CRYSTALLIZATION OF BASIC-PROTEINS BY ION-PAIRING

We tested a number of salts against their ability to crystallize hen egg white lysozyme [M.M. Ries-Kautt and A.F. Ducruix, J. Biol. Chem. 264 (1989) 745] and showed that, in the case of lysozyme, the effectiveness of the ions toward crystallization is linked to the reverse order of Hofmeister series, i.e SCN- > NO3- > Cl- > HC6H5O72- > CH3COO- > H2PO4- > SO42-. The ability of lysozyme to crystallize at very low concentration of SCN- is probably due to ion pairing between the anion and positively charged residues of the protein. The possibility of crystallizing basic proteins with a family of various anionic species has been studied in the present work and linked to Pearson's theory of "hard" and "soft" acid and base interactions. Although the nature of interactions between ions and proteins has not yet been measured, we demonstrate the possibility to rationalize crystallization conditions for basic proteins at acidic pH. As a consequence, it is suggested that ion pairing should be possible for acidic proteins at basic pH in the presence of appropriate cationic species. Some applications are developed.