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However, proteins have a rough surface, are often not perfectly spherical, and the ionic charge has an effect to the diffusion of a molecule in solution. The hydrodynamic radius $R_h$, defined as the radius of a hard sphere that diffuses at the same rate as that solute, takes these effects into account. The hydrodynamic radius is important in predicting transretinal penetration.\cite{Jackson2003} For example, Ambati et al.\ reported that the globular protein albumin, with a MW of 69~kDa and $R_h$ of 3.62~nm, had approximately twice the scleral permeability of a linear dextran of 40~kDa and $R_h$ of 4.5~nm.\cite{Ambati2000a}   Small-angle scattering studies using X-rays (SAXS) or neutrons (SANS) \cite{Svergun_2013} as well as dynamic light scattering (DLS) \cite{1985, \cite{Pecora_1985,  Hong_2009} and NMR techniques \cite{Wilkins1999} have been used for measuring $R_h$. Global analysis of hundreds of proteins has led to the definition of empirical relationships between $R_h$ and the number of amino acids $N$, related to the MW by \(N = \frac{\text{MW}}{110 \text{ Da}}\). Such formulas have been defined, for example, by Wilkins \cite{Wilkins1999} \begin{equation}  R_h=4.75\cdot N^{0.29} \label{eq:Wilkins}  \end{equation}