Nanoscale and mesoscopic protein assemblies in lysozyme-NaCl solutions probed by dynamic light scattering

Dynamic light scattering was used to analyze nanoscale and mesoscopic protein-associated objects formed in lysozyme-NaCl solutions under crystallization conditions. The study focuses on the sequence of particle-size populations detected after salt addition and on their relation to the initial state of the protein solution. NaCl reproducibly induced a small population with hydrodynamic diameters of 5.6-6.5 nm, which is consistent with nanoscale oligomeric lysozyme assemblies and with previous SAXS/SANS evidence for dimers and octamers in crystallization solutions. Mesoscopic particles in the tens-to-hundreds of nanometers range and large submicron-to-micron aggregates appeared at selected stages of the experiments. The samples that produced crystals showed different mesoscopic-particle dynamics, whereas a sample with similar early DLS behavior did not yield crystals. These observations suggest that, in this limited series, the evolution of protein-associated nano- and mesoscopic objects was related to the initial particle-size distribution and the full temporal trajectory rather than to any single DLS peak. The DLS data are interpreted qualitatively, since intensity-weighted distributions do not provide direct number or mass fractions of the detected populations. The results characterize a multiscale and nonstationary self-organization process in a concentrated protein solution, involving a small nanoscale population consistent with oligomeric precursor clusters, mesoscopic aggregated states, and large sedimenting objects. This work may be useful for understanding soft biological nanoobjects and mesoscopic protein assemblies formed during salt-induced association and crystallization-related restructuring of concentrated protein solutions.