Origins of the pH-Responsive Photoluminescence of Peptide-Functionalized Au Nanoclusters

Ultrasmall peptide-protected gold nanoclusters are a promising class of bioresponsive material exhibiting pH-sensitive photoluminescence. We present a theoretical insight into the effect peptide-ligand environment has on pH-responsive fluorescence, with the aim of enhancing the rational design of gold nanoclusters for bioapplications. Employing a hybrid quantum/classical computational methodology, we systematically calculate deprotonation free energies of N-terminal cysteine amine groups in proximity to the inherently fluorescent core of Au25(Peptide)18 nanoclusters. We find that subtle changes in hexapeptide sequence alter the electrostatic environment and significantly shift the conventional N-terminal amine pKa expected for amino acids free-in-solution. Our findings provide an insight into how the deprotonation equilibrium of N-terminal amine and side chain carboxyl groups cooperatively respond to solution pH changes, explaining the experimentally observed, yet elusive, pH-responsive fluorescence of peptidefunctionalized Au25 clusters.

Automated summary

Through theoretical investigation, we have revealed the impact of peptide-ligand environment on the pH-responsive fluorescence of gold nanoclusters. Our findings show that subtle changes in the hexapeptide sequence can significantly alter the pH response of the clusters, providing insights into the underlying mechanism.