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Ji X Y, Sun S, Zhang Y. Recognition of chiral substances by terahertz circular dichroism[J]. Opto-Electron Eng, 2024, 51(5): 240005. doi: 10.12086/oee.2024.240005
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Recognition of chiral substances by terahertz circular dichroism
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Abstract
Biomolecules with different chirality have different or even opposite biological and pharmacological activities. Since vibration and rotation energy levels of many biomolecules lie within the terahertz range, terahertz spectroscopy has emerged as a useful tool for biomolecular identification. Nevertheless, linearly polarized light sources are used in terahertz time-domain spectroscopy, which is ineffective for identifying chiral compounds. We theoretically constructed a circularly polarized Jones matrix using a linearly polarized Jones matrix model. We also calculated the transmission circular dichroism spectrum of the sample based on the difference in transmittance of circularly polarized light, offering a useful technique for describing various chiral compounds. The spectra of (R)-(-)-Ibuprofen and (S)-(+)-Ibuprofen were investigated using a transmission terahertz time-domain spectroscopy system, and the linear polarization biased transmittance and circular polarization transmittance of (R)-(-)-Ibuprofen and (S)-(+)-Ibuprofen were computed. Additionally, the transmittance circular dichroic spectra were calculated, and the two chiral compounds' circular dichroic values reached 0.015, successfully achieving the (R)-(-)-Ibuprofen and (S)-(+)-Ibuprofen recognition effect. This technique serves as a guide for chiral molecule identification and detection using terahertz spectroscopy technology.-
Keywords:
- Jones matrix /
- circular dichroism /
- terahertz /
- chiral recognition
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References
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Overview
Chiral molecules are defined as materials that have chiral centers, also referred to as asymmetric centers in chemical structure. Biomolecules with different chirality exhibit distinct or even opposing biological and pharmacological effects, even when their structural groups are the same. Chirality detection and recognition have long been a significant issue in the realm of life sciences. Circular dichroism, a method of identifying chiral substances, is produced when left- and right-handed circularly polarized light are absorbed differently by different chiral molecules. This results in different amplitudes of circularly polarized light passing through different rotations. Currently, available techniques for detecting chiral drugs include high performance liquid chromatography, chiral Raman spectroscopy, nuclear magnetic resonance spectroscopy, and polarimeter detection; nevertheless, there are drawbacks, such as costly and difficult-to-use procedures. The terahertz spectroscopy technique becomes an excellent way to identify biological macromolecules since many of them have vibrational and rotational energy levels that fall in the terahertz range. Currently, linearly polarized light sources are used in the terahertz time-domain spectroscopy system, which is unable to detect chiral compounds. Theoretically, we can synthesize the Jones matrix under circular polarization using the Jones matrix model of linear polarization. We can then determine the circular dichroism spectrum of the sample based on the transmission difference of circular polarization light. This approach offers a useful way to characterize various chiral molecules. By rotating the crystal and the experimental sample through four measurements, the time-domain spectra under various polarization states are obtained based on the transmitted-terahertz time-domain spectroscopy system. The Jones matrix of the sample online polarization space is obtained, which is then converted into the circular polarization space. The circular polarization space's Jones matrix component is utilized to compute the sample's circular dichroism (CD) spectrum. This study presents the measurement of the spectra of (R)-(-)-Ibuprofen and (S)-(+)-Ibuprofen, together with the calculation of their linear and circular bias transmittances. The circular dichroism spectra of the two chiral substances are calculated, and the circular dichroism values of the two chiral substances reach 0.015, which effectively realizes the recognition effect of (R)-(-)-Ibuprofen and (S)-(+)-Ibuprofen. This work provides a reference for the following chiral recognition in the terahertz band by using the superstructure surface and improving the local intensity of the light field through high-quality resonance to improve the accuracy and sensitivity of chiral recognition.
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Ji X Y, Sun S, Zhang Y. Recognition of chiral substances by terahertz circular dichroism[J]. Opto-Electron Eng, 2024, 51(5): 240005. doi: 10.12086/oee.2024.240005
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Figure 1.
(a) Schematic diagram of transmissive terahertz time-domain spectroscopy. HWP: half-wave plate, PBS: polarization splitting prism, L: lens, PM: parabolic mirror, QWP: quarter wave plate, and P: polarizer; (b) Experimental tablet sample and corresponding molecular formula
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Figure 2.
(a) Time domain signal and (b) frequency domain spectrum of the THz-TDS
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Figure 3.
Transmittances of (R)-(-)-ibuprofen and (S)-(+) -ibuprofen for linear polarized terahertz wave. (a) Transmittances of (R)-(-)-Ibuprofen for same linear polarization; (b) Transmittances of (R)-(-)-Ibuprofen for cross linear polarization; (c) Transmittances of (S)-(+)-Ibuprofen for same linear polarization; (d) Transmittance of (S)-(+)-Ibuprofen for cross linear polarization
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Figure 4.
Transmittances of (R)-(-)-Ibuprofen and (S)-(+) -ibuprofen for circular polarized wave. (a) Transmittances of (R)-(-)-Ibuprofen for same circular polarization; (b) Transmittance of (R)-(-)-Ibuprofen for cross circular polarization; (c) Transmittance of (S)-(+)-Ibuprofen for same circular polarization; (d) Transmittance of (S)-(+)-Ibuprofen for cross circular polarization
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Figure 5.
Terahertz CD spectra of (R)-(-)-Ibuprofen and (S)-(+)-ibuprofen
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