Electrostatic Selectivity in Protein–Nanoparticle Interactions

Abstract

The binding of bovine serum albumin (BSA) and β-lactoglobulin (BLG) to TTMA (a cationic gold nanoparticle coupled to 3, 6, 9, 12-Tetraoxatricosan-1-Aminium, 23-mercapto-N, N, N-TriMethyl)- was studied by high-resolution turbidimetry (to observe a critical pH for binding), dynamic light scattering (to monitor particle growth), and isothermal titration calorimetry (to measure binding energetics), all as a function of pH and ionic strength. Distinctively higher affinities observed for BLG vs. BSA, despite the lower pI of the latter, were explained in terms of their different charge anisotropies, namely the negative charge patch of BLG. To confirm this effect, we studied two isoforms of BLG that differ in only two amino acids. Significantly stronger binding to BLGA could be attributed to the presence of the additional aspartates in that negative charge domain for dimer, best portrayed in DelPhi. This selectivity decreases with ionic strength, for which both isoforms bind well below pI, but increases with ionic strength for BLG vs. BSA which binds above pI. This result points to the diminished role of long-range repulsions for binding above pI. Dynamic light scattering reveals a tendency for higher-order aggregation for TTMA–BSA at pH above the pI of BSA, due to its ability to bridge nanoparticles, whereas soluble BLG–TTMA complexes were stable over a range of pH because the charge anisotropy of this protein at pH < pI makes it unable to bridge nanoparticles. Finally, isothermal titration calorimetry shows endoenthalpic binding for all proteins; the higher affinity of TTMA for BLGA vs. BLGB comes from a difference in the dominant entropy term.

Pullulan-based nanoparticles as carriers for transmucosal protein delivery

“…On the contrary, binding of BSA to polycations -on the wrong side‖ of the pI is not favored because its negative domains are fragmented, thus being ineffective at binding polycations, with the long-range repulsion prevailing over the short-range attraction. Only at pH above pI, where wider negative domains form, soluble complexes are observed, with coacervation occurring at even higher pH values (Chen et al, 2011).…”

“…Therefore, it is possible to assume that protein association does not neutralise significantly the positive surface charge of chitosan or aminated pullulan molecules (Gan and Wang, 2007). Also, although there is the possibility of further protein adsorption on the surface of formed nanoparticles, this would occur by interaction of the negative domains of the former, with the positive charge of the latter, therefore partially or totally neutralizing the protein negative global charge (Chen et al, 2011). Moreover, in the case of SP/CS nanoparticles, the final solution pH is 3.3, well below BSA pI.…”

Nanoparticles Carrying Biological Molecules: Recent Advances and Applications

“…For instance, immobilisation of lysozyme and β-lactoglobulin, a globular protein onto negatively charged silica nanoparticles and binding of bovine serum albumin and β-lactoglobulin to cationic gold nanoparticle functionalised with 3,6,9,12-tetraoxatricosan1-1-aminium, 23-mercapto-N,N,N-trimethyl, under different pH and ionic conditions (Chen K. et al, 2011;Meissner J. et al, 2015). Generally, the maximum adsorption occurs at the protein isoelectric point.…”

Differential interactions of halophilic and non-halophilic proteases with nanoparticles

“…It may therefore be inferred that nanoparticles are adsorbed differentially in halophilic and non-halophilic proteases due to the influence of charge. Stronger binding of cationic gold nanoparticles to β-lactoglobulin (BLG) isomer BLGA over BLGB was attributed to the presence of additional aspartate residues [21]. The higher affinity of the nanoparticle for the protein may arise due to strong electrostatic interaction between positive charge density of the nanoparticle with the negatively charged domain on the protein.…”