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\section{Discussion}  The radii obtained by anisotropy measurements are in the expected range and consistent with calculations based on MW, see Fig~\ref{fig:theor_radii}. Our measured radius 3.49$\pm$0.03~nm for well-characterised protein BSA is in excellent agreement with the accepted hydrodynamic radius of BSA 3.48~nm.\cite{Axelsson1978} The cubic dependence of the radius in on  the rotational correlation time means that this technique is more sensitive to small changes in radius than methods where the radius depends on the measurement linearly. Dynamic light scattering (DLS), based on the measurement of fluctuating scattered light intensity due to Brownian particles motion, is a well established method for determining the size of small particles in solution, including macromolecular drugs. Wen \textit{et al} \cite{Wen2013} report hydrodynamic radii of 4.2~nm for ranibizumab and 6.3~nm for bevacizumab using DLS, and 5.4~nm for BSA measured as a control, and Li \textit{et al} \cite{Li2011} 4.1~nm, 6.5~nm and 4.8~nm for the same molecules, respectively. These results are slightly higher than estimates based on MW, which they suspect could be due to aggregation. DLS is limited to very low particle concentrations, and susceptible to artefacts arising from dust and other contamination.