Investigating the assembly of multiple nanoparticles encapsulated in a viral capsid

Viral capsids are nanoscale structures that spontaneously self-assemble from many copies of proteins, often around a cargo. The cargo can be genetic material, enzymes, and smaller nanoparticles (NPs), that ultimately determine the properties of the associated co-assembled products. Unlike the co-assembly of a capsid around a single NP, the co-assembly of multiple small NPs encapsulating within a single capsid is under-explored. The encapsulation and arrangement of multiple NPs within capsids is of both fundamental interest and practical importance with applications in sensing and imaging. Utilizing a biological template derived from the Brome Mosaic Virus (BMV) capsid and gold NPs, we explore the stability of many-NP-encapsulated capsid complexes via experiments and simulations. A coarse-grained model is developed to capture the steric and electrostatic interactions between NPs, and between NPs and the capsid. Molecular dynamics simulations of this model are performed for NPs of different sizes and charges in a broad range of solution conditions that mimic experimental conditions. The distribution of NPs within the capsid is extracted using simulations and compared to high-resolution transmission electron microscopy (TEM) and cryo-EM reconstruction images obtained experimentally. The factors influencing the stability of the many-NP-encapsulated capsids are discussed.