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 ...