect the hyper-Raman optical activity at the sensitivity level of 1%. We also explored parasitic thermal effects which can be mitigating by varying the repetition rate of the laser source used for excitation of hyper-Raman scattering.
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- Award ID(s):
- 1810995
- PAR ID:
- 10173527
- Date Published:
- Journal Name:
- Hyper-Raman optical activity of biologically relevant chiral molecules
- Page Range / eLocation ID:
- 84
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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null (Ed.)Light-absorbing chromophores in photoreceptors contain a π-electron system and are intrinsically planar molecules. However, within a protein environment these cofactors often become nonplanar and chiral in a manner that is widely believed to be functionally important. When the same chromophore is out-of-plane distorted in opposite directions in different members of a protein family, such conformers become a set of enantiomers. In techniques using chiral optical spectroscopy such as Raman optical activity (ROA), such proteins are expected to show opposite signs in their spectra. Here we use two microbial rhodopsins, Gloeobacter rhodopsin and sodium ion pump rhodopsin (NaR), to provide the first experimental and theoretical evidence that the twist direction of the retinal chromophore indeed determines the sign of the ROA spectrum. We disrupt the hydrogen bond responsible for the distortion of the retinal in NaR and show that the sign of the ROA signals of this nonfunctional mutant is flipped. The reported ROA spectra are monosignate, a property that has been seen for a variety of photoreceptors, which we attribute to an energetically favorable gradual curvature of the chromophore.more » « less
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Abstract Experimental vibrational Raman and Raman optical activity (ROA) spectra for diacetyl L‐tartaric acid (DAT), two of its esters, namely, monomethyl and lauryl esters (T1OH and T12OH), and corresponding sodium salts (DATNa, T1ONa, and T12ONa), are measured. T12OH and T12ONa represent the first chiral surfactants investigated using ROA spectroscopy. The quantum chemical (QC) predictions using B3LYP functional and 6‐311++G(2d,2p) basis set are used to interpret the ROA spectra for DAT, DATNa, T1OH, and T1ONa. It is found that the use of implicit solvation, as represented in polarizable continuum model (PCM), for predicting the experimental ROA spectra in aqueous solutions is inadequate for DAT and T1OH. However, the same PCM predicts the experimental ROA spectra satisfactorily for the DATNa and T1ONa. This favorable observation for the latter is attributed to the absence of intra‐ and inter‐molecular hydrogen bonding interactions for sodium salts in aqueous solutions. The overwhelming number of conformations resulting from 12‐carbon alkyl chain, in T12OH and T12ONa, makes it impractical to undertake QC predictions for them. Nevertheless, it is found that the predictions made for shorter alkyl chain analogs, namely, T1OH and T1ONa, may be used to explain the experimental ROA spectra of T12OH and T12ONa. The current work highlights the importance of converting carboxylic acids to corresponding sodium salts and of QC predictions for shorter achiral alkyl chain analogs to interpret the ROA spectra of chiral surfactants that contain long achiral alkyl chains.
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Abstract Vibrational Raman optical activity (ROA) spectra were calculated under off‐resonance, near‐resonance, and at‐resonance conditions for
( A ) and under off‐resonance conditions for( B ) using a new driver software for calculating the ROA intensities from complex (damped) time‐dependent linear response Kohn‐Sham theory. The off‐resonance spectra ofA andB show many similarities. At an incident laser wavelength of 532 nm, used in commercial ROA spectrometers, the spectrum ofA is enhanced by near‐resonance with the ligand‐field transitions of the complex. The near‐resonance spectrum exhibits many qualitative differences compared with the off‐resonance case, but it remains bi‐signate. Even under full resonance with the ligand‐field electronic transitions, the ROA spectrum ofA remains bi‐signate when the electronic transitions are broadened such as to yield absorption line widths that are comparable with those in the experimental UV‐vis absorption and electronic circular dichroism spectra. -
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Abstract Stimulated Raman scattering (SRS) microscopy allows for high-speed label-free chemical imaging of biomedical systems. The imaging sensitivity of SRS microscopy is limited to ~10 mM for endogenous biomolecules. Electronic pre-resonant SRS allows detection of sub-micromolar chromophores. However, label-free SRS detection of single biomolecules having extremely small Raman cross-sections (~10−30 cm2sr−1) remains unreachable. Here, we demonstrate plasmon-enhanced stimulated Raman scattering (PESRS) microscopy with single-molecule detection sensitivity. Incorporating pico-Joule laser excitation, background subtraction, and a denoising algorithm, we obtain robust single-pixel SRS spectra exhibiting single-molecule events, verified by using two isotopologues of adenine and further confirmed by digital blinking and bleaching in the temporal domain. To demonstrate the capability of PESRS for biological applications, we utilize PESRS to map adenine released from bacteria due to starvation stress. PESRS microscopy holds the promise for ultrasensitive detection and rapid mapping of molecular events in chemical and biomedical systems.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.1117/12.2545530
