Highlights d The regulatory C-tail/N-domain interaction of arrestin-1 is resolved d Arrestin-1 binds InsPs using many of the same residues that hold the C-tail d InsP binding does not activate arrestin-1, but partially removes the C-tail

The three-dimensional structure of protein is determined by analyzing diffraction data collected using X-ray beams. However, X-ray beam can damage protein crystals during data collection, lowering the quality of the crystal data. A way to prevent such damage is by treating protein crystals with cryoprotectants. The cryoprotectant stabilizes the protein crystal and prevents lowering the quality of the diffraction data. Many kinds of cryoprotectants are commercially available, and various treatment methods have also been reported. However, incorrect selection or treatment of such cryoprotectants may lead to deterioration of crystal diffraction data when using X-ray beams.

Structural analysis of proteins using X-ray crystallography is crucial for discovering their biochemical functionalities. However, growing X-ray-quality crystals of proteins is often a challenging task that requires complicated and tedious processes, especially for the formation of crystalline seeds in the early stage of the process. In the present study, graphene quantum dots (GQDs) were investigated as a nanomaterial nucleant for protein crystallization in the aspects of the process accelerations and their possible underlying mechanisms, where lysozyme was employed for a model system. Compared to that without GQDs, dramatically faster formation of crystalline seeds was observed in the presence of GQDs within 2 h of incubation. The hydrodynamic size of lysozyme increased by about 30% when GQDs were included according to dynamic light scattering measurements, implying a possible binding of GQDs to the protein. X-ray diffraction analysis of lysozyme crystals also implies the possible binding of GQDs to the protein by showing reduced thermal B-factors when GQDs were included, suggesting flexibility reducing interactions of GQDs with random coil sites. In spite of the prevalent existence of GQDs in the crystal, the X-ray structural analysis cell parameters of lysozyme grown with GQDs showed negligible differences compared to those grown without GQDs, suggesting that GQDs might be an ideal nucleant for protein crystallization.

In the present study, we used the electrochemical transparency of graphene to show that the direct intercalation of alkali-metal cations is not a prerequisite for the redox reaction of Prussian blue (PB). PB thin films passivated with monolayer graphene still underwent electrochemical redox reactions in the presence of alkali-metal ions (K+ or Na+) despite the inability of the cations to penetrate the graphene and be incorporated into the PB. Graphene passivation not only preserved the electrochemical activity of the PB but also substantially enhanced the stability of the PB. As a proof of concept, we showed that a transparent graphene electrode covering PB can be used as an excellent hydrogen peroxide transducer, thereby demonstrating the possibility of realizing an electrochemical sensor capable of long-term measurements.