Quantitative investigation of the interaction between proteins and charged functional groups on the polyglycerol-grafted nanodiamond surface

Interaction at the interface between protein and nanoparticle (NP) surface is of fundamental importance in nanomedicine, because protein corona formation on the NP surface affects the biological identity of the NP. Recently, polyglycerol (PG) has been revealed as a protein free coating for NPs, enabling us to evaluate the net affinity of the specific functional group on the “transparent” NP surface to protein. Herein, PG grafted nanodiamond (ND-PG) was functionalized with carboxyl and amino groups at various densities, and their interaction with proteins including lysozyme (LYS), bovine serum albumin (BSA) and γ-globulin (γ-GLO) were investigated in phosphate buffer saline (PBS). The results show that the carboxylate (-COO^–) and ammonium (-NH₃ ⁺) groups interact exclusively with positively charged LYS, and negatively charged BSA and γ-GLO, respectively, and that the interaction becomes stronger as the functional group density increases. Moreover, the association constants (K) between BSA and the ammonium group on ND-PG are determined and found to correlate linearly with the ammonium group density. The slopes in the linear relationship are proposed to be indices to represent the interaction strength of ammonium group to BSA, enabling the quantitative comparison of the affinity between proteins and various NP surface coatings.