Tryptophan-Gold Nanoparticle Interaction: A First-Principles Quantum Mechanical Study

The nature of interaction between tryptophan (Trp) and gold (Au) nanoparticles is studied using first-principles density functional theory calculations and described in terms of equilibrium configurations, interaction energies, density of states, molecular orbitals, and charge density. The calculated results find the binding involving both carboxyl and indole functional groups with mixed salt-bridge and charge-solvation structure to be energetically preferred and is attributed to the stronger-than-expected pi interactions facilitated by the indole group in Trp. In the Trp-Au conjugates, modification of the molecular orbitals associated with Trp occurs because states associated with Au and the hybrid orbitals have mixed metal-molecule character. We find that the nonradiative energy transfer from excited states of Trp to hybrid states of Trp-Au may reduce intrinsic fluorescence intensity of Trp in the conjugated system.