Agradecemos a Robert Berger (Philipps-Universität Marburg, Alemanha) para inspirar as discussões sobre a interpretação de nossos dados e quiralidade molecular em geral. Estamos gratos a Julia Kiedrowski, Alexander Schießer e Michael Reggelin de TU Darmstadt (Alemanha), bem como Benjamin Spenger, Manuel Mazenauer e Jürgen Stohner de faculdade Wädenswil (Suíça) para fornecer a amostra.
O projeto foi apoiado pela iniciativa do estado de Hessen para científico e económico excelência sob o foco ELCH (dinâmica de elétrons de sistemas quirais) e o Ministério Federal da educação e pesquisa (BMBF). MS reconhece o apoio financeiro da Fundação de Adolf Messer.

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H., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Imaging+the+Absolute+Configuration+of+a+Chiral+Epoxide+in+the+Gas+Phase.">Imaging the Absolute Configuration of a Chiral Epoxide in the Gas Phase.</a> Science. 342 (6162), 1084-1186 (2013).</li><li>D&#246;rner, R., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cold+Target+Recoil+Ion+Momentum+Spectroscopy:+a+'momentum+microscope'+to+view+atomic+collision+dynamics.">Cold Target Recoil Ion Momentum Spectroscopy: a 'momentum microscope' to view atomic collision dynamics.</a> Physics Reports. 330, 95-192 (2000).</li><li>Ullrich, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Recoil-ion+and+electron+momentum+spectroscopy:+reaction-microscopes.">Recoil-ion and electron momentum spectroscopy: reaction-microscopes.</a> Rep. 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Phys. 66 (9), 1463-1545 (2003).</li><li>Jagutzki, O., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Multiple+Hit+Readout+of+a+Microchannel+Plate+Detector+With+a+Three-Layer+Delay-Line+Anode.">Multiple Hit Readout of a Microchannel Plate Detector With a Three-Layer Delay-Line Anode.</a> IEEE Trans Nucl Sci. 49 (5), 2477-2483 (2002).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> RoentDek GmbH MCP Delay Line Detector Manual. , (2017).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> RoentDek GmbH The RoentDek Constant Fraction Discriminators CFD8c, CFD7x, CFD4c, CFD1c and CFD1x. , (2017).</li><li>Zeller, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Imaging+the+He2+quantum+halo+state+using+a+free+electron+laser.">Imaging the He2 quantum halo state using a free electron laser.</a> PNAS. 113 (51), 14651-14655 (2016).</li><li>Pitzer, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Stereochemical+configuration+and+selective+excitation+of+the+chiral+molecule+halothane.">Stereochemical configuration and selective excitation of the chiral molecule halothane.</a> J. 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A. 667, 11-15 (2012).</li><li>Kastirke, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Konstruktion und Aufbau einer UHV-tauglichen COLTRIMS-Kammer. , (2014).</li><li>Calegari, F., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Charge+migration+induced+by+attosecond+pulses+in+bio-relevant+molecules.">Charge migration induced by attosecond pulses in bio-relevant molecules.</a> J. Phys. B.: At. Mol. Opt. Phys. 49 (14), 142001 (2016).</li><li>Gaie-Levrel, F., Garcia, G. A., Schwell, M., Nahon, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=VUV+state-selected+photoionization+of+thermally-desorbed+biomolecules+by+coupling+an+aerosol+source+to+an+imaging+photoelectron/photoion+coincidence+spectrometer:+case+of+the+amino+acids+tryptophan+and+phenylalanine.">VUV state-selected photoionization of thermally-desorbed biomolecules by coupling an aerosol source to an imaging photoelectron/photoion coincidence spectrometer: case of the amino acids tryptophan and phenylalanine.</a> Phys. Chem. Chem. Phys. 13, 7024-7036 (2010).</li><li>Ablikim, U., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Identification+of+absolute+geometries+of+cis+and+trans+molecular+isomers+by+Coulomb+Explosion+Imaging.">Identification of absolute geometries of cis and trans molecular isomers by Coulomb Explosion Imaging.</a> Sci. Rep. 6, 38202 (2016).</li></ol>

Os autores declaram não interesses concorrentes.

Devido à variedade de componentes, uma instalação COLTRIMS requer um nível bastante elevado de conhecimentos técnicos, especialmente nas áreas de técnica de vácuo, detecção de partículas, rápido eletrônica e análise de dados. Antes de virar para a investigação de espécies complexas, deve, portanto, completamente verificar-se a instalação está sendo executado corretamente, por exemplo, executar e analisar uma medição em uma espécie diatômica ou triatômica.
Otim...

Quiralidade é uma característica da nossa natureza que tem sido fascinantes pesquisadores de mais de 150 anos. Em 19th século, Pasteur, Van ' t Hoff e outros descobriram que as moléculas podem ocorrer em duas estruturas de imagem de espelho que não são super-imposable - como a nossa esquerda e mão direita. Esta propriedade foi denominada 'quiral', da palavra grega para 'mão'.
Até agora, não há diferença entre propriedades termodinâmicas ou em níveis de energia da mão esquerda e direita formulários (os dois ' enantiômeros') foi encontrada. A fim de analisar a destreza manual de uma determinada amostra e para separar os enantiômeros, interação com outras moléculas quirais pode ser usada, como por exemplo é feito em várias abordagens chromatographical. 1 Chiroptical métodos como Dicroísmo circular (vibracional), CD (V) e dispersão rotatory óptico, ORD, são rotineiramente empregados para distinguir entre os enantiômeros. 2
Quando se trata da determinação da estrutura microscópica, estas técnicas exigem informações adicionais, por exemplo, dos cálculos de química quântica. A única técnica que é amplamente aceito para determinar diretamente a configuração absoluta é anômala de difração de raios x. 3
Recentemente mostrou que a configuração absoluta de espécies quirais simples pode ser determinada por Coulomb explosão Imaging. 4 , 5 nesta abordagem, as moléculas na fase gasosa são multiplicar ionizado para que os restantes núcleos fortemente repelem ao outro. Esta repulsão leva à fragmentação rápida ('explosão') das moléculas. A direção e a magnitude do fragmento correlato de momentos para a estrutura da molécula – para pequenas moléculas, as direções de impulso correspondem surpreendentemente bem para os eixos de ligação. Explosão de Coulomb para determinação da estrutura molecular tem sido pioneira usando feixes de íon molecular de um acelerador. 6 esta técnica de folha do feixe tem recentemente sido também solicitou o reconhecimento quiral. 7
Ao contrário da difração de raios x anômalos, a amostra não deve ser cristalino mas fornecido na fase gasosa. Isso torna a abordagem de Coulomb explosão ideal para espécies voláteis e, portanto, complementar à difração de raios x. Em certos casos, a destreza manual nem pode ser determinada por moléculas individuais.
Na prática, a reconstrução exata da estrutura molecular provou difícil mesmo para derivados do metano, por exemplo, moléculas com um carbono central e diferentes substituintes. Isto é atribuído ao fato de que a interação entre os fragmentos não é exatamente Coulombic e que nem todos os títulos quebraram simultaneamente. Para obter informações de estereoquímicas, especialmente para distinguir entre os enantiômeros, esta reconstrução Felizmente não é necessária. Em vez disso, os vetores da dinâmica dos diferentes fragmentos podem ser correlacionados para render uma quantidade que é distinta para as moléculas de mão esquerda e direita. Para obter resultados fiáveis, pelo menos quatro momentos de fragmento tem que ser gravado.
Para medir esta informação dinâmica, os fragmentos de um – e única – molecular rompimento precisa ser detectada em uma etapa de medição. Esta condição é geralmente referida como 'detecção coincidente'. Além disso, as direções de emissão precisa ser analisado, que montantes na prática para gravar o tempo e a posição do fragmento de impacto em um formato de dados de lista-modo.
Em física atômica e molecular, foram desenvolvidas técnicas que implementam essa abordagem de medição empregando eletrostáticos espectrômetros para detectores de multi-hit separação e tempo e posição-sensíveis à massa. O exemplo mais proeminente é a instalação COLTRIMS (frio alvo Recoil Ion Momentum espectroscopia) – também conhecido como microscópio de reação. 8 , 9 um esboço para este tipo de experiência é dada na Figura 1. Ao contrário de um padrão COLTRIMS que pode gravar os elétrons também, Coulomb explosão Imaging requer apenas o detector de íons.
Espectrômetro e detector são montados sob vácuo ultra elevado (&#60; 1 x 10-9 hPa) para evitar a criação de íons do gás residual. Moléculas simples da amostra são fornecidas através de um jato de molecular gasoso gratuito criado por expansão supersônica: em virtude da pressão de vapor, as moléculas se expandir através de um bocal pequeno (cerca de 50 µm de diâmetro) no vácuo. Esta parte do experimento, a câmara de origem, é separado da região de interação por geralmente duas escumadeiras e estágios diferencialmente bombeados. Diferencialmente bombeado seção adicional está localizado atrás da região de interação para despejar o jato de gás e, assim, evitar o gás de fundo na região de interação.
A radiação ionizante cruza-se com o jato molecular abaixo dos 90°. A maioria dos laboratórios atualmente usam pulsos de laser de femtosecond, apesar de radiação síncrotron, íons rápidos ou impacto do elétron são possíveis 'projéteis' para induzir a explosão de Coulomb.
O protocolo seguinte faz a suposição de que uma configuração de execução para a imagem latente coincidente de íons e um laser de femtosecond estão disponíveis no laboratório. O pico de intensidade necessário para induzir a explosão de Coulomb em quatro ou mesmo cinco fragmentos deve ser da ordem de 6 x 1014 W/cm2. Para evitar medidas excessivamente longas, a taxa de repetência do laser deve ser igual ou superior a 10 kHz. Isto é crucial porque, por um lado, deteção coincidente só pode ser verificada se a probabilidade de fragmentação no foco do laser é significativamente inferior a 1 por pulso de laser (idealmente não mais de 10%). A taxa de fragmentação total, por outro lado, deve não ser inferior a poucos kHz porque o compartilhamento das vias multifragmentation relevantes é geralmente inferior a 10-4. Fato tão encorajador, deve ser mencionado que em princípio já um evento único de fragmentação é suficiente para identificar a configuração de uma amostra de enantiopure, e a deteção de algumas centenas permite determinar a abundância dos enantiômeros em um amostra de composição enantiomérica desconhecida.

<table><tbody><tr><td>CHBrCl2</td><td>SigmaAldrich</td><td>139181-10G</td><td>or other suitable sample</td></tr><tr><td>femtosecond laser system</td><td>KMLabs</td><td>Wyvern500</td><td></td></tr><tr><td>High-reflective mirrors</td><td>EKSMA</td><td>042-0800</td><td></td></tr><tr><td>mirror mounts</td><td>Newport</td><td>U100-A-LH-2K&amp;nbsp;&amp;nbsp;</td><td></td></tr><tr><td>focusing mirror (protected silver, f = 75 mm)</td><td>Thorlabs&amp;nbsp;</td><td>CM254-075-P01</td><td>(if available: f = 60 mm)</td></tr><tr><td>COLTRIMS spectrometer, including electronics and data acquisition system</td><td>RoentDek</td><td>custom</td><td>contrary to the standard COLTRIMS, only one detector is needed</td></tr></tbody></table>

coulomb explosion imaging as a tool to distinguish between stereoisomers

Este artigo mostra como o COLTRIMS (frio alvo Recoil Ion Momentum espectroscopia) ou a técnica de "microscópio de reação" pode ser usada para distinguir entre enantiômeros (estereoisômeros) das espécies quirais simples no nível de moléculas individuais. Nessa abordagem, um jato gasoso molecular da amostra se expande em uma câmara de vácuo e cruza com pulsos de laser de femtosecond (fs). A alta intensidade dos pulsos leva à rápida ionização múltipla, acendendo uma explosão de Coulomb chamados que produz vários fragmentos (carregados positivamente) catiônicos. Campo eletrostático orienta estes cátions para detectores e posição-urgente. Semelhante a um espectrômetro de massa de tempo-de-voo, o tempo de chegada de cada íon produz informações sobre sua massa. Como um excedente, o campo eletrostático é regulado de forma que a direção de emissão e a energia cinética após a fragmentação da levam a variações no tempo-de-voo e na posição de impacto sobre o detector.
Cada impacto de íon cria um sinal eletrônico no detector; Este sinal é tratado pela eletrônica de alta frequência e gravado pelo evento por um computador. Os dados registados correspondem aos tempos de impacto e posições. Com esses dados, podem ser calculadas a energia e a direção de emissão de cada fragmento. Estes valores estão relacionados com as propriedades estruturais da molécula sob investigação, ou seja, para os comprimentos de ligação e as posições relativas dos átomos, que permite para determinar a molécula por molécula a lateralidade das espécies quirais simples e outras características isoméricos.

Nesta parte, mostramos os resultados obtidos para halomethanes. Estas espécies são ideais para prova de princípio experiências devido à sua simplicidade e alta pressão de vapor. Entretanto, o halotano espécies mais complexo foi investigado usando fótons de raio-x-macio único de uma fonte de síncrotron para induzir múltiplas de ionização. 14
CHBrClF
<p class="jove_content" fo:keep-together...

Watch this Scientific Journal Video about Coulomb Explosion Imaging as a Tool to Distinguish Between Stereoisomers at JoVE.com

Coulomb Explosion Imaging as a Tool to Distinguish Between Stereoisomers

Imagens de Explosão de Coulomb como uma Ferramenta para Distinguir Estereoisômeros

Para pequenas espécies quirais, Coulomb explosão Imaging fornece uma nova abordagem para determinar a lateralidade de moléculas individuais.

This article shows how the COLTRIMS (Cold Target Recoil Ion Momentum Spectroscopy) or the "reaction microscope" technique can be used to distinguish ...

coulomb-explosion-imaging-as-tool-to-distinguish-between

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Organic Chemistry

Chemistry

Unit 1

Transition Metals and Coordination Complexes

Stereoisomerism

SCIENTISTS IN ACTION

9.0K visualizações.  Goethe-Universität Frankfurt am Main. Para pequenas espécies quirais, Coulomb explosão Imaging fornece uma nova abordagem para determinar a lateralidade de moléculas individuais.

Vídeo: Coulomb Explosion Imaging as a Tool to Distinguish Between Stereoisomers

Spatial Separation of Molecular Conformers and Clusters

Gas-phase molecular physics and physical chemistry experiments commonly use supersonic expansions through pulsed valves for the production of cold molecular beams. However, these beams often contain multiple conformers and clusters, even at low rotational temperatures. We present an experimental methodology that allows the spatial separation of these constituent parts of a molecular beam expansion. Using an electric deflector the beam is separated by its mass-to-dipole moment ratio, analogous to a bender or an electric sector mass spectrometer spatially dispersing charged molecules on the basis of their mass-to-charge ratio. This deflector exploits the Stark effect in an inhomogeneous electric field and allows the separation of individual species of polar neutral molecules and clusters. It furthermore allows the selection of the coldest part of a molecular beam, as low-energy rotational quantum states generally experience the largest deflection. Different structural isomers (conformers) of a species can be separated due to the different arrangement of functional groups, which leads to distinct dipole moments. These are exploited by the electrostatic deflector for the production of a conformationally pure sample from a molecular beam. Similarly, specific cluster stoichiometries can be selected, as the mass and dipole moment of a given cluster depends on the degree of solvation around the parent molecule. This allows experiments on specific cluster sizes and structures, enabling the systematic study of solvation of neutral molecules.

We present a technique that allows the spatial separation of different conformers or clusters present in a molecular beam. An electrostatic deflector is used to separate species by their mass-to-dipole moment ratio, leading to the production of gas-phase ensembles of a single conformer or cluster stoichiometry.

Separação Espacial de Conformadores Moleculares e Clusters

Gas-phase molecular physics and physical chemistry experiments commonly use supersonic expansions through pulsed valves for the production of cold ...

Engenharia

Direct Imaging of Laser-driven Ultrafast Molecular Rotation

We present a method for visualizing laser-induced, ultrafast molecular rotational wave packet dynamics. We have developed a new 2-dimensional Coulomb explosion imaging setup in which a hitherto-impractical camera angle is realized. In our imaging technique, diatomic molecules are irradiated with a circularly polarized strong laser pulse. The ejected atomic ions are accelerated perpendicularly to the laser propagation. The ions lying in the laser polarization plane are selected through the use of a mechanical slit and imaged with a high-throughput, 2-dimensional detector installed parallel to the polarization plane. Because a circularly polarized (isotropic) Coulomb exploding pulse is used, the observed angular distribution of the ejected ions directly corresponds to the squared rotational wave function at the time of the pulse irradiation. To create a real-time movie of molecular rotation, the present imaging technique is combined with a femtosecond pump-probe optical setup in which the pump pulses create unidirectionally rotating molecular ensembles. Due to the high image throughput of our detection system, the pump-probe experimental condition can be easily optimized by monitoring a real-time snapshot. As a result, the quality of the observed movie is sufficiently high for visualizing the detailed wave nature of motion. We also note that the present technique can be implemented in existing standard ion imaging setups, offering a new camera angle or viewpoint for the molecular systems without the need for extensive modification.

We present a protocol for creating a real-time movie of a molecular rotational wave packet using a high-resolution Coulomb explosion imaging setup.

Direct Imaging da Laser-driven Ultrafast rotação Molecular

We present a method for visualizing laser-induced, ultrafast molecular rotational wave packet dynamics. We have developed a new 2-dimensional Coulomb ...

Química

Photoelectron Imaging of Anions Illustrated by 310 Nm Detachment of F&#8722;

Anion photoelectron imaging is a very efficient method for the study of energy states of bound negative ions, neutral species and interactions of unbound electrons with neutral molecules/atoms. State-of-the-art in vacuo anion generation techniques allow application to a broad range of atomic, molecular, and cluster anion systems. These are separated and selected using time-of-flight mass spectrometry. Electrons are removed by linearly polarized photons (photo detachment) using table-top laser sources which provide ready access to excitation energies from the infra-red to the near ultraviolet. Detecting the photoelectrons with a velocity mapped imaging lens and position sensitive detector means that, in principle, every photoelectron reaches the detector and the detection efficiency is uniform for all kinetic energies. Photoelectron spectra extracted from the images via mathematical reconstruction using an inverse Abel transformation reveal details of the anion internal energy state distribution and the resultant neutral energy states. At low electron kinetic energy, typical resolution is sufficient to reveal energy level differences on the order of a few millielectron-volts, i.e., different vibrational levels for molecular species or spin-orbit splitting in atoms. Photoelectron angular distributions extracted from the inverse Abel transformation represent the signatures of the bound electron orbital, allowing more detailed probing of electronic structure. The spectra and angular distributions also encode details of the interactions between the outgoing electron and the residual neutral species subsequent to excitation. The technique is illustrated by the application to an atomic anion (F&#8722;), but it can also be applied to the measurement of molecular anion spectroscopy, the study of low lying anion resonances (as an alternative to scattering experiments) and femtosecond (fs) time resolved studies of the dynamic evolution of anions.

Here, we present a protocol for photoelectron imaging of anionic species. Anions generated in vacuo and separated by mass spectrometry are probed using velocity mapped photoelectron imaging, providing details of anion and neutral energy levels, anion and neutral structure and the nature of the anion electronic state.

Imagem de fotoelétron de ânions ilustrado por 310 Nm destacamento de F−

Anion photoelectron imaging is a very efficient method for the study of energy states of bound negative ions, neutral species and interactions of unbound ...

Using a Cyclic Ion Mobility Spectrometer for Tandem Ion Mobility Experiments

Accurate characterization of chemical structures is important to understand their underlying biological mechanisms and functional properties. Mass spectrometry (MS) is a popular tool but is not always sufficient to completely unveil all structural features. For example, although carbohydrates are biologically relevant, their characterization is complicated by numerous levels of isomerism. Ion mobility spectrometry (IMS) is an interesting complement because it is sensitive to ion conformations and, thus, to isomerism. 
Furthermore, recent advances have significantly improved the technique: the last generation of Cyclic IMS instruments offers additional capabilities compared to linear IMS instruments, such as an increased resolving power or the possibility to perform tandem ion mobility (IMS/IMS) experiments. During IMS/IMS, an ion is selected based on its ion mobility, fragmented, and reanalyzed to obtain ion mobility information about its fragments. Recent work showed that the mobility profiles of the fragments contained in such IMS/IMS data can act as a fingerprint of a particular glycan and can be used in a molecular networking strategy to organize glycomics datasets in a structurally relevant way. 
The goal of this protocol is thus to describe how to generate IMS/IMS data, from sample preparation to the final Collision Cross Section (CCS) calibration of the ion mobility dimension that yields reproducible spectra. Taking the example of one representative glycan, this protocol will show how to build an IMS/IMS control sequence on a Cyclic IMS instrument, how to account for this control sequence to translate IMS arrival time into drift time (i.e., the effective separation time applied to the ions), and how to extract the relevant mobility information from the raw data. This protocol is designed to clearly explain the critical points of an IMS/IMS experiment and thus help new Cyclic IMS users perform straightforward and reproducible acquisitions.

Ion mobility spectrometry (IMS) is an interesting complement to mass spectrometry for the characterization of biomolecules, notably because it is sensitive to isomerism. This protocol describes a tandem IMS (IMS/IMS) experiment, which allows the isolation of a molecule and the generation of the mobility profiles of its fragments.

Usando um espectrômetro de mobilidade de íons cíclicos para experimentos de mobilidade de íons tandem

Accurate characterization of chemical structures is important to understand their underlying biological mechanisms and functional properties. Mass ...

Direct Comparison of Hyperspectral Stimulated Raman Scattering and Coherent Anti-Stokes Raman Scattering Microscopy for Chemical Imaging

Stimulated Raman scattering (SRS) and coherent anti-Stokes Raman scattering (CARS) microscopy are the most widely used coherent Raman scattering imaging technologies. Hyperspectral SRS and CARS imaging offer Raman spectral information at every pixel, which enables better separation of different chemical compositions. Although both techniques require two excitation lasers, their signal detection schemes and spectral properties are quite different. The goal of this protocol is to perform both hyperspectral SRS and CARS imaging on a single platform and compare the two microscopy techniques for imaging different biological samples. The spectral focusing method is employed to acquire spectral information using femtosecond lasers. By using standard chemical samples, the sensitivity, spatial resolution, and spectral resolution of SRS and CARS in the same excitation conditions (i.e., power at the sample, pixel dwell time, objective lens, pulse energy) are compared. The imaging contrasts of CARS and SRS for biological samples are juxtaposed and compared. The direct comparison of CARS and SRS performances would allow for optimal selection of the modality for chemical imaging.

This paper directly compares the resolution, sensitivity, and imaging contrasts of stimulated Raman scattering (SRS) and coherent anti-Stokes Raman scattering (CARS) integrated into the same microscope platform. The results show that CARS has a better spatial resolution, SRS gives better contrasts and spectral resolution, and both methods have similar sensitivity.

Comparação direta da dispersão de Raman estimulada hiperespectral e microscopia de dispersão de Raman coerente para imagem química

Stimulated Raman scattering (SRS) and coherent anti-Stokes Raman scattering (CARS) microscopy are the most widely used coherent Raman scattering imaging ...

Differential Imaging of Biological Structures with Doubly-resonant Coherent Anti-stokes Raman Scattering (CARS)

Coherent Raman imaging techniques have seen a dramatic increase in activity over the past decade due to their promise to enable label-free optical imaging with high molecular specificity 1. The sensitivity of these techniques, however, is many orders of magnitude weaker than fluorescence, requiring milli-molar molecular concentrations 1,2. Here, we describe a technique that can enable the detection of weak or low concentrations of Raman-active molecules by amplifying their signal with that obtained from strong or abundant Raman scatterers. The interaction of short pulsed lasers in a biological sample generates a variety of coherent Raman scattering signals, each of which carry unique chemical information about the sample. Typically, only one of these signals, e.g. Coherent Anti-stokes Raman scattering (CARS), is used to generate an image while the others are discarded. However, when these other signals, including 3-color CARS and four-wave mixing (FWM), are collected and compared to the CARS signal, otherwise difficult to detect information can be extracted 3. For example, doubly-resonant CARS (DR-CARS) is the result of the constructive interference between two resonant signals 4. We demonstrate how tuning of the three lasers required to produce DR-CARS signals to the 2845 cm-1 CH stretch vibration in lipids and the 2120 cm-1 CD stretching vibration of a deuterated molecule (e.g. deuterated sugars, fatty acids, etc.) can be utilized to probe both Raman resonances simultaneously. Under these conditions, in addition to CARS signals from each resonance, a combined DR-CARS signal probing both is also generated. We demonstrate how detecting the difference between the DR-CARS signal and the amplifying signal from an abundant molecule's vibration can be used to enhance the sensitivity for the weaker signal. We further demonstrate that this approach even extends to applications where both signals are generated from different molecules, such that e.g. using the strong Raman signal of a solvent can enhance the weak Raman signal of a dilute solute.

Differential Imaging of Biological Structures with Doubly-resonant Coherent Anti-stokes Raman Scattering CARS

A combination of three single wavelength short-pulsed lasers is used to generate coherent anti-Stokes Raman scattering (CARS) and doubly-resonant CARS (DR-CARS). The difference between these signals provides enhanced sensitivity for otherwise difficult to detect coherent Raman signals, enabling imaging of weak Raman scatterers.

Imagem diferencial de estruturas biológicas com Duplamente-ressonante Scattering Anti-Stokes Raman coerente (CARS)

Coherent Raman imaging techniques have seen a dramatic increase in activity over the past decade due to their promise to enable label-free optical imaging ...

Biologia

Usando um microscópio VSFG para análise química multimodal, rápida e hiperespectral

Multimodal Nonlinear Hyperspectral Chemical Imaging Using Line-Scanning Vibrational Sum-Frequency Generation Microscopy

Vibrational sum-frequency generation (VSFG), a second-order nonlinear optical signal, has traditionally been used to study molecules at interfaces as a spectroscopy technique with a spatial resolution of ~100 &#181;m. However, the spectroscopy is not sensitive to the heterogeneity of a sample. To study mesoscopically heterogeneous samples, we, along with others, pushed the resolution limit of VSFG spectroscopy down to ~1 &#181;m level and constructed the VSFG microscope. This imaging technique not only can resolve sample morphologies through imaging, but also record a broadband VSFG spectrum at every pixel of the images. Being a second-order nonlinear optical technique, its selection rule enables the visualization of non-centrosymmetric or chiral self-assembled structures commonly found in biology, materials science, and bioengineering, among others. In this article, the audience will be guided through an inverted transmission design that allows for imaging unfixed samples. This work also showcases that VSFG microscopy can resolve chemical-specific geometric information of individual self-assembled sheets by combining it with a neural network function solver. Lastly, the images obtained under brightfield, SHG, and VSFG configurations of various samples briefly discuss the unique information revealed by VSFG imaging.

Autor em destaque: Revelando o potencial da microscopia VSFG no estudo de estruturas automontadas mesoscopicamente heterogêneas 

A multimodal, rapid hyperspectral imaging framework was developed to obtain broadband vibrational sum-frequency generation (VSFG) images, along with brightfield, second harmonic generation (SHG) imaging modalities. Due to the infrared frequency being resonant with molecular vibrations, microscopic structural and mesoscopic morphology knowledge is revealed of symmetry-allowed samples.

Imagem Química Hiperespectral Não Linear Multimodal Usando Microscopia de Geração de Soma e Frequência Vibracional de Varredura de Linha

Vibrational sum-frequency generation (VSFG), a second-order nonlinear optical signal, has traditionally been used to study molecules at interfaces as a ...

Stereoisomers

On the basis of mirror symmetry, stereoisomers of an organic molecule can be further classified into diastereomers and enantiomers. Diastereomers are stereoisomers that are not mirror images of each other. Substituted alkenes, such as the cis and trans isomers of 2-butene, are diastereomers, as these molecules exhibit different spatial orientations of their constituent atoms, are not mirror images of each other,&#160;and do not interconvert. Here, the interconversion is suppressed due to restricted rotation around the &#960; bond. Another class of diastereomers &#8212; those without &#960; bonds &#8212; are molecules that have non-superposable spatial orientations and exhibit different configurations of their substituent groups at some, but not all, stereocenters. For example, cis-1,2-dimethylcyclohexane and trans-1,2-dimethylcyclohexane are diastereomers, as these molecules are not superposable and have a different configuration of the methyl and hydrogen groups at only one out of their two stereocenters.
Enantiomers are stereoisomers that are mirror images of each other. As only chiral molecules can have non-superposable mirror images, enantiomers are chiral molecules. For example, the chiral molecule 2-butanol and its mirror image are enantiomers, as these molecules exhibit non-superposable spatial orientations of their constituent atoms and are mirror images of each other. A chiral molecule and its mirror image are collectively referred to as an enantiomeric pair, or a pair of enantiomers. The enantiomer of a chiral molecule can be drawn by taking its mirror image from any position or&#160;exchanging the positions of two of the substituents at each stereocenter of the molecule.

Stereoisomers: Diastereomers and Enantiomers

Estereoisômeros

On the basis of mirror symmetry, stereoisomers of an organic molecule can be further classified into diastereomers and enantiomers. Diastereomers are ...

Imagens de Explosão de Coulomb como uma Ferramenta para Distinguir Estereoisômeros

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