The penetration coefficient of fluorescein can be used to estimate the accessible free volume of the framework as discussed in Ref. For l = 42 bp the fraction of free volume reaches 85%, a value well above what can be reached with alternative nanoscale DNA building blocks and that compares favourably even against ultrahigh porosity metal-organic frameworks. Large SAv molecules are completely excluded from frameworks with l = 24 and 28 bp, for which ξ values are not significantly higher than the measured background, indicating no loading into crystals. A sharp penetration transition is then observed upon increasing l to 35 and 42 bp, which produce similar values of ξ. This indicates that the pores in the l = 24 and 28 bp crystals are too small to accommodate the rather bulky protein, whereas the larger pores of the l = 35 and 42 bp crystals can. As an additional check to show that SAv was completely excluded from crystals with smaller pore size, we added FAM to each of the samples previously exposed to SAv. As shown in Fig. (bi-color symbols), and further highlighted in Fig. -stars with l = 35 and 42 bp show a decrease in the ξ measured for FAM in crystals with SAv in comparison to crystals not soaked in streptavidin -a clear proof that the bulky protein is excluding a fraction of the available free volume to fluorescein. For samples with l = 24 and 28 bp, no change in ξ is observed, confirming that streptavidin does not penetrate these frameworks. Note that SAv displays an affinity for the surface of the crystals and coats them even in conditions in which the mesh size is too small to allow penetration (Fig. ). The affinity is likely due to hydrophobic patches on the proteins. Rhodamine B, a small hydrophobic molecule, was observed to strongly partition within the crystals regardless of arm length, likely due to an affinity for the cholesterol-rich cores, as is clearly visible in Fig. .