Dynamic light scattering (DLS)
Dynamic light scattering (DLS) is a non-invasive, quick and robust technique to determine the hydrodynamic size of molecules or particles.
Based on intensity fluctuations of laser light scattered by the molecules/particles, moving in Brownian motion, the diffusion coefficient is determined and converted to particle size via the Stokes-Einstein equation. DLS can determine the hydrodynamic size of protein monomers, small aggregates in the nanometer range and partially also particles in the high nanometer/low micrometer range. The technique can also be employed for the analysis of colloidal systems, such as liposomes, nanoparticles, polymers and virus-like particles.
The particle size range of DLS depends on the properties of the analyzed species, such as refractive index or density, as well as the surrounding formulation, mainly the viscosity. For protein monomers and aggregates, a size range from about 1 nm to several µm can be covered. Particle concentrations cannot be obtained by DLS. For aggregated protein samples, DLS results can show that the aggregate distribution in the sample is polydisperse, but it is not feasible to obtain detailed quantitative information on monomer and aggregate content.
DLS is mainly used as a research tool during formulation development and to generate supportive data on formulation purity (mainly with respect to aggregate and particle content) for comparability exercises or stability studies.
Because of its high sensitivity to detect minute amounts of aggregates and the possibility to measure small sample volumes (e.g., using plate readers), DLS is a suitable technique for (formulation) screening purposes. Another quantity that DLS can deliver is the interaction parameter (kd) derived from the determination of the diffusion coefficient, which can be a surrogate for the second virial coefficient (B22). The kd can be used to predict the colloidal stability of a protein, which may impact the aggregation propensity of a protein within a formulation.
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S. Zölls, R. Tantipolphan, M. Wiggenhorn, G. Winter, W. Jiskoot, W. Friess, A. Hawe, Particles in therapeutic protein formulations, Part 1: overview of analytical methods., J. Pharm. Sci. 101  914–35. doi:10.1002/jps.23001.