One- and two-photon brightness of proteins interacting with gold. A closer look at gold–insulin conjugates

Abstract

Red luminophores displaying large Stokes shift and high-quantum yields are obtained when gold salts are reacted with proteins under strongly alkaline conditions. Although bovine serum albumin (BSA) has mainly been used as a protein template, other attempts to prepare red luminophores have been proposed using other proteins. Here, we report on the structural characterization and nonlinear optical properties of insulin–gold conjugates. Such conjugates display strong luminescence at ∼670 nm with quantum yields that reach 5.4%. They also display long luminescence lifetimes allowing efficient reactive oxygen species generation, with a quantum yield of ¹O₂ generation reaching 13%. In addition, they exhibit remarkable nonlinear optical properties and in particular a strong two-photon excited fluorescence (TPEF) cross section in the range of 800–1100 nm. By combining experimental studies and time-dependent density functional theory simulations (TD–DFT), we show the formation of insulin–Au(III) conjugates. The interaction of Au(III) ions with the aromatic rings of tyrosine induces charge transfer-like excitation in the visible range. Experimental investigations, together with molecular dynamics simulations of insulin and calculations of electronic properties in a model system, are performed to explore the origin of optical features and the structure–optical property relationship, leading the way to new concepts for nonlinear optics using protein–Au(III) conjugates.