Large Optical Activity of Gold Nanocluster Enantiomers Induced by a Pair of Optically Active Penicillamines

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.