Kinetically Limited Fluorescent Labeling of a Small Icosahedral Capsid for Superradiant Virus-like Particles (srVLPs).

Superradiant virus-like particles (srVLPs) consist of hundreds of chromophores covalently attached to a nanoscale viral capsid. Upon excitation with a short laser pulse, srVLPs emit a picosecond burst of light at room temperature. Their emission dynamics exhibit superradiance (SR), a collective phenomenon where weakly coupled quantum emitters synchronize to emit in phase (1, 2). Due to their unique optical properties, srVLPs hold promise for biomedical imaging and nanotechnology. However, key aspects of their photophysical behavior remain unclear, specifically how chromophore location and local environment within the capsid affect emission.

To address this, we use the Brome Mosaic Virus (BMV) capsid as a model for stepwise chemical modification targeting lysine residues classified by reactivity, accessibility, and location. Leveraging amine modification kinetics, we vary reactive dye concentration to optimize labeling and characterize how physical and optical properties evolve with dye loading. Fluorescent conjugation occurs sequentially, with dye binding first to the capsid exterior, then populating interior sites beyond an occupancy threshold. Passivation of accessible residues prior to labeling reduces dye loading, increases capsid flexibility, and alters chromophore environment and dynamics. Kinetic fitting reveals cooperative interactions between lysine sites across the capsid are needed to accurately model conjugation.