  A quantitative understanding of aggregation mechanisms leading to the
formation of inorganic nanoparticles (NPs) and protein composites in aqueous
media is of paramount interest for colloid chemistry. In particular, the
interactions between silica (SiO2) NPs and lysozyme (LZM) have attracted
attention, because LZM is well-known to adsorb strongly to silica NPs, while at
the same time preserving its enzymatic activity. The inherent nature of the
aggregation processes leading to NP-LZM composites involves structural changes
at length-scales from few to hundreds of nanometres but also time scales << 1
second. To unravel these we used in situ synchrotron-based small-angle X-ray
scattering (SAXS) and followed the subtle interparticle interactions in
solution at a time resolution of 50 ms/frame (20 fps). We show that if the size
of silica NPs (~5 nm diameter) is matched by the dimensions of LZM, the
evolving scattering patterns contain a unique structure factor contribution
originating from the presence of LZM. We developed a scattering model and
applied it to analyse this structure function, which allowed us to extract
structural information on the deformation of lysozyme molecules during
aggregation, as well as to derive the mechanisms of composite formation.
