Encapsulation of guest molecules into the hollow spaces of crystals has been applied for a variety of purposes such as structure determination, separation, and catalysis of the guest. Although host-guest studies have been developed mainly in crystals of small molecules, those of biomacromolecules have recently been applied. In those reports, a huge hollow space in the protein crystal is commonly used for encapsulation of the guest. Our previous study revealed that cylindrical hemocyanins stack inside the crystal as a linear hollow structure. The diameter of the linear hollow is approximately 110 Å, which is large enough for most proteins to pass through. In the present study, we evaluated the potential of hemocyanin crystals as a host to encapsulate biomacromolecules. Confocal microscopy revealed that hemocyanin crystals encapsulate proteins of molecular mass up to 250 kDa, i.e., 27 kDa green fluorescence protein, 105 kDa allophycocyanin, 220 kDa C-phycocyanin, and 250 kDa phycoerythrin, and DNAs up to 200-bp long, whereas 440 kDa ferritin not. Further analysis revealed that hemocyanin crystals prefer a negatively charged guest rather than a positive charge to encapsulate. Moreover, a photobleaching experiment showed that the guest does not move once entrapped. This knowledge of the host-guest study using the hollow hemocyanin crystal should be of significance for further application of hollow proteinaceous crystals as a host.
Keywords: Encapsulation; Hemocyanin; Hollow space; Protein crystal; host−guest.
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