High-pressure circular dichroism spectroscopy up to 400 MPa using polycrystalline yttrium aluminum garnet (YAG) as pressure-resistant optical windows

Abstract

Circular dichroism (CD) spectroscopy at high pressure (≤400 MPa) was accomplished by using polycrystalline yttrium aluminum garnet (Y₃Al₅O₁₂, YAG) as pressure-resistant optical windows. Conformational changes, including main-chain helix inversions of poly(quinoxaline-2,3-diyl)s (PQXs), in organic solvents at high hydrostatic pressure (>200 MPa) using a newly developed high-pressure CD cell with polycrystalline YAG windows were demonstrated.

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Pressure is one of the most fundamental thermodynamic parameters used in order to determine the conformation and higher-order structure of molecules.¹ To date, high-pressure in situ measurements of UV-vis-IR absorption,² fluorescence,³ nuclear magnetic resonance (NMR),⁴ as well as X-ray⁵ or neutron⁶ scattering have provided important information on the dynamics of molecular structure upon pressurization. In contrast, high-pressure circular dichroism (CD) spectroscopy,⁷ which is a sensitive method that elucidates the chiral higher-order structure of solute molecules, still remains less explored due to the limitations associated with pressure-resistant optical window materials for CD measurements.

The key requirements of a pressure-resistant optical window for CD spectroscopy are: (i) high transmittance for a wide wavelength range; (ii) high durability at high pressure, which can be estimated from the Young's modulus (YM) or apparent elastic limit of the window material; (iii) optical isotropy, so that artifact CD signals can be excluded. It should be noted here that (i) and (ii) are also common requirements for high-pressure optical windows for UV-vis-IR absorption and fluorescence measurements. So far, single-crystal alumina (α-Al₂O₃, sapphire) has been used as window material for high-pressure optical measurements, as it exhibits high transmittance for a broad wavelength range as well as a high YM (400 GPa).⁸ However, α-Al₂O₃ is not suitable as an optical window for CD spectroscopy due to its optical anisotropy arising from its crystal structure (trigonal crystal system, R3c).

In 1976, Harris and co-workers reported a pressure cell for CD measurements using fused silica (quartz glass, SiO₂) windows,^7a albeit that the pressure was limited to 17 MPa. In 2002, Hayashi and co-workers developed a universal high-pressure cell with exchangeable optical windows (Fig. 1a).^7b In this system, the sample solution is enclosed in an inner quartz cell with a diaphragm tube, and the cell is pressurized by water from a high-pressure pump. The sample solution is pressurized through the deformation of the diaphragm tube (Fig. 1b).

Fig. 1 (a) Schematic illustration of the universal high-pressure CD cell developed by Hayashi and co-workers. (b) Pressurization of a sample solution through deformation of the diaphragm tube.

Such a high-pressure cell with windows made of diamond (cubic carbon; thickness, T = 1.0 mm), which exhibits an extremely high YM (1100 GPa)⁸ and optical isotropy (cubic crystal system, Fd3m), tolerated up to 400 MPa. However, as optical grade diamonds are very expensive, alternative materials are desirable. Hayashi's group also showed that a similar cell could tolerate pressures up to 200 MPa when using thick SiO₂ windows (T = 8.0 mm, YM = 95 GPa).⁸

Yttrium aluminum garnet (Y₃Al₅O₁₂, YAG) is widely used as a lasing medium for solid-state lasers upon doping with various lanthanides.⁹ Recently, transparent polycrystalline YAG has been developed as an efficient lasing medium,¹⁰ which is now commercially available at low cost. Undoped polycrystalline YAG shows good transmittance in a wide wavelength range and a high YM (308 GPa).¹¹ Furthermore, because YAG exhibits optical isotropy (cubic crystal system, m3m), it should be a good prospective material for optical windows in high-pressure CD spectroscopy. In this paper, we report the optical properties of prospective materials for pressure-resistant optical windows including fused silica, sapphire, diamond, and polycrystalline YAG. In addition, we demonstrate high-pressure CD measurements using a pressure cell equipped with polycrystalline YAG windows.

Initially, we prepared disc-shaped windows of fused silica (1, SiO₂ glass, T = 6.5 mm), sapphire (2, α-Al₂O₃, T = 7.0 mm), diamond (3, cubic C, T = 0.5 mm), and polycrystalline YAG (4, T = 5.0 mm) to measure UV-vis transmittance spectra (Fig. 2a). While 1 and 2 showed high transmittance in the entire UV-vis region (200–700 nm), 3 exhibited strong absorption despite its thinness (T = 0.5 mm). Although 4 also exhibited an absorption peak in the short-wavelength region (220 nm), the transmittance of 4 was higher than that of 3 (3: 1.2%, 4: 23.6%), even though the thickness of 4 was ten times higher than that of 3.

Fig. 2 (a) Transmittance and (b) CD spectra using 1–4 as window materials. 1: SiO₂ glass, T = 6.5 mm; 2: α-Al₂O₃, T = 7.0 mm; 3: cubic C, T = 0.5 mm; 4: polycrystalline YAG, T = 5.0 mm. Note that the CD spectrum of 3 in the short wavelength region (200–215 nm) is not accurate because of the low transmittance.

Subsequently, we carried out CD measurements on 1–4 (Fig. 2b). While 1 showed weak artifact signals arising from striae or interior strain of the fused silica, 2 exhibited strong artifact peaks due to its optical anisotropy. In addition, the sign of the artifact signal was dependent on the orientation of the disc-shaped sample (for details, see ESI†). Where its transmittance is sufficient (>10%), 3 afforded an almost flat CD spectrum (230–700 nm). It should also be noted that 4 furnished a CD spectrum without artifact peaks and high transmittance for a wider range of the UV-vis region (215–700 nm) than 3 (Fig. 2a), even though the thickness of 4 was ten times higher than that of 3.

Next, the mechanical strength of polycrystalline YAG as an optical window material was investigated (appropriate safety measures must be taken, see ESI†). Polycrystalline YAG discs with different thickness (T = 0.5 and 1.0 mm) were prepared and introduced in an unclamped-type flange with a circular hole (diameter, D = 5.0 mm). The hydrostatic pressure (P) was increased to reach the burst pressure of the disc (Fig. 3a). Average burst pressure values of 67 MPa and 125 MPa were found for T = 0.5 mm and T = 1.0 mm, respectively (Fig. 3b). The apparent elastic limit F_a can be expressed by the equation:

F_a = P × (D/T)² × K/4 (1)

where K is an empirical factor (∼1.125 for an unclamped circular window).¹² According to eqn (1), F_a values of 1890 MPa and 880 MPa were estimated for T = 0.5 and T = 1.0 mm, respectively. For a polycrystalline YAG window (T = 5.0 mm), burst pressures of 6710 (from 0.5 mm disc) or 3120 MPa (from 1.0 mm disc) were estimated, which is sufficient for measurements up to 400 MPa. Subsequently, a repetitive pressurization/depressurization test (≤50 MPa) of a polycrystalline YAG window (T = 1.0 mm) was carried out (Fig. 3c). After ten cycles of pressurization, including two cycles where pressure was maintained for 10 min, no apparent change could be detected by microscopic observation. After the breaking test of a polycrystalline YAG window (T = 0.5 mm), many cracks were observed in the peripheral part of the disc, and the center part was ruptured into fine pieces and powder. In contrast, the SiO₂ window broke into relatively large pieces, which suggests that this is a characteristic behavior of polycrystalline materials.

Fig. 3 (a) Schematic illustration of a breaking test of a polycrystalline YAG window. P, T, and D refer to pressure, thickness, and diameter, respectively. (b) Burst pressure values for polycrystalline YAG windows (T = 0.5 and 1.0 mm). The error bars represent the standard deviation obtained from three independent experiments. (c) Repeated exposure of a polycrystalline YAG window (T = 1.0 mm) to pressurization/depressurization (≤50 MPa). (d) Photograph of a polycrystalline YAG window (T = 0.5 mm) after the breaking test (top view).

Finally, we performed high-pressure CD measurements on poly(quinoxaline-2,3-diyl)s (PQXs)¹³ dissolved in organic solvents using polycrystalline YAG windows (T = 5.0 mm). Recently, we reported solvent-¹⁴ and pressure-dependent helix inversion¹⁵ of the helical main chains in PQXs. For example, a PQX bearing (S)-2-butoxymethyl side chains (P1; Fig. 4) adopted P- or M-helical structures in a mixture of CHCl₃/1,1,2-trichloroethane (CHCl₃/1,1,2-TCE, v/v = 35/65) at ambient (0.1 MPa) or high pressure (200 MPa), respectively. However, its screw-sense induction behavior at pressures beyond 200 MPa could not be measured, given that we used SiO₂ windows.

Fig. 4 Variable pressure CD spectra of (a) a mixed 1,1,2-TCE/CHCl₃ solvent (30/70) for a baseline correction, (b) P1 (12.9 × 10⁻² g L⁻¹) in the mixed 1,1,2-TCE/CHCl₃ solvent (30/70) after the baseline correction. (c) g_abs values of P1 in a mixed 1,1,2-TCE/CHCl₃ solvent, and of (d) P1 in 1,2-DCE or 1-BuCl, and of P2 in 1,2-DCE.

P1 was dissolved in CHCl₃/1,1,2-TCE mixtures of varying proportion and subjected to CD measurements at 0.1, 200, and 400 MPa. The solvent without chiral solute showed almost flat CD spectra at these pressures, suggesting that the YAG windows were not distorted even at the high pressure (Fig. 4a). After the baseline correction, CD spectra of P1 at 0.1, 200, and 400 MPa were obtained (Fig. 4b, see ESI† for other samples). At ambient pressure (0.1 MPa), P1 showed a positive signal at 368.0 nm in a mixture consisting of 20–35% CHCl₃, indicating that P1 adopted a P-helical conformation therein (Fig. 4c). In mixtures consisting of 40–100% CHCl₃, P1 adopted an M-helical conformation at 0.1 MPa. At 200 MPa, the conformation of P1 was inverted to M-helical in a mixture containing 35% CHCl₃. Finally, at 400 MPa, P1 adopted an M-helical conformation in mixtures consisting of 30–100% CHCl₃.

We then carry out CD measurements on P1 in 1,2-dichloroethane (1,2-DCE) or 1-butylchloride (1-BuCl), and on P2 bearing (S)-2-pentyloxymethyl side chains in 1,2-DCE (Fig. 4d). P1 in 1,2-DCE exhibited a helix inversion around 100 MPa, and a negative CD signal was intensified at 300 MPa. Although the g_abs value slightly increased up to 400 MPa, the screw-sense excess reached saturation at 300–400 MPa. P1 in 1-BuCl adopted an M-helical conformation at 0.1–200 MPa, but the main chain was inverted to a P-helical conformation at 400 MPa. The g_abs value of P2 in 1,2-DCE gradually decreased with pressurization, albeit that helix inversion was not observed up to 400 MPa. These results suggest that the use of the high-pressure-resistant YAG windows affords more insight into the effect of pressure on the conformational change of helical PQXs.

Conclusions

In summary, we have demonstrated that polycrystalline yttrium aluminum garnet (Y₃Al₅O₁₂, YAG) can be used as optical window material in high-pressure CD measurements due to its high mechanical strength, transmittance over a wide wavelength range, and optical isotropy. We have also developed a high-pressure cell equipped with YAG windows and carried out CD measurements of poly(quinoxaline-2,3-diyl)s at high pressure. The pressure-induced conformational changes of these polymers were clearly observed in the CD spectra recorded using the YAG windows. Although there is still a limitation on the transmittance in the short wavelength region, we believe that high-pressure CD spectroscopy with YAG optical windows should thus reveal various pressure-induced conformational changes of not only helical polymers, but also of chiral small molecules, supramolecules, and biomolecules in living systems.

Acknowledgements

The authors would like to thank Dr Masamitsu Matsumoto and Mr Makoto Nagasawa of Syn Corporation Ltd. (Kyoto, Japan) for their kind help with the breaking tests. Financial support for this research was provided by the JST (CREST, “Establishment of Molecular Technology towards the Creation of New Function”) and by a Grant-in-Aid for Scientific Research on Innovative Areas “π-System Figuration” (No. 15H00994) from MEXT.

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures, and UV-vis absorption and CD spectra. See DOI: 10.1039/c6ra23736c

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