Utilization of Cyclodextrins and Its Derivative Particles as Nucleants for Protein Crystallization

Finding new nucleants to promote protein crystallization is an important task, especially for purposes other than structural determination. Here, we investigated cyclodextrins and its derivative particles, as potential nucleants for protein crystallization. β-Cyclodextrin (β-CD) and its derivatives (including p-toluenesulfonyl-β-cyclodextrin (PTCD), polymer-β-cyclodextrin (PCD), mono-(6-(1,6-hexamethylenediamine)-6-deoxy)-β-cyclodextrin (MHCD) and mercapto-β-cyclodextrin (MCD)) were used as nucleants. The experimental results confirmed that β-CD and its derivatives showed significantly positive effects, promoting protein crystallization and improving crystal quality. A larger number of protein molecules (including lysozyme, catalase, subtilisin A VIII, concanavalin A VI, α-chymotrypsinogen, proteinase K, cellulase, papain, glucose isomerase, hemoglobin, and ribonuclease A XII) attached to the particles usually corresponded to a higher crystallization success rate. More detailed analysis showed that cyclodextrins exhibited the best performance when the overall charge of protein in solution was the opposite to zeta potential of the cyclodextrins particle. Our results showed that cyclodextrins can be useful as nucleants due to the ease of modifying them to suit the crystallization of different proteins, and they can be explored for use in the mass purification of proteins for the biopharmaceutical industry. Furthermore, the phenomenon discovered in this study pointed toward a way to find new nucleants based on the overall charge of proteins in a solution: the nucleants should preferably be the opposite between the overall charge of target protein and the zeta potential of the cyclodextrin particle.