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Download Author Version (PDF) Analyst Accepted Manuscript This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. www.rsc.org/analyst Page 1 of 7 Please do notAnalyst adjust margins 1 2 3 4 Analyst 5 6 7 8 COMMUNICATION 9 10 11 12 Quantitative enantioselective Raman spectroscopy 13 14 J. Kiefera,b,c 15 Received 00th January 20xx, 16 Accepted 00th January 20xx 17 DOI: 10.1039/x0xx00000x 18 19 www.rsc.org/ 20 21 Analytical methods for quantitative enantioselective measurements are identical physicochemical properties. Methods capable of 22 highly desirable in the life sciences. Existing technologies have differentiating enantiomers include NMR spectroscopy,4 23 disadvantages such as limited temporal resolution, the need for molecular microwave5 and fluorescence6 spectroscopy, vibrational circular 24 labeling, or high experimental complexity. To overcome these limitations, dichroism (VCD),7 and cavity ringdown polarimetry.8 However, all 25 this work presents a method based on conventional Raman spectroscopy. these methods have disadvantages. Some of them are time- 26 A systematic investigation of the key parameters is carried out. It is consuming and hence do not provide sufficient time-resolution for Manuscript 27 demonstrated that their careful choice provides an opportunity for process monitoring, some require molecular labeling or the 28 enantioselective and quantitative analysis of enantiopure systems as well addition of a chemical agent, and some are expensive and 29 as enantiomer mixtures. experimentally complicated. 30 Analytical techniques based on the Raman effect are interesting 31 Many biologically and pharmaceutically active molecules are chiral,1 alternatives, as they have the potential to provide detailed and 32 for instance amino acids and sugars. Chemical synthesis of such quantitative information about the chemical structure and 33 compounds usually generates the racemate, which is a 1:1 mixture composition. The most established tool in the context of chiral 34 of both enantiomers. However, the enantiomers of a chiral systems is Raman optical activity (ROA).9-11 In a ROA experiment, 35 substance may exhibit very different physiological effects. the sample is exposed to circularly polarized light and the difference 36 Prominent examples are the nonsteroidal anti-inflammatory drug intensity spectrum obtained from right- and left-handed circular 37 ibuprofen and the immune-modulatory drug thalidomide. In case of irradiation yields enantioselective structural information.10 A 38 ibuprofen, one enantiomer is the drug exhibiting the desired drawback, however, can be the measurement time, which is usually 39 pharmaceutical activity, while its counterpart is virtually inactive. in the order of minutes to hours; hence the potential for process 40 Hence, there is no need to purify one enantiomer as the racemate monitoring is limited. Another Raman method is surface enhanced 41 can be marketed.2 On the other hand, thalidomide is a completely Raman spectroscopy (SERS), which has been demonstrated as a tool 42 different story: one enantiomer possesses the desired effect, while for enantiomeric discrimination recently.12 SERS utilizes the Raman 43 the other can cause birth defects in children.3 Therefore, it is signal enhancement in the close vicinity of a plasmonic material, 44 important that such a chiral substance is produced with high e.g. a metal nanoparticle. When this material interacts with the 45 Accepted enantiomeric purity. target species in an enantioselective way, enantiomeric 46 The key to ensure enantiomeric purity is to have effective methods discrimination is possible. Disadvantages of SERS for process 47 in place for monitoring the synthesis and purification process. A monitoring are that the plasmonic agent needs to be added. This 48 suitable analytical technology should facilitate speciation/molecular may be expensive and not practicable. Furthermore, signal 49 identification, enantiomeric discrimination, and quantification of quantification is difficult.13 The disadvantages of both ROA and SERS 50 the enantiomeric ratio. These requirements represent true 51 can be overcome using conventional Raman spectroscopy with challenges, as the enantiomers of a chiral substance possess 52 polarization-resolved signal detection. Raman scattering is 14, 15 16, 17 53 frequently used as a tool for structural and compositional analysis. However, enantioselective measurements were believed 54 a. Technische Thermodynamik, Universität Bremen, Badgasteiner Str. 1, 28359 55 Bremen, Germany to be impossible in the past. This limitation has been overcome only b. School of Engineering, University of Aberdeen, Fraser Noble Building, Aberdeen recently.18 56 AB24 3UE, UK 18 57 c. Erlangen Graduate School in Advanced Optical Technologies, Universität In a recent Communication, we have proposed a concept that 58 Erlangen-Nürnberg, 91052 Erlangen, Germany allows the qualitative discrimination of the enantiomers of a chiral 59 Electronic Supplementary Information (ESI) available: [details of any substance using Raman spectroscopy for the first time. For this supplementary information available should be included here]. See purpose, a simple half-wave retarder was inserted in a polarization- 60 Analyst DOI: 10.1039/x0xx00000x This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 1 Please do not adjust margins Please do notAnalyst adjust margins Page 2 of 7 1 COMMUNICATION Journal Name 2 3 resolved Raman setup and the polarization behavior of the laser signal components as function of the rotation angle of the half- 4 and the Raman signal was exploited.19 The present article extends wave retarder. The data have been simulated assuming a constant 5 the qualitative enantioselective approach to facilitate quantitative depolarization ratio of 0.2 and the maximum concentration of 1 for 6 measurements. Hence, it makes an important step towards using one or the other enantiomer. The latter means absolute values for 7 this new method in practical applications. For completeness, it and of 5° and 10°, respectively, where the D-enantiomer will 8 should be pointed out that the proposed method utilizes the effect exhibit a negative sign. The angle is the angle by which the laser 9 of optical activity on the Raman signal regardless of the origin of the polarization is rotated at the measurement point in the sample with 10 optical activity. This means that enantioselective Raman respect to the z-axis. 11 spectroscopy can be used to directly monitor the Raman peaks of a The angle between the optical axis of the half-wave plate and the z- 12 chiral substance or to indirectly detect the chirality via the peaks of axis has been varied from -20° to +20°, in other words -2 to +2. 13 an achiral species in the sample, e.g. a solvent. For the =0°, the signals of both enantiomers are rotated by the 14 same angle in terms of absolute value, and the remaining symmetry 15 Quantitative measurements in a chiral system can have two main of the polarization diagram results in identical vertical and 16 objectives: (1) to determine the overall concentration of a target horizontal signal components for both enantiomers. The other 17 species, and (2) to determine the concentration ratio of the extreme values considered are -20° and +20°. In both cases, the 18 enantiomers of the target species. In order to achieve these goals, polarized signal component, Ipol, of one of the enantiomers is 19 the effects of all key parameters on the signal behavior must be rotated such that it coincides with the z-axis, and the other one 20 understood for the proposed enantioselective Raman technique. ends up at +40° or -40°, respectively. This situation represents the 21 The main experimental parameter to be taken into account is the highest possible asymmetry; hence, the largest differences in signal 22 rotation angle of the half-wave retarder. On the sample side, the intensities and the best discrimination of the enantiomers. 23 Raman depolarization ratio is crucial. The effects of both need to be However, it must be kept in mind that the data displayed 24 analyzed for varying enantiomer concentration. In the present correspond to one specific Raman depolarization ratio and a given 25 work, these effects are studied computationally in order to avoid concentration. In the full Raman spectrum of a chiral molecule, 26 Manuscript 27 any bias from experimental artifacts. The influence of the there will be numerous peaks exhibiting different depolarization 28 parameters of interest on the signal polarization and intensity are ratios, and the concentration may be unknown. Consequently, 29 modeled using first principles of physics. A detailed description of selecting an angle for an experiment may be not as 30 the method and the underlying mathematical framework are laid straightforward as Fig. 1 would suggest at first glance. Generally 31 out in the Electronic Supplementary Information together with a speaking, any angle that does not equal zero works in principle, but 32 description of all the parameters. In the following, we analyze the a test/calibration measurement using a defined sample is helpful to 33 effects of retarder orientation, Raman depolarization and optimize the alignment. 34 concentration on a single isolated peak in the Raman spectrum.
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