Structural and thermal analyses of a hydrolysis compound of thalidomide

Thalidomide (TD) is a historically famous chiral drug. After prescription as a safe hypnotic, TD was suspected of contributing to teratogenicity, resulting in prohibition of the use of TD. TD and its derivatives, however, have attracted a renewed attention since their therapeutic effects for Hansen's disease and multiple myeloma were demonstrated. Meanwhile TD has been known to suffer from spontaneous hydrolysis with complicated pathways, leading to the production of various metabolites of TD. Therefore, we are now facing the difficulty in specifying the compounds which cause desired and/or adverse effects in the drug mechanism of TD. In a previous study, pure hydrolytic products of TD were synthesized and assayed for production-inhibitory activity of TNF-α, a kind of cytokine that induces inflammation. This study has showed that some products, especially α-(2-carboxybenzamido)glutarimide (CBG), exhibit high potency for the inhibition of TNF-α production compared to original TD. The hydrolytic products of TD thus are found to greatly attribute to the pharmacological effect of TD. For comprehending effects of the hydrolytic products, it is much significant to perform physicochemical analysis of them because their properties are deeply related to molecular stability and interaction with receptor proteins. In this study, we thus aim to investigate the physicochemical properties of CBG employing X-ray crystal structural analysis and thermal analyses. Single crystals of racemic and enantiomeric CBG were grown by solvent evaporation methods. On the crystallization, we chose alcoholic solvents such as methanol and ethanol. Surprisingly, crystals grown from the CBG solutions have indicated the same crystal structure of TD with high reproducibility. This unique result is likely to represent dehydration of CBG in non-aqueous solvents, which has not been reported so far. The detailed mechanism is under investigation.

Optical properties of chiral photomechanical salicylideneaniline crystal

Mechanical motion caused by UV/VIS radiation onto bulk materials is called a photomechanical effect. In other words, the photomechanical effect is a type of energy conversion systems. Recently, a mechanical bending of photochromic diarylethene crystals was reported, and the molecular-scale motion was found to produce the macroscale bending of the crystals (Kobatake et al., 2007). Subsequently, several photomechanical crystals have been reported to provide promising opportunities for artificial molecular machinery. The mechanism of the conversion of light energy to mechanical energy, however, has not quantitatively been understood. In photomechanical effect, microscale structural change and stress should be involved with optical properties of anisotropy and chirality; linear birefringence (LB), linear dichroism (LD), circular dichroism (CD) and optical rotatory power (ORP). CD and ORP in a chiral anisotropic crystal are extremely difficult to be measured owing exclusively to the contribution of strong linear anisotropy. The High Accuracy Universal Polarimeter generalized by our group (G-HAUP) enables us to measure LB, LD, CD, and ORP simultaneously and quantitatively (Tanaka et al., 2012). The purpose of our study is to investigate the relationship between microscale structural change or stress induced by UV/VIS radiation and the four optical properties. We synthesized chiral N-3,5-di-tert-butylsalicylidene-1-phenylethylamine photochromic crystal, as shown in Scheme 1, because the mechanism of photomechanical effect in this crystal has been revealed qualitatively by analyzing single-crystal structure under UV/VIS radiation (Koshima et al., 2013). The LB, LD, CD, and ORP spectra in the direction perpendicular to (001) were successfully measured under VIS radiation. Furthermore, LD spectrum was found to change by UV radiation in 30mW/cm². These results could contribute to an elucidation of the mechanism of photomechanical effect quantitatively.

Chiroptical study on benzil crystal using G-HAUP

There has been a great interest in understanding chiroptical properties such as optical rotatory power (ORP) and circular dichroism (CD) in anisotropic materials because these properties provide valuable information on molecular/crystal structure. In particular, generation of chiroptical properties in crystals composed of achiral compounds is considered as one of the most intriguing topics in the field of crystal optics. Benzil (C6H5COCOC6H5), which consists of two phenyl rings and two carbonyl groups, is achiral in solution, but exhibits chirality in the crystalline state. In the crystalline state, the molecules are arranged in a helical form and are put along three-fold left-handed screw axis or right-handed one. The benzil crystal belongs to trigonal crystal system with non-centrosymmetric space group P3121 or P3221. Hence, the crystal symmetry allows the benzil crystal to exhibit ORP and CD and indeed it is often used as a chiral reference crystal in visible/ultraviolet region (S. Chandrasekhar, 1954; N. K. Chaudhuri & M. A. El-Sayed, 1967) as well as α-quartz and nickel sulphate hexahydrate crystals. Meanwhile, ORP and CD reflect spatial dispersion of a second-rank permittivity tensor that depends on not only frequency but also wave vector of an electromagnetic wave. Furthermore, the magnitude of ORP and CD usually vary with the direction of an incident light. It is therefore much important in chiral science to discuss the relationship between ORP and CD along the principal axes and the molecular arrangements along them, respectively. In the case of benzil crystal, it is expected to show different ORP and CD along the a and c axes because of its trigonal crystal system, D3. In this study, we attempt to measure the ORP and CD of benzil crystal in the direction of the c and a axes with the Generalized-High Accuracy Universal Polarimeter (G-HAUP)(M. Tanaka et al., 2012).

Determination of Chirality of the Chiral Space Groups with Two-fold Screw Axis

Crystals are classified according to their symmetry operations into 230 space groups. A two-fold screw axis, 2₁, a symmetry operation, is frequently found in the crystal, especially non-centrosymmetric organic crystals. The two-fold screw axis shows a chiral character when molecules in the crystal tilt against the screw axis as shown in Figure. The crystals belonging to a chiral space group with two-fold screw axis such as a P2₁ or P2₁2₁2₁ exhibit two different types with opposite chirality. However, although the two crystals are not identical in a molecular arrangement, we cannot distinguish them with the present notation of space group. Recently, the idea of determining the handedness of two-fold screw axis have been successfully proposed using hierarchical interpretation of the crystals.(I. Hisaki, T. Sasaki et al., 2012) Nevertheless, the issue on a notation to distinguish chiral crystals with two-fold screw axis still remains unsettled as far as we know. Therefore, we attempt to propose a novel notation for the crystals belonging to chiral space groups with two-fold screw axis. We focus on the relationship between the absolute structure and optical activity of the crystals. We have selected alanine crystals, which belong to P2₁2₁2₁, as model crystals to discuss the notation. We have determined the absolute structure of the alanine crystals by X-ray diffraction and have measured their optical activity with Generalized High Accuracy Universal Polarimeter (G-HAUP).(M. Tanaka, N. Nakamura et al., 2012) G-HAUP is an apparatus that can measure simultaneously the linear birefringence, linear dichroism, circular birefringence, i.e., optical activity, and circular dichroism in any solid material. These experimental results have successfully correlated the absolute structure to the optical activity of the alanine crystals.

Measurement of chiroptical properties of nickel sulfate hexahydrate with G-HAUP

A crystal, which belongs to a lower crystal system than cubic one, exhibits optical anisotropies. The optical anisotropies originate from the difference in refraction and absorption between orthogonally linearly polarized lights. When molecules forming a crystal are enantiomers with the same chirality or form helical structures with the same handedness, the crystal shows chiroptical properties, which originate from the difference in refraction and absorption between right and left circularly polarized lights. The four optical phenomena are called linear birefringence (LB), linear dichroism (LD), circular birefringence (CB) and circular dichroism (CD), respectively. It had been difficult to measure CB and CD in chiral crystals with optical anisotropies because the signals of the anisotropies are three or four orders of magnitude larger than those of chiroptical properties. The Generalized High Accuracy Universal Polarimeter (G-HAUP) [1] enables us to simultaneously measure LB, LD, CB and CD of any anisotropic crystal. Nickel sulfate (NS) is achiral in the solution state. However, in the crystalline state, it forms hexahydrate and exhibits chirality since molecules are put in helical arrangements. The NS crystal belongs to an enantiomorphous space group, P4₁2₁2 or P4₃2₁2. Many researchers have reported the optical properties of NS crystal because large and good-quality crystals are readily grown. However, we consider the LB, LD, CB and CD in NS crystal should be simultaneously and completely investigated. The purpose of this study is to obtain LB, LD, CB and CD along the a axis with G-HAUP and compare the CB and CD with the results along the c axis. We measured optical rotatory power (ORP) along the c axis with G-HAUP, which agrees with the previous results [2,3]. We then prepared for some samples with chirality and anisotropy. We measured LB, LD, CB and CD spectra, respectively and will demonstrate the relation between their optical properties and structures.