Protein nanoparticle electrostatic interaction: size dependent counterions induced conformational change of hen egg white lysozyme.

In our earlier paper (Ghosh et al., 2013), we have shown that (i) the positively charged hen egg white lysozyme (HEWL), dispersed in water, binds electrostatically with the negatively functionalized iron oxide nanoparticles (IONPs), and (ii) the Na(+) counterions, associated with functionalized IONPs, diffuse into bound proteins and irreversibly unfold them. Having this information, we have extended our investigation and report here the effect of the size and the charge of alkaline metal counterions on the conformational modification of HEWL. In order to obtain a negative functional 'shell' on IONPs and the counterions of different size and charge we have functionalized IONPs with different derivatives of citrate, namely, tri-lithium citrate (TLC, Li3C6H5O7), tri-sodium citrate (TSC, Na3C6H5O7), tri-potassium citrate (TKC, K3C6H5O7) and tri-magnesium citrate (TMC, Mg3C12H10O14). The size of counterions varies as Mg(2+)