We have succeeded for the first time in preparing a pair of gold nanocluster enantiomers protected by optically active thiols: D- and L-penicillamine (D-Pen and L-Pen). Circular dichroism (CD) spectroscopy confirmed the mirror image relationship between the D-Pen-capped and the L-Pen-capped gold nanoclusters, suggesting that the surface modifier acts as a chiral selector, and that the nanoclusters have well-defined stereostructures as common chiral molecules do. No CD signals could be obtained when the gold nanoclusters were synthesized by using a racemic mixture (rac-Pen). These chiroptical properties were investigated for the three separated fractions of each of the gold nanoclusters (D-Pen-capped, L-Pen-capped, or rac-Pen-capped clusters) by polyacrylamide gel electrophoresis (PAGE). Each fractioned component has the mean diameter of 0.57, 1.18, or 1.75 nm that was determined by a solution-phase small-angle X-ray scattering. With a decrease in the mean cluster diameter, optical activity or anisotropy factors gradually increased. On the basis of the kinetic and the structural considerations, the origins of large optical activity of the gold nanocluster enantiomers are discussed.

The effect of solvent on the crystallization behavior of the polymorphs of 2-(3-cyano-4-isobutyloxyphenyl)-4-methyl-5-thiazolecarboxylic acid (BPT) was investigated under rapid cooling. From methanol (MeOH) and ethanol (EtOH) solutions, only the solvated crystals of the D forms of methanol (D(MeOH)) and ethanol (D(EtOH)) crystallized. Both D forms are stable and have similar crystal structures. However, the solubility of the D(EtOH) form is 1.5 times higher than that of the D(MeOH) form. With the release of alcohol molecules, both D forms transformed to the C form with an increase in temperature for the DSC measurement. After that, the C form transformed to the A form via a melt-mediated mechanism. The release temperature of alcohol was higher for D(EtOH) than for D(MeOH). When the crystallization was performed in 1-propanol (1-PrOH) and 2-propanol (2-PrOH), the metastable A form preferentially crystallized. On the other hand, in acetonitrile (MeCN) solutions the stable C form was selectively obtained. These crystallization behaviors in each solvent did not depend on supersaturation in solutions. The FTIR spectra of BPT in EtOH and 1-PrOH suggested that BPT molecules in solution take a conformation similar to that in each crystal. These results suggest that the solvent effect is controlled by the thermodynamic equilibrium properties such as the conformation of the solute and the solute–solvent interactions rather than the crystallization kinetics of the polymorphs. Furthermore, the solution-mediated transformation rate from the A form to the C form is higher in MeCN than those in 1-PrOH and 2-PrOH. In the mixed solvents of 1-PrOH and MeCN with water, the same polymorphs crystallized as those obtained in pure solvents in the water volume fraction up to the range of 0.1. However, the hydrated crystals (BH form) predominantly crystallized with further addition of water. Solubility measurements suggested that such behavior is related to the solvated structure surrounding the BPT molecule.

The three types of inclusion compounds of cholanamide (CAM, 3α, 7α, 12α-trihydroxy-5β-cholan-24-amide) have been crystallized from the solutions of (S)-butan-2-ol (CAMSB), (R)-butan-2-ol (CAMRB) and racemic butan-2-ol (CAMSRB), respectively. The crystal structures have been determined. The three crystal structures are isomorphous to each other and revealed that the host CAM molecules form the same layered arrangements, providing channel spaces for the guest butan-2-ol molecules. As expected, the CAMSB and CAMRB crystals include the pure (S)- and (R)-enantiomers of butan-2-ol, whereas the (S)-enriched mixture of enantiomers is accommodated in CAMSRB with a molar ratio between the host CAM and guest butan-2-ol molecules of 1:1. The hydrogen-bond network is rigidly formed between the CAM molecules and also between CAM and butan-2-ol molecules. CAMSB and CAMRB have slightly different unit-cell dimensions: the channels in CAMRB have a larger section, resulting in a larger unit-cell volume. In CAMSRB, although both enantiomers of the guest alcohol are included, the (S)-enantiomer is more abundant, indicating that the optical resolution occurs during the crystallization step.