Confinement-Enhanced Luminescence in Protein-Gold Nanoclusters.

Confinement has profound effects on protein functions. Nanoscale probes for confinement or excluded volume interactions could help us understand how these interactions influence protein functions. This work reports on the increased luminescence of BSA-gold nanoclusters when confined. Confinement of the BSA-gold nanoclusters occurred within reverse micelles (RMs), where the size of the RMs determined the degree of confinement. The confinement-enhanced luminescence is reversible, i.e., the emission returns to its original value following cyclic changes in RM size. Circular dichroism measurements show an increase in alpha-helical character of the BSA-stabilized nanoclusters with confinement, which could provide a mechanism for the increase in luminescence. The alpha-helical character of the native proteins also increases with confinement, suggesting that the protein-nanocluster might sense confinement in an analogous fashion as the proteins. When the RMs approach the size of the protein, the intensity becomes independent of alpha-helical character, suggesting a different mechanism for the luminescence increase.